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itemprop=\"text\"\u003e\u003cp dir=\"auto\"\u003ePEP: 638\nTitle: Syntactic Macros\nAuthor: Mark Shannon \u0026lt;\u003ca href=\"mailto:mark@hotpy.org\"\u003emark@hotpy.org\u003c/a\u003e\u0026gt;\nDiscussions-To: \u003ca href=\"https://mail.python.org/archives/list/python-dev@python.org/thread/U4C4XHNRC4SHS3TPZWCTY4SN4QU3TT6V/\" rel=\"nofollow\"\u003ehttps://mail.python.org/archives/list/python-dev@python.org/thread/U4C4XHNRC4SHS3TPZWCTY4SN4QU3TT6V/\u003c/a\u003e\nStatus: Draft\nType: Standards Track\nCreated: 24-Sep-2020\nPost-History: 26-Sep-2020\u003c/p\u003e\n\u003ca name=\"user-content-abstract\"\u003e\u003c/a\u003e\n\u003cdiv class=\"markdown-heading\" dir=\"auto\"\u003e\u003ch2 tabindex=\"-1\" class=\"heading-element\" dir=\"auto\"\u003eAbstract\u003c/h2\u003e\u003ca id=\"user-content-abstract\" class=\"anchor\" aria-label=\"Permalink: Abstract\" href=\"#abstract\"\u003e\u003csvg class=\"octicon octicon-link\" viewBox=\"0 0 16 16\" version=\"1.1\" width=\"16\" height=\"16\" aria-hidden=\"true\"\u003e\u003cpath d=\"m7.775 3.275 1.25-1.25a3.5 3.5 0 1 1 4.95 4.95l-2.5 2.5a3.5 3.5 0 0 1-4.95 0 .751.751 0 0 1 .018-1.042.751.751 0 0 1 1.042-.018 1.998 1.998 0 0 0 2.83 0l2.5-2.5a2.002 2.002 0 0 0-2.83-2.83l-1.25 1.25a.751.751 0 0 1-1.042-.018.751.751 0 0 1-.018-1.042Zm-4.69 9.64a1.998 1.998 0 0 0 2.83 0l1.25-1.25a.751.751 0 0 1 1.042.018.751.751 0 0 1 .018 1.042l-1.25 1.25a3.5 3.5 0 1 1-4.95-4.95l2.5-2.5a3.5 3.5 0 0 1 4.95 0 .751.751 0 0 1-.018 1.042.751.751 0 0 1-1.042.018 1.998 1.998 0 0 0-2.83 0l-2.5 2.5a1.998 1.998 0 0 0 0 2.83Z\"\u003e\u003c/path\u003e\u003c/svg\u003e\u003c/a\u003e\u003c/div\u003e\n\u003cp dir=\"auto\"\u003eThis PEP adds support for syntactic macros to Python.\nA macro is a compile-time function that transforms\na part of the program to allow functionality that cannot be\nexpressed cleanly in normal library code.\u003c/p\u003e\n\u003cp dir=\"auto\"\u003eThe term \"syntactic\" means that this sort of macro operates on the program's\nsyntax tree. This reduces the chance of mistranslation that can happen\nwith text-based substitution macros, and allows the implementation\nof \u003ca href=\"https://en.wikipedia.org/wiki/Hygienic_macro\" rel=\"nofollow\"\u003ehygienic macros\u003c/a\u003e.\u003c/p\u003e\n\u003cp dir=\"auto\"\u003eSyntactic macros allow libraries to modify the abstract syntax tree during compilation,\nproviding the ability to extend the language for specific domains without\nadding to complexity to the language as a whole.\u003c/p\u003e\n\u003ca name=\"user-content-motivation\"\u003e\u003c/a\u003e\n\u003cdiv class=\"markdown-heading\" dir=\"auto\"\u003e\u003ch2 tabindex=\"-1\" class=\"heading-element\" dir=\"auto\"\u003eMotivation\u003c/h2\u003e\u003ca id=\"user-content-motivation\" class=\"anchor\" aria-label=\"Permalink: Motivation\" href=\"#motivation\"\u003e\u003csvg class=\"octicon octicon-link\" viewBox=\"0 0 16 16\" version=\"1.1\" width=\"16\" height=\"16\" aria-hidden=\"true\"\u003e\u003cpath d=\"m7.775 3.275 1.25-1.25a3.5 3.5 0 1 1 4.95 4.95l-2.5 2.5a3.5 3.5 0 0 1-4.95 0 .751.751 0 0 1 .018-1.042.751.751 0 0 1 1.042-.018 1.998 1.998 0 0 0 2.83 0l2.5-2.5a2.002 2.002 0 0 0-2.83-2.83l-1.25 1.25a.751.751 0 0 1-1.042-.018.751.751 0 0 1-.018-1.042Zm-4.69 9.64a1.998 1.998 0 0 0 2.83 0l1.25-1.25a.751.751 0 0 1 1.042.018.751.751 0 0 1 .018 1.042l-1.25 1.25a3.5 3.5 0 1 1-4.95-4.95l2.5-2.5a3.5 3.5 0 0 1 4.95 0 .751.751 0 0 1-.018 1.042.751.751 0 0 1-1.042.018 1.998 1.998 0 0 0-2.83 0l-2.5 2.5a1.998 1.998 0 0 0 0 2.83Z\"\u003e\u003c/path\u003e\u003c/svg\u003e\u003c/a\u003e\u003c/div\u003e\n\u003cp dir=\"auto\"\u003eNew language features can be controversial, disruptive and sometimes divisive.\nPython is now sufficiently powerful and complex, that many proposed additions\nare a net loss for the language due to the additional complexity.\u003c/p\u003e\n\u003cp dir=\"auto\"\u003eAlthough a language change may make certain patterns easy to express,\nit will have a cost. Each new feature makes the language larger,\nharder to learn and harder to understand.\nPython was once described as \u003ca href=\"https://www.linuxjournal.com/article/4731\" rel=\"nofollow\"\u003ePython Fits Your Brain\u003c/a\u003e,\nbut that becomes less and less true as more and more features are added.\u003c/p\u003e\n\u003cp dir=\"auto\"\u003eBecause of the high cost of adding a new feature,\nit is very difficult or impossible to add a feature that would benefit only\nsome users, regardless of how many users, or how beneficial that feature would\nbe to them.\u003c/p\u003e\n\u003cp dir=\"auto\"\u003eThe use of Python in data science and machine learning has grown very rapidly\nover the last few years.\nHowever, most of the core developers of Python do not have a background in\ndata science or machine learning.\nThis makes it extremely difficult for the core developers to determine whether a\nlanguage extension for machine learning is worthwhile.\u003c/p\u003e\n\u003cp dir=\"auto\"\u003eBy allowing language extensions to be modular and distributable, like libraries,\ndomain-specific extensions can be implemented without negatively impacting\nusers outside of that domain.\nA web developer is likely to want a very different set of extensions from\na data scientist.\nWe need to let the community develop their own extensions.\u003c/p\u003e\n\u003cp dir=\"auto\"\u003eWithout some form of user-defined language extensions,\nthere will be a constant battle between those wanting to keep the\nlanguage compact and fitting their brains, and those wanting a new feature\nthat suits their domain or programming style.\u003c/p\u003e\n\u003ca name=\"user-content-improving-the-expressiveness-of-libraries-for-specific-domains\"\u003e\u003c/a\u003e\n\u003cdiv class=\"markdown-heading\" dir=\"auto\"\u003e\u003ch3 tabindex=\"-1\" class=\"heading-element\" dir=\"auto\"\u003eImproving the expressiveness of libraries for specific domains\u003c/h3\u003e\u003ca id=\"user-content-improving-the-expressiveness-of-libraries-for-specific-domains\" class=\"anchor\" aria-label=\"Permalink: Improving the expressiveness of libraries for specific domains\" href=\"#improving-the-expressiveness-of-libraries-for-specific-domains\"\u003e\u003csvg class=\"octicon octicon-link\" viewBox=\"0 0 16 16\" version=\"1.1\" width=\"16\" height=\"16\" aria-hidden=\"true\"\u003e\u003cpath d=\"m7.775 3.275 1.25-1.25a3.5 3.5 0 1 1 4.95 4.95l-2.5 2.5a3.5 3.5 0 0 1-4.95 0 .751.751 0 0 1 .018-1.042.751.751 0 0 1 1.042-.018 1.998 1.998 0 0 0 2.83 0l2.5-2.5a2.002 2.002 0 0 0-2.83-2.83l-1.25 1.25a.751.751 0 0 1-1.042-.018.751.751 0 0 1-.018-1.042Zm-4.69 9.64a1.998 1.998 0 0 0 2.83 0l1.25-1.25a.751.751 0 0 1 1.042.018.751.751 0 0 1 .018 1.042l-1.25 1.25a3.5 3.5 0 1 1-4.95-4.95l2.5-2.5a3.5 3.5 0 0 1 4.95 0 .751.751 0 0 1-.018 1.042.751.751 0 0 1-1.042.018 1.998 1.998 0 0 0-2.83 0l-2.5 2.5a1.998 1.998 0 0 0 0 2.83Z\"\u003e\u003c/path\u003e\u003c/svg\u003e\u003c/a\u003e\u003c/div\u003e\n\u003cp dir=\"auto\"\u003eMany domains see repeated patterns that are difficult or impossible\nto express as a library.\nMacros can express those patterns in a more concise and less error-prone way.\u003c/p\u003e\n\u003ca name=\"user-content-trialing-new-language-features\"\u003e\u003c/a\u003e\n\u003cdiv class=\"markdown-heading\" dir=\"auto\"\u003e\u003ch3 tabindex=\"-1\" class=\"heading-element\" dir=\"auto\"\u003eTrialing new language features\u003c/h3\u003e\u003ca id=\"user-content-trialing-new-language-features\" class=\"anchor\" aria-label=\"Permalink: Trialing new language features\" href=\"#trialing-new-language-features\"\u003e\u003csvg class=\"octicon octicon-link\" viewBox=\"0 0 16 16\" version=\"1.1\" width=\"16\" height=\"16\" aria-hidden=\"true\"\u003e\u003cpath d=\"m7.775 3.275 1.25-1.25a3.5 3.5 0 1 1 4.95 4.95l-2.5 2.5a3.5 3.5 0 0 1-4.95 0 .751.751 0 0 1 .018-1.042.751.751 0 0 1 1.042-.018 1.998 1.998 0 0 0 2.83 0l2.5-2.5a2.002 2.002 0 0 0-2.83-2.83l-1.25 1.25a.751.751 0 0 1-1.042-.018.751.751 0 0 1-.018-1.042Zm-4.69 9.64a1.998 1.998 0 0 0 2.83 0l1.25-1.25a.751.751 0 0 1 1.042.018.751.751 0 0 1 .018 1.042l-1.25 1.25a3.5 3.5 0 1 1-4.95-4.95l2.5-2.5a3.5 3.5 0 0 1 4.95 0 .751.751 0 0 1-.018 1.042.751.751 0 0 1-1.042.018 1.998 1.998 0 0 0-2.83 0l-2.5 2.5a1.998 1.998 0 0 0 0 2.83Z\"\u003e\u003c/path\u003e\u003c/svg\u003e\u003c/a\u003e\u003c/div\u003e\n\u003cp dir=\"auto\"\u003eIt is possible to demonstrate potential language extensions using macros.\nFor example, macros would have enabled the \u003ccode\u003ewith\u003c/code\u003e statement and\n\u003ccode\u003eyield from\u003c/code\u003e expression to have been trialed.\nDoing so might well have lead to a higher quality implementation\nat first release, by allowing more testing\nbefore those features were included in the language.\u003c/p\u003e\n\u003cp dir=\"auto\"\u003eIt is nearly impossible to make sure that a new feature is completely reliable\nbefore it is released; bugs relating to the \u003ccode\u003ewith\u003c/code\u003e and \u003ccode\u003eyield from\u003c/code\u003e\nfeatures were still being fixed many years after they were released.\u003c/p\u003e\n\u003ca name=\"user-content-long-term-stability-for-the-bytecode-interpreter\"\u003e\u003c/a\u003e\n\u003cdiv class=\"markdown-heading\" dir=\"auto\"\u003e\u003ch3 tabindex=\"-1\" class=\"heading-element\" dir=\"auto\"\u003eLong term stability for the bytecode interpreter\u003c/h3\u003e\u003ca id=\"user-content-long-term-stability-for-the-bytecode-interpreter\" class=\"anchor\" aria-label=\"Permalink: Long term stability for the bytecode interpreter\" href=\"#long-term-stability-for-the-bytecode-interpreter\"\u003e\u003csvg class=\"octicon octicon-link\" viewBox=\"0 0 16 16\" version=\"1.1\" width=\"16\" height=\"16\" aria-hidden=\"true\"\u003e\u003cpath d=\"m7.775 3.275 1.25-1.25a3.5 3.5 0 1 1 4.95 4.95l-2.5 2.5a3.5 3.5 0 0 1-4.95 0 .751.751 0 0 1 .018-1.042.751.751 0 0 1 1.042-.018 1.998 1.998 0 0 0 2.83 0l2.5-2.5a2.002 2.002 0 0 0-2.83-2.83l-1.25 1.25a.751.751 0 0 1-1.042-.018.751.751 0 0 1-.018-1.042Zm-4.69 9.64a1.998 1.998 0 0 0 2.83 0l1.25-1.25a.751.751 0 0 1 1.042.018.751.751 0 0 1 .018 1.042l-1.25 1.25a3.5 3.5 0 1 1-4.95-4.95l2.5-2.5a3.5 3.5 0 0 1 4.95 0 .751.751 0 0 1-.018 1.042.751.751 0 0 1-1.042.018 1.998 1.998 0 0 0-2.83 0l-2.5 2.5a1.998 1.998 0 0 0 0 2.83Z\"\u003e\u003c/path\u003e\u003c/svg\u003e\u003c/a\u003e\u003c/div\u003e\n\u003cp dir=\"auto\"\u003eHistorically, new language features have been implemented by naive compilation\nof the AST into new, complex bytecode instructions.\nThose bytecodes have often had their own internal flow-control, performing\noperations that could, and should, have been done in the compiler.\u003c/p\u003e\n\u003cp dir=\"auto\"\u003eFor example,\nuntil recently flow control within the \u003ccode\u003etry\u003c/code\u003e-\u003ccode\u003efinally\u003c/code\u003e and \u003ccode\u003ewith\u003c/code\u003e\nstatements was managed by complicated bytecodes with context-dependent semantics.\nThe control flow within those statements is now implemented in the compiler, making\nthe interpreter simpler and faster.\u003c/p\u003e\n\u003cp dir=\"auto\"\u003eBy implementing new features as AST transformations, the existing compiler can\ngenerate the bytecode for a feature without having to modify the interpreter.\u003c/p\u003e\n\u003cp dir=\"auto\"\u003eA stable interpreter is necessary if we are to improve the performance and\nportability of the CPython VM.\u003c/p\u003e\n\u003ca name=\"user-content-rationale\"\u003e\u003c/a\u003e\n\u003cdiv class=\"markdown-heading\" dir=\"auto\"\u003e\u003ch2 tabindex=\"-1\" class=\"heading-element\" dir=\"auto\"\u003eRationale\u003c/h2\u003e\u003ca id=\"user-content-rationale\" class=\"anchor\" aria-label=\"Permalink: Rationale\" href=\"#rationale\"\u003e\u003csvg class=\"octicon octicon-link\" viewBox=\"0 0 16 16\" version=\"1.1\" width=\"16\" height=\"16\" aria-hidden=\"true\"\u003e\u003cpath d=\"m7.775 3.275 1.25-1.25a3.5 3.5 0 1 1 4.95 4.95l-2.5 2.5a3.5 3.5 0 0 1-4.95 0 .751.751 0 0 1 .018-1.042.751.751 0 0 1 1.042-.018 1.998 1.998 0 0 0 2.83 0l2.5-2.5a2.002 2.002 0 0 0-2.83-2.83l-1.25 1.25a.751.751 0 0 1-1.042-.018.751.751 0 0 1-.018-1.042Zm-4.69 9.64a1.998 1.998 0 0 0 2.83 0l1.25-1.25a.751.751 0 0 1 1.042.018.751.751 0 0 1 .018 1.042l-1.25 1.25a3.5 3.5 0 1 1-4.95-4.95l2.5-2.5a3.5 3.5 0 0 1 4.95 0 .751.751 0 0 1-.018 1.042.751.751 0 0 1-1.042.018 1.998 1.998 0 0 0-2.83 0l-2.5 2.5a1.998 1.998 0 0 0 0 2.83Z\"\u003e\u003c/path\u003e\u003c/svg\u003e\u003c/a\u003e\u003c/div\u003e\n\u003cp dir=\"auto\"\u003ePython is both expressive and easy to learn;\nit is widely recognized as the easiest-to-learn, widely used programming language.\nHowever, it is not the most flexible. That title belongs to lisp.\u003c/p\u003e\n\u003cp dir=\"auto\"\u003eBecause lisp is homoiconic, meaning that lisp programs are lisp data structures,\nlisp programs can be manipulated by lisp programs.\nThus much of the language can be defined in itself.\u003c/p\u003e\n\u003cp dir=\"auto\"\u003eWe would like that ability in Python,\nwithout the many parentheses that characterize lisp.\nFortunately, homoiconicity is not needed for a language to be able to\nmanipulate itself, all that is needed is the ability to manipulate programs\nafter parsing, but before translation to an executable form.\u003c/p\u003e\n\u003cp dir=\"auto\"\u003ePython already has the components needed.\nThe syntax tree of Python is available through the \u003ccode\u003east\u003c/code\u003e module.\nAll that is needed is a marker to tell the compiler that a macro is present,\nand the ability for the compiler to callback into user code to manipulate the AST.\u003c/p\u003e\n\u003ca name=\"user-content-specification\"\u003e\u003c/a\u003e\n\u003cdiv class=\"markdown-heading\" dir=\"auto\"\u003e\u003ch2 tabindex=\"-1\" class=\"heading-element\" dir=\"auto\"\u003eSpecification\u003c/h2\u003e\u003ca id=\"user-content-specification\" class=\"anchor\" aria-label=\"Permalink: Specification\" href=\"#specification\"\u003e\u003csvg class=\"octicon octicon-link\" viewBox=\"0 0 16 16\" version=\"1.1\" width=\"16\" height=\"16\" aria-hidden=\"true\"\u003e\u003cpath d=\"m7.775 3.275 1.25-1.25a3.5 3.5 0 1 1 4.95 4.95l-2.5 2.5a3.5 3.5 0 0 1-4.95 0 .751.751 0 0 1 .018-1.042.751.751 0 0 1 1.042-.018 1.998 1.998 0 0 0 2.83 0l2.5-2.5a2.002 2.002 0 0 0-2.83-2.83l-1.25 1.25a.751.751 0 0 1-1.042-.018.751.751 0 0 1-.018-1.042Zm-4.69 9.64a1.998 1.998 0 0 0 2.83 0l1.25-1.25a.751.751 0 0 1 1.042.018.751.751 0 0 1 .018 1.042l-1.25 1.25a3.5 3.5 0 1 1-4.95-4.95l2.5-2.5a3.5 3.5 0 0 1 4.95 0 .751.751 0 0 1-.018 1.042.751.751 0 0 1-1.042.018 1.998 1.998 0 0 0-2.83 0l-2.5 2.5a1.998 1.998 0 0 0 0 2.83Z\"\u003e\u003c/path\u003e\u003c/svg\u003e\u003c/a\u003e\u003c/div\u003e\n\u003ca name=\"user-content-syntax\"\u003e\u003c/a\u003e\n\u003cdiv class=\"markdown-heading\" dir=\"auto\"\u003e\u003ch3 tabindex=\"-1\" class=\"heading-element\" dir=\"auto\"\u003eSyntax\u003c/h3\u003e\u003ca id=\"user-content-syntax\" class=\"anchor\" aria-label=\"Permalink: Syntax\" href=\"#syntax\"\u003e\u003csvg class=\"octicon octicon-link\" viewBox=\"0 0 16 16\" version=\"1.1\" width=\"16\" height=\"16\" aria-hidden=\"true\"\u003e\u003cpath d=\"m7.775 3.275 1.25-1.25a3.5 3.5 0 1 1 4.95 4.95l-2.5 2.5a3.5 3.5 0 0 1-4.95 0 .751.751 0 0 1 .018-1.042.751.751 0 0 1 1.042-.018 1.998 1.998 0 0 0 2.83 0l2.5-2.5a2.002 2.002 0 0 0-2.83-2.83l-1.25 1.25a.751.751 0 0 1-1.042-.018.751.751 0 0 1-.018-1.042Zm-4.69 9.64a1.998 1.998 0 0 0 2.83 0l1.25-1.25a.751.751 0 0 1 1.042.018.751.751 0 0 1 .018 1.042l-1.25 1.25a3.5 3.5 0 1 1-4.95-4.95l2.5-2.5a3.5 3.5 0 0 1 4.95 0 .751.751 0 0 1-.018 1.042.751.751 0 0 1-1.042.018 1.998 1.998 0 0 0-2.83 0l-2.5 2.5a1.998 1.998 0 0 0 0 2.83Z\"\u003e\u003c/path\u003e\u003c/svg\u003e\u003c/a\u003e\u003c/div\u003e\n\u003ca name=\"user-content-lexical-analysis\"\u003e\u003c/a\u003e\n\u003cdiv class=\"markdown-heading\" dir=\"auto\"\u003e\u003ch4 tabindex=\"-1\" class=\"heading-element\" dir=\"auto\"\u003eLexical analysis\u003c/h4\u003e\u003ca id=\"user-content-lexical-analysis\" class=\"anchor\" aria-label=\"Permalink: Lexical analysis\" href=\"#lexical-analysis\"\u003e\u003csvg class=\"octicon octicon-link\" viewBox=\"0 0 16 16\" version=\"1.1\" width=\"16\" height=\"16\" aria-hidden=\"true\"\u003e\u003cpath d=\"m7.775 3.275 1.25-1.25a3.5 3.5 0 1 1 4.95 4.95l-2.5 2.5a3.5 3.5 0 0 1-4.95 0 .751.751 0 0 1 .018-1.042.751.751 0 0 1 1.042-.018 1.998 1.998 0 0 0 2.83 0l2.5-2.5a2.002 2.002 0 0 0-2.83-2.83l-1.25 1.25a.751.751 0 0 1-1.042-.018.751.751 0 0 1-.018-1.042Zm-4.69 9.64a1.998 1.998 0 0 0 2.83 0l1.25-1.25a.751.751 0 0 1 1.042.018.751.751 0 0 1 .018 1.042l-1.25 1.25a3.5 3.5 0 1 1-4.95-4.95l2.5-2.5a3.5 3.5 0 0 1 4.95 0 .751.751 0 0 1-.018 1.042.751.751 0 0 1-1.042.018 1.998 1.998 0 0 0-2.83 0l-2.5 2.5a1.998 1.998 0 0 0 0 2.83Z\"\u003e\u003c/path\u003e\u003c/svg\u003e\u003c/a\u003e\u003c/div\u003e\n\u003cp dir=\"auto\"\u003eAny sequence of identifier characters followed by an exclamation point\n(exclamation mark, UK English) will be tokenized as a \u003ccode\u003eMACRO_NAME\u003c/code\u003e.\u003c/p\u003e\n\u003ca name=\"user-content-statement-form\"\u003e\u003c/a\u003e\n\u003cdiv class=\"markdown-heading\" dir=\"auto\"\u003e\u003ch4 tabindex=\"-1\" class=\"heading-element\" dir=\"auto\"\u003eStatement form\u003c/h4\u003e\u003ca id=\"user-content-statement-form\" class=\"anchor\" aria-label=\"Permalink: Statement form\" href=\"#statement-form\"\u003e\u003csvg class=\"octicon octicon-link\" viewBox=\"0 0 16 16\" version=\"1.1\" width=\"16\" height=\"16\" aria-hidden=\"true\"\u003e\u003cpath d=\"m7.775 3.275 1.25-1.25a3.5 3.5 0 1 1 4.95 4.95l-2.5 2.5a3.5 3.5 0 0 1-4.95 0 .751.751 0 0 1 .018-1.042.751.751 0 0 1 1.042-.018 1.998 1.998 0 0 0 2.83 0l2.5-2.5a2.002 2.002 0 0 0-2.83-2.83l-1.25 1.25a.751.751 0 0 1-1.042-.018.751.751 0 0 1-.018-1.042Zm-4.69 9.64a1.998 1.998 0 0 0 2.83 0l1.25-1.25a.751.751 0 0 1 1.042.018.751.751 0 0 1 .018 1.042l-1.25 1.25a3.5 3.5 0 1 1-4.95-4.95l2.5-2.5a3.5 3.5 0 0 1 4.95 0 .751.751 0 0 1-.018 1.042.751.751 0 0 1-1.042.018 1.998 1.998 0 0 0-2.83 0l-2.5 2.5a1.998 1.998 0 0 0 0 2.83Z\"\u003e\u003c/path\u003e\u003c/svg\u003e\u003c/a\u003e\u003c/div\u003e\n\u003cpre\u003emacro_stmt = MACRO_NAME testlist [ \"import\" NAME ] [ \"as\" NAME ] [ \":\" NEWLINE suite ]\n\u003c/pre\u003e\n\u003ca name=\"user-content-expression-form\"\u003e\u003c/a\u003e\n\u003cdiv class=\"markdown-heading\" dir=\"auto\"\u003e\u003ch4 tabindex=\"-1\" class=\"heading-element\" dir=\"auto\"\u003eExpression form\u003c/h4\u003e\u003ca id=\"user-content-expression-form\" class=\"anchor\" aria-label=\"Permalink: Expression form\" href=\"#expression-form\"\u003e\u003csvg class=\"octicon octicon-link\" viewBox=\"0 0 16 16\" version=\"1.1\" width=\"16\" height=\"16\" aria-hidden=\"true\"\u003e\u003cpath d=\"m7.775 3.275 1.25-1.25a3.5 3.5 0 1 1 4.95 4.95l-2.5 2.5a3.5 3.5 0 0 1-4.95 0 .751.751 0 0 1 .018-1.042.751.751 0 0 1 1.042-.018 1.998 1.998 0 0 0 2.83 0l2.5-2.5a2.002 2.002 0 0 0-2.83-2.83l-1.25 1.25a.751.751 0 0 1-1.042-.018.751.751 0 0 1-.018-1.042Zm-4.69 9.64a1.998 1.998 0 0 0 2.83 0l1.25-1.25a.751.751 0 0 1 1.042.018.751.751 0 0 1 .018 1.042l-1.25 1.25a3.5 3.5 0 1 1-4.95-4.95l2.5-2.5a3.5 3.5 0 0 1 4.95 0 .751.751 0 0 1-.018 1.042.751.751 0 0 1-1.042.018 1.998 1.998 0 0 0-2.83 0l-2.5 2.5a1.998 1.998 0 0 0 0 2.83Z\"\u003e\u003c/path\u003e\u003c/svg\u003e\u003c/a\u003e\u003c/div\u003e\n\u003cpre\u003emacro_expr = MACRO_NAME \"(\" testlist \")\"\n\u003c/pre\u003e\n\u003ca name=\"user-content-resolving-ambiguity\"\u003e\u003c/a\u003e\n\u003cdiv class=\"markdown-heading\" dir=\"auto\"\u003e\u003ch4 tabindex=\"-1\" class=\"heading-element\" dir=\"auto\"\u003eResolving ambiguity\u003c/h4\u003e\u003ca id=\"user-content-resolving-ambiguity\" class=\"anchor\" aria-label=\"Permalink: Resolving ambiguity\" href=\"#resolving-ambiguity\"\u003e\u003csvg class=\"octicon octicon-link\" viewBox=\"0 0 16 16\" version=\"1.1\" width=\"16\" height=\"16\" aria-hidden=\"true\"\u003e\u003cpath d=\"m7.775 3.275 1.25-1.25a3.5 3.5 0 1 1 4.95 4.95l-2.5 2.5a3.5 3.5 0 0 1-4.95 0 .751.751 0 0 1 .018-1.042.751.751 0 0 1 1.042-.018 1.998 1.998 0 0 0 2.83 0l2.5-2.5a2.002 2.002 0 0 0-2.83-2.83l-1.25 1.25a.751.751 0 0 1-1.042-.018.751.751 0 0 1-.018-1.042Zm-4.69 9.64a1.998 1.998 0 0 0 2.83 0l1.25-1.25a.751.751 0 0 1 1.042.018.751.751 0 0 1 .018 1.042l-1.25 1.25a3.5 3.5 0 1 1-4.95-4.95l2.5-2.5a3.5 3.5 0 0 1 4.95 0 .751.751 0 0 1-.018 1.042.751.751 0 0 1-1.042.018 1.998 1.998 0 0 0-2.83 0l-2.5 2.5a1.998 1.998 0 0 0 0 2.83Z\"\u003e\u003c/path\u003e\u003c/svg\u003e\u003c/a\u003e\u003c/div\u003e\n\u003cp dir=\"auto\"\u003eThe statement form of a macro takes precedence, so that the code\n\u003ccode\u003emacro_name!(x)\u003c/code\u003e will be parsed as a macro statement,\nnot as an expression statement containing a macro expression.\u003c/p\u003e\n\u003ca name=\"user-content-semantics\"\u003e\u003c/a\u003e\n\u003cdiv class=\"markdown-heading\" dir=\"auto\"\u003e\u003ch3 tabindex=\"-1\" class=\"heading-element\" dir=\"auto\"\u003eSemantics\u003c/h3\u003e\u003ca id=\"user-content-semantics\" class=\"anchor\" aria-label=\"Permalink: Semantics\" href=\"#semantics\"\u003e\u003csvg class=\"octicon octicon-link\" viewBox=\"0 0 16 16\" version=\"1.1\" width=\"16\" height=\"16\" aria-hidden=\"true\"\u003e\u003cpath d=\"m7.775 3.275 1.25-1.25a3.5 3.5 0 1 1 4.95 4.95l-2.5 2.5a3.5 3.5 0 0 1-4.95 0 .751.751 0 0 1 .018-1.042.751.751 0 0 1 1.042-.018 1.998 1.998 0 0 0 2.83 0l2.5-2.5a2.002 2.002 0 0 0-2.83-2.83l-1.25 1.25a.751.751 0 0 1-1.042-.018.751.751 0 0 1-.018-1.042Zm-4.69 9.64a1.998 1.998 0 0 0 2.83 0l1.25-1.25a.751.751 0 0 1 1.042.018.751.751 0 0 1 .018 1.042l-1.25 1.25a3.5 3.5 0 1 1-4.95-4.95l2.5-2.5a3.5 3.5 0 0 1 4.95 0 .751.751 0 0 1-.018 1.042.751.751 0 0 1-1.042.018 1.998 1.998 0 0 0-2.83 0l-2.5 2.5a1.998 1.998 0 0 0 0 2.83Z\"\u003e\u003c/path\u003e\u003c/svg\u003e\u003c/a\u003e\u003c/div\u003e\n\u003ca name=\"user-content-compilation\"\u003e\u003c/a\u003e\n\u003cdiv class=\"markdown-heading\" dir=\"auto\"\u003e\u003ch4 tabindex=\"-1\" class=\"heading-element\" dir=\"auto\"\u003eCompilation\u003c/h4\u003e\u003ca id=\"user-content-compilation\" class=\"anchor\" aria-label=\"Permalink: Compilation\" href=\"#compilation\"\u003e\u003csvg class=\"octicon octicon-link\" viewBox=\"0 0 16 16\" version=\"1.1\" width=\"16\" height=\"16\" aria-hidden=\"true\"\u003e\u003cpath d=\"m7.775 3.275 1.25-1.25a3.5 3.5 0 1 1 4.95 4.95l-2.5 2.5a3.5 3.5 0 0 1-4.95 0 .751.751 0 0 1 .018-1.042.751.751 0 0 1 1.042-.018 1.998 1.998 0 0 0 2.83 0l2.5-2.5a2.002 2.002 0 0 0-2.83-2.83l-1.25 1.25a.751.751 0 0 1-1.042-.018.751.751 0 0 1-.018-1.042Zm-4.69 9.64a1.998 1.998 0 0 0 2.83 0l1.25-1.25a.751.751 0 0 1 1.042.018.751.751 0 0 1 .018 1.042l-1.25 1.25a3.5 3.5 0 1 1-4.95-4.95l2.5-2.5a3.5 3.5 0 0 1 4.95 0 .751.751 0 0 1-.018 1.042.751.751 0 0 1-1.042.018 1.998 1.998 0 0 0-2.83 0l-2.5 2.5a1.998 1.998 0 0 0 0 2.83Z\"\u003e\u003c/path\u003e\u003c/svg\u003e\u003c/a\u003e\u003c/div\u003e\n\u003cp dir=\"auto\"\u003eUpon encountering a \u003ccode\u003emacro\u003c/code\u003e during translation to bytecode,\nthe code generator will look up the macro processor registered for the macro,\nand pass the AST rooted at the macro to the processor function.\nThe returned AST will then be substituted for the original tree.\u003c/p\u003e\n\u003cp dir=\"auto\"\u003eFor macros with multiple names,\nseveral trees will be passed to the macro processor,\nbut only one will be returned and substituted,\nshorting the enclosing block of statements.\u003c/p\u003e\n\u003cp dir=\"auto\"\u003eThis process can be repeated,\nto enable macros to return AST nodes including other macros.\u003c/p\u003e\n\u003cp dir=\"auto\"\u003eThe compiler will not look up a macro processor until that macro is reached,\nso that inner macros do not need to have processors registered.\nFor example, in a \u003ccode\u003eswitch\u003c/code\u003e macro, the \u003ccode\u003ecase\u003c/code\u003e and \u003ccode\u003edefault\u003c/code\u003e macros wouldn't\nneed processors registered as they would be eliminated by the \u003ccode\u003eswitch\u003c/code\u003e processor.\u003c/p\u003e\n\u003cp dir=\"auto\"\u003eTo enable definition of macros to be imported,\nthe macros \u003ccode\u003eimport!\u003c/code\u003e and \u003ccode\u003efrom!\u003c/code\u003e are predefined.\nThey support the following syntax:\u003c/p\u003e\n\u003cpre\u003e\"import!\" dotted_name \"as\" name\n\n\"from!\" dotted_name \"import\" name [ \"as\" name ]\n\u003c/pre\u003e\n\u003cp dir=\"auto\"\u003eThe \u003ccode\u003eimport!\u003c/code\u003e macro performs a compile-time import of \u003ccode\u003edotted_name\u003c/code\u003e\nto find the macro processor, then registers it under \u003ccode\u003ename\u003c/code\u003e\nfor the scope currently being compiled.\u003c/p\u003e\n\u003cp dir=\"auto\"\u003eThe \u003ccode\u003efrom!\u003c/code\u003e macro performs a compile-time import of \u003ccode\u003edotted_name.name\u003c/code\u003e\nto find the macro processor, then registers it under \u003ccode\u003ename\u003c/code\u003e\n(using the \u003ccode\u003ename\u003c/code\u003e following \"as\", if present)\nfor the scope currently being compiled.\u003c/p\u003e\n\u003cp dir=\"auto\"\u003eNote that, since \u003ccode\u003eimport!\u003c/code\u003e and \u003ccode\u003efrom!\u003c/code\u003e only define the macro for the\nscope in which the import is present, all uses of a macro must be preceded by\nan explicit \u003ccode\u003eimport!\u003c/code\u003e or \u003ccode\u003efrom!\u003c/code\u003e to improve clarity.\u003c/p\u003e\n\u003cp dir=\"auto\"\u003eFor example, to import the macro \"compile\" from \"my.compiler\":\u003c/p\u003e\n\u003cpre\u003efrom! my.compiler import compile\n\u003c/pre\u003e\n\u003ca name=\"user-content-defining-macro-processors\"\u003e\u003c/a\u003e\n\u003cdiv class=\"markdown-heading\" dir=\"auto\"\u003e\u003ch4 tabindex=\"-1\" class=\"heading-element\" dir=\"auto\"\u003eDefining macro processors\u003c/h4\u003e\u003ca id=\"user-content-defining-macro-processors\" class=\"anchor\" aria-label=\"Permalink: Defining macro processors\" href=\"#defining-macro-processors\"\u003e\u003csvg class=\"octicon octicon-link\" viewBox=\"0 0 16 16\" version=\"1.1\" width=\"16\" height=\"16\" aria-hidden=\"true\"\u003e\u003cpath d=\"m7.775 3.275 1.25-1.25a3.5 3.5 0 1 1 4.95 4.95l-2.5 2.5a3.5 3.5 0 0 1-4.95 0 .751.751 0 0 1 .018-1.042.751.751 0 0 1 1.042-.018 1.998 1.998 0 0 0 2.83 0l2.5-2.5a2.002 2.002 0 0 0-2.83-2.83l-1.25 1.25a.751.751 0 0 1-1.042-.018.751.751 0 0 1-.018-1.042Zm-4.69 9.64a1.998 1.998 0 0 0 2.83 0l1.25-1.25a.751.751 0 0 1 1.042.018.751.751 0 0 1 .018 1.042l-1.25 1.25a3.5 3.5 0 1 1-4.95-4.95l2.5-2.5a3.5 3.5 0 0 1 4.95 0 .751.751 0 0 1-.018 1.042.751.751 0 0 1-1.042.018 1.998 1.998 0 0 0-2.83 0l-2.5 2.5a1.998 1.998 0 0 0 0 2.83Z\"\u003e\u003c/path\u003e\u003c/svg\u003e\u003c/a\u003e\u003c/div\u003e\n\u003cp dir=\"auto\"\u003eA macro processor is defined by a four-tuple, consisting of\n\u003ccode\u003e(func, kind, version, additional_names)\u003c/code\u003e:\u003c/p\u003e\n\u003cul dir=\"auto\"\u003e\n\u003cli\u003e\u003ccode\u003efunc\u003c/code\u003e must be a callable that takes \u003ccode\u003elen(additional_names)+1\u003c/code\u003e arguments, all of which are abstract syntax trees, and returns a single abstract syntax tree.\u003c/li\u003e\n\u003cli\u003e\u003ccode\u003ekind\u003c/code\u003e must be one of the following:\u003cul dir=\"auto\"\u003e\n\u003cli\u003e\u003ccode\u003emacros.STMT_MACRO\u003c/code\u003e: A statement macro where the body of the macro is indented. This is the only form allowed to have additional names.\u003c/li\u003e\n\u003cli\u003e\u003ccode\u003emacros.SIBLING_MACRO\u003c/code\u003e: A statement macro where the body of the macro is the next statement in the same block. The following statement is moved into the macro as its body.\u003c/li\u003e\n\u003cli\u003e\u003ccode\u003emacros.EXPR_MACRO\u003c/code\u003e: An expression macro.\u003c/li\u003e\n\u003c/ul\u003e\n\u003c/li\u003e\n\u003cli\u003e\u003ccode\u003eversion\u003c/code\u003e is used to track versions of macros, so that generated bytecodes can be correctly cached. It must be an integer.\u003c/li\u003e\n\u003cli\u003e\u003ccode\u003eadditional_names\u003c/code\u003e are the names of the additional parts of the macro, and must be a tuple of strings.\u003c/li\u003e\n\u003c/ul\u003e\n\u003cpre\u003e# (func, _ast.STMT_MACRO, VERSION, ())\nstmt_macro!:\n multi_statement_body\n\n# (func, _ast.SIBLING_MACRO, VERSION, ())\nsibling_macro!\nsingle_statement_body\n\n# (func, _ast.EXPR_MACRO, VERSION, ())\nx = expr_macro!(...)\n\n# (func, _ast.STMT_MACRO, VERSION, (\"subsequent_macro_part\",))\nmulti_part_macro!:\n multi_statement_body\nsubsequent_macro_part!:\n multi_statement_body\n\u003c/pre\u003e\n\u003cp dir=\"auto\"\u003eThe compiler will check that the syntax used matches the declared kind.\u003c/p\u003e\n\u003cp dir=\"auto\"\u003eFor convenience, the decorator \u003ccode\u003emacro_processor\u003c/code\u003e is provided in the \u003ccode\u003emacros\u003c/code\u003e module to mark a function as a macro processor:\u003c/p\u003e\n\u003cpre\u003edef macro_processor(kind, version, *additional_names):\n def deco(func):\n return func, kind, version, additional_names\n return deco\n\u003c/pre\u003e\n\u003cp dir=\"auto\"\u003eWhich can be used to help declare macro processors, for example:\u003c/p\u003e\n\u003cpre\u003e@macros.macro_processor(macros.STMT_MACRO, 1_08)\ndef switch(astnode):\n ...\n\u003c/pre\u003e\n\u003ca name=\"user-content-ast-extensions\"\u003e\u003c/a\u003e\n\u003cdiv class=\"markdown-heading\" dir=\"auto\"\u003e\u003ch4 tabindex=\"-1\" class=\"heading-element\" dir=\"auto\"\u003eAST extensions\u003c/h4\u003e\u003ca id=\"user-content-ast-extensions\" class=\"anchor\" aria-label=\"Permalink: AST extensions\" href=\"#ast-extensions\"\u003e\u003csvg class=\"octicon octicon-link\" viewBox=\"0 0 16 16\" version=\"1.1\" width=\"16\" height=\"16\" aria-hidden=\"true\"\u003e\u003cpath d=\"m7.775 3.275 1.25-1.25a3.5 3.5 0 1 1 4.95 4.95l-2.5 2.5a3.5 3.5 0 0 1-4.95 0 .751.751 0 0 1 .018-1.042.751.751 0 0 1 1.042-.018 1.998 1.998 0 0 0 2.83 0l2.5-2.5a2.002 2.002 0 0 0-2.83-2.83l-1.25 1.25a.751.751 0 0 1-1.042-.018.751.751 0 0 1-.018-1.042Zm-4.69 9.64a1.998 1.998 0 0 0 2.83 0l1.25-1.25a.751.751 0 0 1 1.042.018.751.751 0 0 1 .018 1.042l-1.25 1.25a3.5 3.5 0 1 1-4.95-4.95l2.5-2.5a3.5 3.5 0 0 1 4.95 0 .751.751 0 0 1-.018 1.042.751.751 0 0 1-1.042.018 1.998 1.998 0 0 0-2.83 0l-2.5 2.5a1.998 1.998 0 0 0 0 2.83Z\"\u003e\u003c/path\u003e\u003c/svg\u003e\u003c/a\u003e\u003c/div\u003e\n\u003cp dir=\"auto\"\u003eTwo new AST nodes will be needed to express macros, \u003ccode\u003emacro_stmt\u003c/code\u003e and \u003ccode\u003emacro_expr\u003c/code\u003e.\u003c/p\u003e\n\u003cpre\u003eclass macro_stmt(_ast.stmt):\n _fields = \"name\", \"args\", \"importname\", \"asname\", \"body\"\n\nclass macro_expr(_ast.expr):\n _fields = \"name\", \"args\"\n\u003c/pre\u003e\n\u003cp dir=\"auto\"\u003eIn addition, macro processors will need a means to express control flow or side-effecting code, that produces a value.\nA new AST node called \u003ccode\u003estmt_expr\u003c/code\u003e will be added, combining a statement and an expression.\nThis new ast node will be a subtype of \u003ccode\u003eexpr\u003c/code\u003e, but include a statement to allow side effects.\nIt will be compiled to bytecode by compiling the statement, then compiling the value.\u003c/p\u003e\n\u003cpre\u003eclass stmt_expr(_ast.expr):\n _fields = \"stmt\", \"value\"\n\u003c/pre\u003e\n\u003ca name=\"user-content-hygiene-and-debugging\"\u003e\u003c/a\u003e\n\u003cdiv class=\"markdown-heading\" dir=\"auto\"\u003e\u003ch4 tabindex=\"-1\" class=\"heading-element\" dir=\"auto\"\u003eHygiene and debugging\u003c/h4\u003e\u003ca id=\"user-content-hygiene-and-debugging\" class=\"anchor\" aria-label=\"Permalink: Hygiene and debugging\" href=\"#hygiene-and-debugging\"\u003e\u003csvg class=\"octicon octicon-link\" viewBox=\"0 0 16 16\" version=\"1.1\" width=\"16\" height=\"16\" aria-hidden=\"true\"\u003e\u003cpath d=\"m7.775 3.275 1.25-1.25a3.5 3.5 0 1 1 4.95 4.95l-2.5 2.5a3.5 3.5 0 0 1-4.95 0 .751.751 0 0 1 .018-1.042.751.751 0 0 1 1.042-.018 1.998 1.998 0 0 0 2.83 0l2.5-2.5a2.002 2.002 0 0 0-2.83-2.83l-1.25 1.25a.751.751 0 0 1-1.042-.018.751.751 0 0 1-.018-1.042Zm-4.69 9.64a1.998 1.998 0 0 0 2.83 0l1.25-1.25a.751.751 0 0 1 1.042.018.751.751 0 0 1 .018 1.042l-1.25 1.25a3.5 3.5 0 1 1-4.95-4.95l2.5-2.5a3.5 3.5 0 0 1 4.95 0 .751.751 0 0 1-.018 1.042.751.751 0 0 1-1.042.018 1.998 1.998 0 0 0-2.83 0l-2.5 2.5a1.998 1.998 0 0 0 0 2.83Z\"\u003e\u003c/path\u003e\u003c/svg\u003e\u003c/a\u003e\u003c/div\u003e\n\u003cp dir=\"auto\"\u003eMacro processors will often need to create new variables.\nThose variables need to named in such as way as to avoid contaminating the original code and other macros.\nNo rules for naming will be enforced, but to ensure hygiene and help debugging, the following naming scheme is recommended:\u003c/p\u003e\n\u003cul dir=\"auto\"\u003e\n\u003cli\u003eAll generated variable names should start with a \u003ccode\u003e$\u003c/code\u003e\u003c/li\u003e\n\u003cli\u003ePurely artificial variable names should start \u003ccode\u003e$$mname\u003c/code\u003e where \u003ccode\u003emname\u003c/code\u003e is the name of the macro.\u003c/li\u003e\n\u003cli\u003eVariables derived from real variables should start \u003ccode\u003e$vname\u003c/code\u003e where \u003ccode\u003evname\u003c/code\u003e is the name of the variable.\u003c/li\u003e\n\u003cli\u003eAll variable names should include the line number and the column offset, separated by an underscore.\u003c/li\u003e\n\u003c/ul\u003e\n\u003cp dir=\"auto\"\u003eExamples:\u003c/p\u003e\n\u003cul dir=\"auto\"\u003e\n\u003cli\u003ePurely generated name: \u003ccode\u003e$$macro_17_0\u003c/code\u003e\u003c/li\u003e\n\u003cli\u003eName derived from a variable for an expression macro: \u003ccode\u003e$var_12_5\u003c/code\u003e\u003c/li\u003e\n\u003c/ul\u003e\n\u003ca name=\"user-content-examples\"\u003e\u003c/a\u003e\n\u003cdiv class=\"markdown-heading\" dir=\"auto\"\u003e\u003ch3 tabindex=\"-1\" class=\"heading-element\" dir=\"auto\"\u003eExamples\u003c/h3\u003e\u003ca id=\"user-content-examples\" class=\"anchor\" aria-label=\"Permalink: Examples\" href=\"#examples\"\u003e\u003csvg class=\"octicon octicon-link\" viewBox=\"0 0 16 16\" version=\"1.1\" width=\"16\" height=\"16\" aria-hidden=\"true\"\u003e\u003cpath d=\"m7.775 3.275 1.25-1.25a3.5 3.5 0 1 1 4.95 4.95l-2.5 2.5a3.5 3.5 0 0 1-4.95 0 .751.751 0 0 1 .018-1.042.751.751 0 0 1 1.042-.018 1.998 1.998 0 0 0 2.83 0l2.5-2.5a2.002 2.002 0 0 0-2.83-2.83l-1.25 1.25a.751.751 0 0 1-1.042-.018.751.751 0 0 1-.018-1.042Zm-4.69 9.64a1.998 1.998 0 0 0 2.83 0l1.25-1.25a.751.751 0 0 1 1.042.018.751.751 0 0 1 .018 1.042l-1.25 1.25a3.5 3.5 0 1 1-4.95-4.95l2.5-2.5a3.5 3.5 0 0 1 4.95 0 .751.751 0 0 1-.018 1.042.751.751 0 0 1-1.042.018 1.998 1.998 0 0 0-2.83 0l-2.5 2.5a1.998 1.998 0 0 0 0 2.83Z\"\u003e\u003c/path\u003e\u003c/svg\u003e\u003c/a\u003e\u003c/div\u003e\n\u003ca name=\"user-content-compile-time-checked-data-structures\"\u003e\u003c/a\u003e\n\u003cdiv class=\"markdown-heading\" dir=\"auto\"\u003e\u003ch4 tabindex=\"-1\" class=\"heading-element\" dir=\"auto\"\u003eCompile-time-checked data structures\u003c/h4\u003e\u003ca id=\"user-content-compile-time-checked-data-structures\" class=\"anchor\" aria-label=\"Permalink: Compile-time-checked data structures\" href=\"#compile-time-checked-data-structures\"\u003e\u003csvg class=\"octicon octicon-link\" viewBox=\"0 0 16 16\" version=\"1.1\" width=\"16\" height=\"16\" aria-hidden=\"true\"\u003e\u003cpath d=\"m7.775 3.275 1.25-1.25a3.5 3.5 0 1 1 4.95 4.95l-2.5 2.5a3.5 3.5 0 0 1-4.95 0 .751.751 0 0 1 .018-1.042.751.751 0 0 1 1.042-.018 1.998 1.998 0 0 0 2.83 0l2.5-2.5a2.002 2.002 0 0 0-2.83-2.83l-1.25 1.25a.751.751 0 0 1-1.042-.018.751.751 0 0 1-.018-1.042Zm-4.69 9.64a1.998 1.998 0 0 0 2.83 0l1.25-1.25a.751.751 0 0 1 1.042.018.751.751 0 0 1 .018 1.042l-1.25 1.25a3.5 3.5 0 1 1-4.95-4.95l2.5-2.5a3.5 3.5 0 0 1 4.95 0 .751.751 0 0 1-.018 1.042.751.751 0 0 1-1.042.018 1.998 1.998 0 0 0-2.83 0l-2.5 2.5a1.998 1.998 0 0 0 0 2.83Z\"\u003e\u003c/path\u003e\u003c/svg\u003e\u003c/a\u003e\u003c/div\u003e\n\u003cp dir=\"auto\"\u003eIt is common to encode tables of data in Python as large dictionaries.\nHowever, these can be hard to maintain and error prone.\nMacros allow such data to be written in a more readable format.\nThen, at compile time, the data can be verified and converted to an efficient format.\u003c/p\u003e\n\u003cp dir=\"auto\"\u003eFor example, suppose we have a two dictionary literals mapping codes to names,\nand vice versa.\nThis is error prone, as the dictionaries may have duplicate keys,\nor one table may not be the inverse of the other.\nA macro could generate the two mappings from a single table and,\nat the same time, verify that no duplicates are present.\u003c/p\u003e\n\u003cpre\u003ecolor_to_code = {\n \"red\": 1,\n \"blue\": 2,\n \"green\": 3,\n}\n\ncode_to_color = {\n 1: \"red\",\n 2: \"blue\",\n 3: \"yellow\", # error\n}\n\u003c/pre\u003e\n\u003cp dir=\"auto\"\u003ewould become:\u003c/p\u003e\n\u003cpre\u003ebijection! color_to_code, code_to_color:\n \"red\" = 1\n \"blue\" = 2\n \"green\" = 3\n\u003c/pre\u003e\n\u003ca name=\"user-content-domain-specific-extensions\"\u003e\u003c/a\u003e\n\u003cdiv class=\"markdown-heading\" dir=\"auto\"\u003e\u003ch4 tabindex=\"-1\" class=\"heading-element\" dir=\"auto\"\u003eDomain-specific extensions\u003c/h4\u003e\u003ca id=\"user-content-domain-specific-extensions\" class=\"anchor\" aria-label=\"Permalink: Domain-specific extensions\" href=\"#domain-specific-extensions\"\u003e\u003csvg class=\"octicon octicon-link\" viewBox=\"0 0 16 16\" version=\"1.1\" width=\"16\" height=\"16\" aria-hidden=\"true\"\u003e\u003cpath d=\"m7.775 3.275 1.25-1.25a3.5 3.5 0 1 1 4.95 4.95l-2.5 2.5a3.5 3.5 0 0 1-4.95 0 .751.751 0 0 1 .018-1.042.751.751 0 0 1 1.042-.018 1.998 1.998 0 0 0 2.83 0l2.5-2.5a2.002 2.002 0 0 0-2.83-2.83l-1.25 1.25a.751.751 0 0 1-1.042-.018.751.751 0 0 1-.018-1.042Zm-4.69 9.64a1.998 1.998 0 0 0 2.83 0l1.25-1.25a.751.751 0 0 1 1.042.018.751.751 0 0 1 .018 1.042l-1.25 1.25a3.5 3.5 0 1 1-4.95-4.95l2.5-2.5a3.5 3.5 0 0 1 4.95 0 .751.751 0 0 1-.018 1.042.751.751 0 0 1-1.042.018 1.998 1.998 0 0 0-2.83 0l-2.5 2.5a1.998 1.998 0 0 0 0 2.83Z\"\u003e\u003c/path\u003e\u003c/svg\u003e\u003c/a\u003e\u003c/div\u003e\n\u003cp dir=\"auto\"\u003eWhere I see macros having real value is in specific domains, not in general-purpose language features.\u003c/p\u003e\n\u003cp dir=\"auto\"\u003eFor example, parsers.\nHere's part of a parser definition for Python, using macros:\u003c/p\u003e\n\u003cpre\u003echoice! single_input:\n NEWLINE\n simple_stmt\n sequence!:\n compound_stmt\n NEWLINE\n\u003c/pre\u003e\n\u003ca name=\"user-content-compilers\"\u003e\u003c/a\u003e\n\u003cdiv class=\"markdown-heading\" dir=\"auto\"\u003e\u003ch4 tabindex=\"-1\" class=\"heading-element\" dir=\"auto\"\u003eCompilers\u003c/h4\u003e\u003ca id=\"user-content-compilers\" class=\"anchor\" aria-label=\"Permalink: Compilers\" href=\"#compilers\"\u003e\u003csvg class=\"octicon octicon-link\" viewBox=\"0 0 16 16\" version=\"1.1\" width=\"16\" height=\"16\" aria-hidden=\"true\"\u003e\u003cpath d=\"m7.775 3.275 1.25-1.25a3.5 3.5 0 1 1 4.95 4.95l-2.5 2.5a3.5 3.5 0 0 1-4.95 0 .751.751 0 0 1 .018-1.042.751.751 0 0 1 1.042-.018 1.998 1.998 0 0 0 2.83 0l2.5-2.5a2.002 2.002 0 0 0-2.83-2.83l-1.25 1.25a.751.751 0 0 1-1.042-.018.751.751 0 0 1-.018-1.042Zm-4.69 9.64a1.998 1.998 0 0 0 2.83 0l1.25-1.25a.751.751 0 0 1 1.042.018.751.751 0 0 1 .018 1.042l-1.25 1.25a3.5 3.5 0 1 1-4.95-4.95l2.5-2.5a3.5 3.5 0 0 1 4.95 0 .751.751 0 0 1-.018 1.042.751.751 0 0 1-1.042.018 1.998 1.998 0 0 0-2.83 0l-2.5 2.5a1.998 1.998 0 0 0 0 2.83Z\"\u003e\u003c/path\u003e\u003c/svg\u003e\u003c/a\u003e\u003c/div\u003e\n\u003cp dir=\"auto\"\u003eRuntime compilers, such as \u003ccode\u003enumba\u003c/code\u003e have to reconstitute the Python source, or attempt to analyze the bytecode.\nIt would be simpler and more reliable for them to get the AST directly:\u003c/p\u003e\n\u003cpre\u003efrom! my.jit.library import jit\n\njit!\ndef func():\n ...\n\u003c/pre\u003e\n\u003ca name=\"user-content-matching-symbolic-expressions\"\u003e\u003c/a\u003e\n\u003cdiv class=\"markdown-heading\" dir=\"auto\"\u003e\u003ch4 tabindex=\"-1\" class=\"heading-element\" dir=\"auto\"\u003eMatching symbolic expressions\u003c/h4\u003e\u003ca id=\"user-content-matching-symbolic-expressions\" class=\"anchor\" aria-label=\"Permalink: Matching symbolic expressions\" href=\"#matching-symbolic-expressions\"\u003e\u003csvg class=\"octicon octicon-link\" viewBox=\"0 0 16 16\" version=\"1.1\" width=\"16\" height=\"16\" aria-hidden=\"true\"\u003e\u003cpath d=\"m7.775 3.275 1.25-1.25a3.5 3.5 0 1 1 4.95 4.95l-2.5 2.5a3.5 3.5 0 0 1-4.95 0 .751.751 0 0 1 .018-1.042.751.751 0 0 1 1.042-.018 1.998 1.998 0 0 0 2.83 0l2.5-2.5a2.002 2.002 0 0 0-2.83-2.83l-1.25 1.25a.751.751 0 0 1-1.042-.018.751.751 0 0 1-.018-1.042Zm-4.69 9.64a1.998 1.998 0 0 0 2.83 0l1.25-1.25a.751.751 0 0 1 1.042.018.751.751 0 0 1 .018 1.042l-1.25 1.25a3.5 3.5 0 1 1-4.95-4.95l2.5-2.5a3.5 3.5 0 0 1 4.95 0 .751.751 0 0 1-.018 1.042.751.751 0 0 1-1.042.018 1.998 1.998 0 0 0-2.83 0l-2.5 2.5a1.998 1.998 0 0 0 0 2.83Z\"\u003e\u003c/path\u003e\u003c/svg\u003e\u003c/a\u003e\u003c/div\u003e\n\u003cp dir=\"auto\"\u003eWhen matching something representing syntax, such a Python \u003ccode\u003east\u003c/code\u003e node, or a \u003ccode\u003esympy\u003c/code\u003e expression,\nit is convenient to match against the actual syntax, not the data structure representing it.\nFor example, a calculator could be implemented using a domain-specific macro for matching syntax:\u003c/p\u003e\n\u003cpre\u003efrom! ast_matcher import match\n\ndef calculate(node):\n if isinstance(node, Num):\n return node.n\n match! node:\n case! a + b:\n return calculate(a) + calculate(b)\n case! a - b:\n return calculate(a) - calculate(b)\n case! a * b:\n return calculate(a) * calculate(b)\n case! a / b:\n return calculate(a) / calculate(b)\n\u003c/pre\u003e\n\u003cp dir=\"auto\"\u003eWhich could be converted to:\u003c/p\u003e\n\u003cpre\u003edef calculate(node):\n if isinstance(node, Num):\n return node.n\n $$match_4_0 = node\n if isinstance($$match_4_0, _ast.Add):\n a, b = $$match_4_0.left, $$match_4_0.right\n return calculate(a) + calculate(b)\n elif isinstance($$match_4_0, _ast.Sub):\n a, b = $$match_4_0.left, $$match_4_0.right\n return calculate(a) - calculate(b)\n elif isinstance($$match_4_0, _ast.Mul):\n a, b = $$match_4_0.left, $$match_4_0.right\n return calculate(a) * calculate(b)\n elif isinstance($$match_4_0, _ast.Div):\n a, b = $$match_4_0.left, $$match_4_0.right\n return calculate(a) / calculate(b)\n\u003c/pre\u003e\n\u003ca name=\"user-content-zero-cost-markers-and-annotations\"\u003e\u003c/a\u003e\n\u003cdiv class=\"markdown-heading\" dir=\"auto\"\u003e\u003ch4 tabindex=\"-1\" class=\"heading-element\" dir=\"auto\"\u003eZero-cost markers and annotations\u003c/h4\u003e\u003ca id=\"user-content-zero-cost-markers-and-annotations\" class=\"anchor\" aria-label=\"Permalink: Zero-cost markers and annotations\" href=\"#zero-cost-markers-and-annotations\"\u003e\u003csvg class=\"octicon octicon-link\" viewBox=\"0 0 16 16\" version=\"1.1\" width=\"16\" height=\"16\" aria-hidden=\"true\"\u003e\u003cpath d=\"m7.775 3.275 1.25-1.25a3.5 3.5 0 1 1 4.95 4.95l-2.5 2.5a3.5 3.5 0 0 1-4.95 0 .751.751 0 0 1 .018-1.042.751.751 0 0 1 1.042-.018 1.998 1.998 0 0 0 2.83 0l2.5-2.5a2.002 2.002 0 0 0-2.83-2.83l-1.25 1.25a.751.751 0 0 1-1.042-.018.751.751 0 0 1-.018-1.042Zm-4.69 9.64a1.998 1.998 0 0 0 2.83 0l1.25-1.25a.751.751 0 0 1 1.042.018.751.751 0 0 1 .018 1.042l-1.25 1.25a3.5 3.5 0 1 1-4.95-4.95l2.5-2.5a3.5 3.5 0 0 1 4.95 0 .751.751 0 0 1-.018 1.042.751.751 0 0 1-1.042.018 1.998 1.998 0 0 0-2.83 0l-2.5 2.5a1.998 1.998 0 0 0 0 2.83Z\"\u003e\u003c/path\u003e\u003c/svg\u003e\u003c/a\u003e\u003c/div\u003e\n\u003cp dir=\"auto\"\u003eAnnotations, either decorators or \u003ca href=\"http://www.python.org/dev/peps/pep-3107\" rel=\"nofollow\"\u003ePEP 3107\u003c/a\u003e function annotations, have a runtime cost\neven if they serve only as markers for checkers or as documentation.\u003c/p\u003e\n\u003cpre\u003e@do_nothing_marker\ndef foo(...):\n ...\n\u003c/pre\u003e\n\u003cp dir=\"auto\"\u003ecan be replaced with the zero-cost macro:\u003c/p\u003e\n\u003cpre\u003edo_nothing_marker!:\ndef foo(...):\n ...\n\u003c/pre\u003e\n\u003ca name=\"user-content-prototyping-language-extensions\"\u003e\u003c/a\u003e\n\u003cdiv class=\"markdown-heading\" dir=\"auto\"\u003e\u003ch4 tabindex=\"-1\" class=\"heading-element\" dir=\"auto\"\u003ePrototyping language extensions\u003c/h4\u003e\u003ca id=\"user-content-prototyping-language-extensions\" class=\"anchor\" aria-label=\"Permalink: Prototyping language extensions\" href=\"#prototyping-language-extensions\"\u003e\u003csvg class=\"octicon octicon-link\" viewBox=\"0 0 16 16\" version=\"1.1\" width=\"16\" height=\"16\" aria-hidden=\"true\"\u003e\u003cpath d=\"m7.775 3.275 1.25-1.25a3.5 3.5 0 1 1 4.95 4.95l-2.5 2.5a3.5 3.5 0 0 1-4.95 0 .751.751 0 0 1 .018-1.042.751.751 0 0 1 1.042-.018 1.998 1.998 0 0 0 2.83 0l2.5-2.5a2.002 2.002 0 0 0-2.83-2.83l-1.25 1.25a.751.751 0 0 1-1.042-.018.751.751 0 0 1-.018-1.042Zm-4.69 9.64a1.998 1.998 0 0 0 2.83 0l1.25-1.25a.751.751 0 0 1 1.042.018.751.751 0 0 1 .018 1.042l-1.25 1.25a3.5 3.5 0 1 1-4.95-4.95l2.5-2.5a3.5 3.5 0 0 1 4.95 0 .751.751 0 0 1-.018 1.042.751.751 0 0 1-1.042.018 1.998 1.998 0 0 0-2.83 0l-2.5 2.5a1.998 1.998 0 0 0 0 2.83Z\"\u003e\u003c/path\u003e\u003c/svg\u003e\u003c/a\u003e\u003c/div\u003e\n\u003cp dir=\"auto\"\u003eAlthough macros would be most valuable for domain-specific extensions, it is possible to\ndemonstrate possible language extensions using macros.\u003c/p\u003e\n\u003ca name=\"user-content-f-strings\"\u003e\u003c/a\u003e\n\u003cdiv class=\"markdown-heading\" dir=\"auto\"\u003e\u003ch5 tabindex=\"-1\" class=\"heading-element\" dir=\"auto\"\u003ef-strings:\u003c/h5\u003e\u003ca id=\"user-content-f-strings\" class=\"anchor\" aria-label=\"Permalink: f-strings:\" href=\"#f-strings\"\u003e\u003csvg class=\"octicon octicon-link\" viewBox=\"0 0 16 16\" version=\"1.1\" width=\"16\" height=\"16\" aria-hidden=\"true\"\u003e\u003cpath d=\"m7.775 3.275 1.25-1.25a3.5 3.5 0 1 1 4.95 4.95l-2.5 2.5a3.5 3.5 0 0 1-4.95 0 .751.751 0 0 1 .018-1.042.751.751 0 0 1 1.042-.018 1.998 1.998 0 0 0 2.83 0l2.5-2.5a2.002 2.002 0 0 0-2.83-2.83l-1.25 1.25a.751.751 0 0 1-1.042-.018.751.751 0 0 1-.018-1.042Zm-4.69 9.64a1.998 1.998 0 0 0 2.83 0l1.25-1.25a.751.751 0 0 1 1.042.018.751.751 0 0 1 .018 1.042l-1.25 1.25a3.5 3.5 0 1 1-4.95-4.95l2.5-2.5a3.5 3.5 0 0 1 4.95 0 .751.751 0 0 1-.018 1.042.751.751 0 0 1-1.042.018 1.998 1.998 0 0 0-2.83 0l-2.5 2.5a1.998 1.998 0 0 0 0 2.83Z\"\u003e\u003c/path\u003e\u003c/svg\u003e\u003c/a\u003e\u003c/div\u003e\n\u003cp dir=\"auto\"\u003eThe f-string \u003ccode\u003ef\"...\"\u003c/code\u003e could be implemented as macro as \u003ccode\u003ef!(\"...\")\u003c/code\u003e.\nNot quite as nice to read, but would still be useful for experimenting with.\u003c/p\u003e\n\u003ca name=\"user-content-try-finally-statement\"\u003e\u003c/a\u003e\n\u003cdiv class=\"markdown-heading\" dir=\"auto\"\u003e\u003ch5 tabindex=\"-1\" class=\"heading-element\" dir=\"auto\"\u003eTry finally statement:\u003c/h5\u003e\u003ca id=\"user-content-try-finally-statement\" class=\"anchor\" aria-label=\"Permalink: Try finally statement:\" href=\"#try-finally-statement\"\u003e\u003csvg class=\"octicon octicon-link\" viewBox=\"0 0 16 16\" version=\"1.1\" width=\"16\" height=\"16\" aria-hidden=\"true\"\u003e\u003cpath d=\"m7.775 3.275 1.25-1.25a3.5 3.5 0 1 1 4.95 4.95l-2.5 2.5a3.5 3.5 0 0 1-4.95 0 .751.751 0 0 1 .018-1.042.751.751 0 0 1 1.042-.018 1.998 1.998 0 0 0 2.83 0l2.5-2.5a2.002 2.002 0 0 0-2.83-2.83l-1.25 1.25a.751.751 0 0 1-1.042-.018.751.751 0 0 1-.018-1.042Zm-4.69 9.64a1.998 1.998 0 0 0 2.83 0l1.25-1.25a.751.751 0 0 1 1.042.018.751.751 0 0 1 .018 1.042l-1.25 1.25a3.5 3.5 0 1 1-4.95-4.95l2.5-2.5a3.5 3.5 0 0 1 4.95 0 .751.751 0 0 1-.018 1.042.751.751 0 0 1-1.042.018 1.998 1.998 0 0 0-2.83 0l-2.5 2.5a1.998 1.998 0 0 0 0 2.83Z\"\u003e\u003c/path\u003e\u003c/svg\u003e\u003c/a\u003e\u003c/div\u003e\n\u003cpre\u003etry_!:\n body\nfinally!:\n closing\n\u003c/pre\u003e\n\u003cp dir=\"auto\"\u003eWould be translated roughly as:\u003c/p\u003e\n\u003cpre\u003etry:\n body\nexcept:\n closing\nelse:\n closing\n\u003c/pre\u003e\n\u003cdl\u003e\n\u003cdt\u003eNote:\u003c/dt\u003e\n\u003cdd\u003eCare must be taken to handle returns, breaks and continues correctly.\nThe above code is merely illustrative.\u003c/dd\u003e\n\u003c/dl\u003e\n\u003ca name=\"user-content-with-statement\"\u003e\u003c/a\u003e\n\u003cdiv class=\"markdown-heading\" dir=\"auto\"\u003e\u003ch5 tabindex=\"-1\" class=\"heading-element\" dir=\"auto\"\u003eWith statement:\u003c/h5\u003e\u003ca id=\"user-content-with-statement\" class=\"anchor\" aria-label=\"Permalink: With statement:\" href=\"#with-statement\"\u003e\u003csvg class=\"octicon octicon-link\" viewBox=\"0 0 16 16\" version=\"1.1\" width=\"16\" height=\"16\" aria-hidden=\"true\"\u003e\u003cpath d=\"m7.775 3.275 1.25-1.25a3.5 3.5 0 1 1 4.95 4.95l-2.5 2.5a3.5 3.5 0 0 1-4.95 0 .751.751 0 0 1 .018-1.042.751.751 0 0 1 1.042-.018 1.998 1.998 0 0 0 2.83 0l2.5-2.5a2.002 2.002 0 0 0-2.83-2.83l-1.25 1.25a.751.751 0 0 1-1.042-.018.751.751 0 0 1-.018-1.042Zm-4.69 9.64a1.998 1.998 0 0 0 2.83 0l1.25-1.25a.751.751 0 0 1 1.042.018.751.751 0 0 1 .018 1.042l-1.25 1.25a3.5 3.5 0 1 1-4.95-4.95l2.5-2.5a3.5 3.5 0 0 1 4.95 0 .751.751 0 0 1-.018 1.042.751.751 0 0 1-1.042.018 1.998 1.998 0 0 0-2.83 0l-2.5 2.5a1.998 1.998 0 0 0 0 2.83Z\"\u003e\u003c/path\u003e\u003c/svg\u003e\u003c/a\u003e\u003c/div\u003e\n\u003cpre\u003ewith! open(filename) as fd:\n return fd.read()\n\u003c/pre\u003e\n\u003cp dir=\"auto\"\u003eThe above would require handling \u003ccode\u003eopen\u003c/code\u003e specially.\nAn alternative that would be more explicit, would be:\u003c/p\u003e\n\u003cpre\u003ewith! open!(filename) as fd:\n return fd.read()\n\u003c/pre\u003e\n\u003ca name=\"user-content-macro-definition-macros\"\u003e\u003c/a\u003e\n\u003cdiv class=\"markdown-heading\" dir=\"auto\"\u003e\u003ch4 tabindex=\"-1\" class=\"heading-element\" dir=\"auto\"\u003eMacro definition macros\u003c/h4\u003e\u003ca id=\"user-content-macro-definition-macros\" class=\"anchor\" aria-label=\"Permalink: Macro definition macros\" href=\"#macro-definition-macros\"\u003e\u003csvg class=\"octicon octicon-link\" viewBox=\"0 0 16 16\" version=\"1.1\" width=\"16\" height=\"16\" aria-hidden=\"true\"\u003e\u003cpath d=\"m7.775 3.275 1.25-1.25a3.5 3.5 0 1 1 4.95 4.95l-2.5 2.5a3.5 3.5 0 0 1-4.95 0 .751.751 0 0 1 .018-1.042.751.751 0 0 1 1.042-.018 1.998 1.998 0 0 0 2.83 0l2.5-2.5a2.002 2.002 0 0 0-2.83-2.83l-1.25 1.25a.751.751 0 0 1-1.042-.018.751.751 0 0 1-.018-1.042Zm-4.69 9.64a1.998 1.998 0 0 0 2.83 0l1.25-1.25a.751.751 0 0 1 1.042.018.751.751 0 0 1 .018 1.042l-1.25 1.25a3.5 3.5 0 1 1-4.95-4.95l2.5-2.5a3.5 3.5 0 0 1 4.95 0 .751.751 0 0 1-.018 1.042.751.751 0 0 1-1.042.018 1.998 1.998 0 0 0-2.83 0l-2.5 2.5a1.998 1.998 0 0 0 0 2.83Z\"\u003e\u003c/path\u003e\u003c/svg\u003e\u003c/a\u003e\u003c/div\u003e\n\u003cp dir=\"auto\"\u003eLanguages that have syntactic macros usually provide a macro for defining macros.\nThis PEP intentionally does not do that, as it is not yet clear what a good design\nwould be, and we want to allow the community to define their own macros.\u003c/p\u003e\n\u003cp dir=\"auto\"\u003eOne possible form could be:\u003c/p\u003e\n\u003cpre\u003emacro_def! name:\n input:\n ... # input pattern, defining meta-variables\n output:\n ... # output pattern, using meta-variables\n\u003c/pre\u003e\n\u003ca name=\"user-content-backwards-compatibility\"\u003e\u003c/a\u003e\n\u003cdiv class=\"markdown-heading\" dir=\"auto\"\u003e\u003ch2 tabindex=\"-1\" class=\"heading-element\" dir=\"auto\"\u003eBackwards Compatibility\u003c/h2\u003e\u003ca id=\"user-content-backwards-compatibility\" class=\"anchor\" aria-label=\"Permalink: Backwards Compatibility\" href=\"#backwards-compatibility\"\u003e\u003csvg class=\"octicon octicon-link\" viewBox=\"0 0 16 16\" version=\"1.1\" width=\"16\" height=\"16\" aria-hidden=\"true\"\u003e\u003cpath d=\"m7.775 3.275 1.25-1.25a3.5 3.5 0 1 1 4.95 4.95l-2.5 2.5a3.5 3.5 0 0 1-4.95 0 .751.751 0 0 1 .018-1.042.751.751 0 0 1 1.042-.018 1.998 1.998 0 0 0 2.83 0l2.5-2.5a2.002 2.002 0 0 0-2.83-2.83l-1.25 1.25a.751.751 0 0 1-1.042-.018.751.751 0 0 1-.018-1.042Zm-4.69 9.64a1.998 1.998 0 0 0 2.83 0l1.25-1.25a.751.751 0 0 1 1.042.018.751.751 0 0 1 .018 1.042l-1.25 1.25a3.5 3.5 0 1 1-4.95-4.95l2.5-2.5a3.5 3.5 0 0 1 4.95 0 .751.751 0 0 1-.018 1.042.751.751 0 0 1-1.042.018 1.998 1.998 0 0 0-2.83 0l-2.5 2.5a1.998 1.998 0 0 0 0 2.83Z\"\u003e\u003c/path\u003e\u003c/svg\u003e\u003c/a\u003e\u003c/div\u003e\n\u003cp dir=\"auto\"\u003eThis PEP is fully backwards compatible.\u003c/p\u003e\n\u003ca name=\"user-content-performance-implications\"\u003e\u003c/a\u003e\n\u003cdiv class=\"markdown-heading\" dir=\"auto\"\u003e\u003ch2 tabindex=\"-1\" class=\"heading-element\" dir=\"auto\"\u003ePerformance Implications\u003c/h2\u003e\u003ca id=\"user-content-performance-implications\" class=\"anchor\" aria-label=\"Permalink: Performance Implications\" href=\"#performance-implications\"\u003e\u003csvg class=\"octicon octicon-link\" viewBox=\"0 0 16 16\" version=\"1.1\" width=\"16\" height=\"16\" aria-hidden=\"true\"\u003e\u003cpath d=\"m7.775 3.275 1.25-1.25a3.5 3.5 0 1 1 4.95 4.95l-2.5 2.5a3.5 3.5 0 0 1-4.95 0 .751.751 0 0 1 .018-1.042.751.751 0 0 1 1.042-.018 1.998 1.998 0 0 0 2.83 0l2.5-2.5a2.002 2.002 0 0 0-2.83-2.83l-1.25 1.25a.751.751 0 0 1-1.042-.018.751.751 0 0 1-.018-1.042Zm-4.69 9.64a1.998 1.998 0 0 0 2.83 0l1.25-1.25a.751.751 0 0 1 1.042.018.751.751 0 0 1 .018 1.042l-1.25 1.25a3.5 3.5 0 1 1-4.95-4.95l2.5-2.5a3.5 3.5 0 0 1 4.95 0 .751.751 0 0 1-.018 1.042.751.751 0 0 1-1.042.018 1.998 1.998 0 0 0-2.83 0l-2.5 2.5a1.998 1.998 0 0 0 0 2.83Z\"\u003e\u003c/path\u003e\u003c/svg\u003e\u003c/a\u003e\u003c/div\u003e\n\u003cp dir=\"auto\"\u003eFor code that doesn't use macros, there will be no effect on performance.\u003c/p\u003e\n\u003cp dir=\"auto\"\u003eFor code that does use macros and has already been compiled to bytecode,\nthere will be some slight overhead to check that the version\nof macros used to compile the code match the imported macro processors.\u003c/p\u003e\n\u003cp dir=\"auto\"\u003eFor code that has not been compiled, or compiled with different versions\nof the macro processors, then there would be the usual overhead of bytecode\ncompilation, plus any additional overhead of macro processing.\u003c/p\u003e\n\u003cp dir=\"auto\"\u003eIt is worth noting that the speed of source to bytecode compilation\nis largely irrelevant for Python performance.\u003c/p\u003e\n\u003ca name=\"user-content-implementation\"\u003e\u003c/a\u003e\n\u003cdiv class=\"markdown-heading\" dir=\"auto\"\u003e\u003ch2 tabindex=\"-1\" class=\"heading-element\" dir=\"auto\"\u003eImplementation\u003c/h2\u003e\u003ca id=\"user-content-implementation\" class=\"anchor\" aria-label=\"Permalink: Implementation\" href=\"#implementation\"\u003e\u003csvg class=\"octicon octicon-link\" viewBox=\"0 0 16 16\" version=\"1.1\" width=\"16\" height=\"16\" aria-hidden=\"true\"\u003e\u003cpath d=\"m7.775 3.275 1.25-1.25a3.5 3.5 0 1 1 4.95 4.95l-2.5 2.5a3.5 3.5 0 0 1-4.95 0 .751.751 0 0 1 .018-1.042.751.751 0 0 1 1.042-.018 1.998 1.998 0 0 0 2.83 0l2.5-2.5a2.002 2.002 0 0 0-2.83-2.83l-1.25 1.25a.751.751 0 0 1-1.042-.018.751.751 0 0 1-.018-1.042Zm-4.69 9.64a1.998 1.998 0 0 0 2.83 0l1.25-1.25a.751.751 0 0 1 1.042.018.751.751 0 0 1 .018 1.042l-1.25 1.25a3.5 3.5 0 1 1-4.95-4.95l2.5-2.5a3.5 3.5 0 0 1 4.95 0 .751.751 0 0 1-.018 1.042.751.751 0 0 1-1.042.018 1.998 1.998 0 0 0-2.83 0l-2.5 2.5a1.998 1.998 0 0 0 0 2.83Z\"\u003e\u003c/path\u003e\u003c/svg\u003e\u003c/a\u003e\u003c/div\u003e\n\u003cp dir=\"auto\"\u003eIn order to allow transformation of the AST at compile time by Python code,\nall AST nodes in the compiler will have to be Python objects.\u003c/p\u003e\n\u003cp dir=\"auto\"\u003eTo do that efficiently, will mean making all the nodes in the \u003ccode\u003e_ast\u003c/code\u003e module\nimmutable, so as not degrade performance by much.\nThey will need to be immutable to guarantee that the AST remains a \u003cem\u003etree\u003c/em\u003e\nto avoid having to support cyclic GC.\nMaking them immutable means they will not have a\n\u003ccode\u003e__dict__\u003c/code\u003e attribute, making them compact.\u003c/p\u003e\n\u003cp dir=\"auto\"\u003eAST nodes in the \u003ccode\u003east\u003c/code\u003e module will remain mutable.\u003c/p\u003e\n\u003cp dir=\"auto\"\u003eCurrently, all AST nodes are allocated using an arena allocator.\nChanging to use the standard allocator might slow compilation down a little,\nbut has advantages in terms of maintenance, as much code can be deleted.\u003c/p\u003e\n\u003ca name=\"user-content-reference-implementation\"\u003e\u003c/a\u003e\n\u003cdiv class=\"markdown-heading\" dir=\"auto\"\u003e\u003ch3 tabindex=\"-1\" class=\"heading-element\" dir=\"auto\"\u003eReference Implementation\u003c/h3\u003e\u003ca id=\"user-content-reference-implementation\" class=\"anchor\" aria-label=\"Permalink: Reference Implementation\" href=\"#reference-implementation\"\u003e\u003csvg class=\"octicon octicon-link\" viewBox=\"0 0 16 16\" version=\"1.1\" width=\"16\" height=\"16\" aria-hidden=\"true\"\u003e\u003cpath d=\"m7.775 3.275 1.25-1.25a3.5 3.5 0 1 1 4.95 4.95l-2.5 2.5a3.5 3.5 0 0 1-4.95 0 .751.751 0 0 1 .018-1.042.751.751 0 0 1 1.042-.018 1.998 1.998 0 0 0 2.83 0l2.5-2.5a2.002 2.002 0 0 0-2.83-2.83l-1.25 1.25a.751.751 0 0 1-1.042-.018.751.751 0 0 1-.018-1.042Zm-4.69 9.64a1.998 1.998 0 0 0 2.83 0l1.25-1.25a.751.751 0 0 1 1.042.018.751.751 0 0 1 .018 1.042l-1.25 1.25a3.5 3.5 0 1 1-4.95-4.95l2.5-2.5a3.5 3.5 0 0 1 4.95 0 .751.751 0 0 1-.018 1.042.751.751 0 0 1-1.042.018 1.998 1.998 0 0 0-2.83 0l-2.5 2.5a1.998 1.998 0 0 0 0 2.83Z\"\u003e\u003c/path\u003e\u003c/svg\u003e\u003c/a\u003e\u003c/div\u003e\n\u003cp dir=\"auto\"\u003eNone as yet.\u003c/p\u003e\n\u003ca name=\"user-content-copyright\"\u003e\u003c/a\u003e\n\u003cdiv class=\"markdown-heading\" dir=\"auto\"\u003e\u003ch2 tabindex=\"-1\" class=\"heading-element\" dir=\"auto\"\u003eCopyright\u003c/h2\u003e\u003ca id=\"user-content-copyright\" class=\"anchor\" aria-label=\"Permalink: Copyright\" href=\"#copyright\"\u003e\u003csvg class=\"octicon octicon-link\" viewBox=\"0 0 16 16\" version=\"1.1\" width=\"16\" height=\"16\" aria-hidden=\"true\"\u003e\u003cpath d=\"m7.775 3.275 1.25-1.25a3.5 3.5 0 1 1 4.95 4.95l-2.5 2.5a3.5 3.5 0 0 1-4.95 0 .751.751 0 0 1 .018-1.042.751.751 0 0 1 1.042-.018 1.998 1.998 0 0 0 2.83 0l2.5-2.5a2.002 2.002 0 0 0-2.83-2.83l-1.25 1.25a.751.751 0 0 1-1.042-.018.751.751 0 0 1-.018-1.042Zm-4.69 9.64a1.998 1.998 0 0 0 2.83 0l1.25-1.25a.751.751 0 0 1 1.042.018.751.751 0 0 1 .018 1.042l-1.25 1.25a3.5 3.5 0 1 1-4.95-4.95l2.5-2.5a3.5 3.5 0 0 1 4.95 0 .751.751 0 0 1-.018 1.042.751.751 0 0 1-1.042.018 1.998 1.998 0 0 0-2.83 0l-2.5 2.5a1.998 1.998 0 0 0 0 2.83Z\"\u003e\u003c/path\u003e\u003c/svg\u003e\u003c/a\u003e\u003c/div\u003e\n\u003cp dir=\"auto\"\u003eThis document is placed in the public domain or under the\nCC0-1.0-Universal license, whichever is more permissive.\u003c/p\u003e\n\n\u003c/article\u003e","renderedFileInfo":null,"shortPath":null,"symbolsEnabled":true,"tabSize":8,"topBannersInfo":{"overridingGlobalFundingFile":false,"globalPreferredFundingPath":"/python/.github/blob/5a0a7cb55767fb3667fe2391bcb66bda638edcb4/FUNDING.yml","showInvalidCitationWarning":false,"citationHelpUrl":"https://docs.github.com/github/creating-cloning-and-archiving-repositories/creating-a-repository-on-github/about-citation-files","actionsOnboardingTip":null},"truncated":false,"viewable":true,"workflowRedirectUrl":null,"symbols":{"timed_out":false,"not_analyzed":true,"symbols":[]}},"copilotInfo":null,"copilotAccessAllowed":false,"modelsAccessAllowed":false,"modelsRepoIntegrationEnabled":false,"csrf_tokens":{"/python/peps/branches":{"post":"yZM2iVHJpjJ0SKrcWsKH40dSEZzOHcp14AisZalLkjxEIS1pT64WV9Hpi3iQzhffwVkDSKz3LBqyVctKf5Dt6A"},"/repos/preferences":{"post":"ZYFhs2P9492PjF7ze6od9n0gyEu36ZrvB8qTS_w3N4H2Ye-fiLtHHs8JKKk82WZy08geWONlbLFmf3VKGduolA"}}},"title":"peps/peps/pep-0638.rst 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3Z"></path></svg></button></div></div></div></div><div></div></div><div class="Box-sc-g0xbh4-0 hGyMdv"><section aria-labelledby="file-name-id-wide file-name-id-mobile" class="Box-sc-g0xbh4-0 fGqKFv"><div class="Box-sc-g0xbh4-0 eoaCFS js-snippet-clipboard-copy-unpositioned undefined" data-hpc="true"><article class="markdown-body entry-content container-lg" itemprop="text"><p dir="auto">PEP: 638 Title: Syntactic Macros Author: Mark Shannon <<a href="mailto:mark@hotpy.org">mark@hotpy.org</a>> Discussions-To: <a href="https://mail.python.org/archives/list/python-dev@python.org/thread/U4C4XHNRC4SHS3TPZWCTY4SN4QU3TT6V/" rel="nofollow">https://mail.python.org/archives/list/python-dev@python.org/thread/U4C4XHNRC4SHS3TPZWCTY4SN4QU3TT6V/</a> Status: Draft Type: Standards Track Created: 24-Sep-2020 Post-History: 26-Sep-2020</p> <a name="user-content-abstract"></a> <div class="markdown-heading" dir="auto"><h2 tabindex="-1" class="heading-element" dir="auto">Abstract</h2><a id="user-content-abstract" class="anchor" aria-label="Permalink: Abstract" href="#abstract"><svg class="octicon octicon-link" viewBox="0 0 16 16" version="1.1" width="16" height="16" aria-hidden="true"><path d="m7.775 3.275 1.25-1.25a3.5 3.5 0 1 1 4.95 4.95l-2.5 2.5a3.5 3.5 0 0 1-4.95 0 .751.751 0 0 1 .018-1.042.751.751 0 0 1 1.042-.018 1.998 1.998 0 0 0 2.83 0l2.5-2.5a2.002 2.002 0 0 0-2.83-2.83l-1.25 1.25a.751.751 0 0 1-1.042-.018.751.751 0 0 1-.018-1.042Zm-4.69 9.64a1.998 1.998 0 0 0 2.83 0l1.25-1.25a.751.751 0 0 1 1.042.018.751.751 0 0 1 .018 1.042l-1.25 1.25a3.5 3.5 0 1 1-4.95-4.95l2.5-2.5a3.5 3.5 0 0 1 4.95 0 .751.751 0 0 1-.018 1.042.751.751 0 0 1-1.042.018 1.998 1.998 0 0 0-2.83 0l-2.5 2.5a1.998 1.998 0 0 0 0 2.83Z"></path></svg></a></div> <p dir="auto">This PEP adds support for syntactic macros to Python. A macro is a compile-time function that transforms a part of the program to allow functionality that cannot be expressed cleanly in normal library code.</p> <p dir="auto">The term "syntactic" means that this sort of macro operates on the program's syntax tree. This reduces the chance of mistranslation that can happen with text-based substitution macros, and allows the implementation of <a href="https://en.wikipedia.org/wiki/Hygienic_macro" rel="nofollow">hygienic macros</a>.</p> <p dir="auto">Syntactic macros allow libraries to modify the abstract syntax tree during compilation, providing the ability to extend the language for specific domains without adding to complexity to the language as a whole.</p> <a name="user-content-motivation"></a> <div class="markdown-heading" dir="auto"><h2 tabindex="-1" class="heading-element" dir="auto">Motivation</h2><a id="user-content-motivation" class="anchor" aria-label="Permalink: Motivation" href="#motivation"><svg class="octicon octicon-link" viewBox="0 0 16 16" version="1.1" width="16" height="16" aria-hidden="true"><path d="m7.775 3.275 1.25-1.25a3.5 3.5 0 1 1 4.95 4.95l-2.5 2.5a3.5 3.5 0 0 1-4.95 0 .751.751 0 0 1 .018-1.042.751.751 0 0 1 1.042-.018 1.998 1.998 0 0 0 2.83 0l2.5-2.5a2.002 2.002 0 0 0-2.83-2.83l-1.25 1.25a.751.751 0 0 1-1.042-.018.751.751 0 0 1-.018-1.042Zm-4.69 9.64a1.998 1.998 0 0 0 2.83 0l1.25-1.25a.751.751 0 0 1 1.042.018.751.751 0 0 1 .018 1.042l-1.25 1.25a3.5 3.5 0 1 1-4.95-4.95l2.5-2.5a3.5 3.5 0 0 1 4.95 0 .751.751 0 0 1-.018 1.042.751.751 0 0 1-1.042.018 1.998 1.998 0 0 0-2.83 0l-2.5 2.5a1.998 1.998 0 0 0 0 2.83Z"></path></svg></a></div> <p dir="auto">New language features can be controversial, disruptive and sometimes divisive. Python is now sufficiently powerful and complex, that many proposed additions are a net loss for the language due to the additional complexity.</p> <p dir="auto">Although a language change may make certain patterns easy to express, it will have a cost. Each new feature makes the language larger, harder to learn and harder to understand. Python was once described as <a href="https://www.linuxjournal.com/article/4731" rel="nofollow">Python Fits Your Brain</a>, but that becomes less and less true as more and more features are added.</p> <p dir="auto">Because of the high cost of adding a new feature, it is very difficult or impossible to add a feature that would benefit only some users, regardless of how many users, or how beneficial that feature would be to them.</p> <p dir="auto">The use of Python in data science and machine learning has grown very rapidly over the last few years. However, most of the core developers of Python do not have a background in data science or machine learning. This makes it extremely difficult for the core developers to determine whether a language extension for machine learning is worthwhile.</p> <p dir="auto">By allowing language extensions to be modular and distributable, like libraries, domain-specific extensions can be implemented without negatively impacting users outside of that domain. A web developer is likely to want a very different set of extensions from a data scientist. We need to let the community develop their own extensions.</p> <p dir="auto">Without some form of user-defined language extensions, there will be a constant battle between those wanting to keep the language compact and fitting their brains, and those wanting a new feature that suits their domain or programming style.</p> <a name="user-content-improving-the-expressiveness-of-libraries-for-specific-domains"></a> <div class="markdown-heading" dir="auto"><h3 tabindex="-1" class="heading-element" dir="auto">Improving the expressiveness of libraries for specific domains</h3><a id="user-content-improving-the-expressiveness-of-libraries-for-specific-domains" class="anchor" aria-label="Permalink: Improving the expressiveness of libraries for specific domains" href="#improving-the-expressiveness-of-libraries-for-specific-domains"><svg class="octicon octicon-link" viewBox="0 0 16 16" version="1.1" width="16" height="16" aria-hidden="true"><path d="m7.775 3.275 1.25-1.25a3.5 3.5 0 1 1 4.95 4.95l-2.5 2.5a3.5 3.5 0 0 1-4.95 0 .751.751 0 0 1 .018-1.042.751.751 0 0 1 1.042-.018 1.998 1.998 0 0 0 2.83 0l2.5-2.5a2.002 2.002 0 0 0-2.83-2.83l-1.25 1.25a.751.751 0 0 1-1.042-.018.751.751 0 0 1-.018-1.042Zm-4.69 9.64a1.998 1.998 0 0 0 2.83 0l1.25-1.25a.751.751 0 0 1 1.042.018.751.751 0 0 1 .018 1.042l-1.25 1.25a3.5 3.5 0 1 1-4.95-4.95l2.5-2.5a3.5 3.5 0 0 1 4.95 0 .751.751 0 0 1-.018 1.042.751.751 0 0 1-1.042.018 1.998 1.998 0 0 0-2.83 0l-2.5 2.5a1.998 1.998 0 0 0 0 2.83Z"></path></svg></a></div> <p dir="auto">Many domains see repeated patterns that are difficult or impossible to express as a library. Macros can express those patterns in a more concise and less error-prone way.</p> <a name="user-content-trialing-new-language-features"></a> <div class="markdown-heading" dir="auto"><h3 tabindex="-1" class="heading-element" dir="auto">Trialing new language features</h3><a id="user-content-trialing-new-language-features" class="anchor" aria-label="Permalink: Trialing new language features" href="#trialing-new-language-features"><svg class="octicon octicon-link" viewBox="0 0 16 16" version="1.1" width="16" height="16" aria-hidden="true"><path d="m7.775 3.275 1.25-1.25a3.5 3.5 0 1 1 4.95 4.95l-2.5 2.5a3.5 3.5 0 0 1-4.95 0 .751.751 0 0 1 .018-1.042.751.751 0 0 1 1.042-.018 1.998 1.998 0 0 0 2.83 0l2.5-2.5a2.002 2.002 0 0 0-2.83-2.83l-1.25 1.25a.751.751 0 0 1-1.042-.018.751.751 0 0 1-.018-1.042Zm-4.69 9.64a1.998 1.998 0 0 0 2.83 0l1.25-1.25a.751.751 0 0 1 1.042.018.751.751 0 0 1 .018 1.042l-1.25 1.25a3.5 3.5 0 1 1-4.95-4.95l2.5-2.5a3.5 3.5 0 0 1 4.95 0 .751.751 0 0 1-.018 1.042.751.751 0 0 1-1.042.018 1.998 1.998 0 0 0-2.83 0l-2.5 2.5a1.998 1.998 0 0 0 0 2.83Z"></path></svg></a></div> <p dir="auto">It is possible to demonstrate potential language extensions using macros. For example, macros would have enabled the <code>with</code> statement and <code>yield from</code> expression to have been trialed. Doing so might well have lead to a higher quality implementation at first release, by allowing more testing before those features were included in the language.</p> <p dir="auto">It is nearly impossible to make sure that a new feature is completely reliable before it is released; bugs relating to the <code>with</code> and <code>yield from</code> features were still being fixed many years after they were released.</p> <a name="user-content-long-term-stability-for-the-bytecode-interpreter"></a> <div class="markdown-heading" dir="auto"><h3 tabindex="-1" class="heading-element" dir="auto">Long term stability for the bytecode interpreter</h3><a id="user-content-long-term-stability-for-the-bytecode-interpreter" class="anchor" aria-label="Permalink: Long term stability for the bytecode interpreter" href="#long-term-stability-for-the-bytecode-interpreter"><svg class="octicon octicon-link" viewBox="0 0 16 16" version="1.1" width="16" height="16" aria-hidden="true"><path d="m7.775 3.275 1.25-1.25a3.5 3.5 0 1 1 4.95 4.95l-2.5 2.5a3.5 3.5 0 0 1-4.95 0 .751.751 0 0 1 .018-1.042.751.751 0 0 1 1.042-.018 1.998 1.998 0 0 0 2.83 0l2.5-2.5a2.002 2.002 0 0 0-2.83-2.83l-1.25 1.25a.751.751 0 0 1-1.042-.018.751.751 0 0 1-.018-1.042Zm-4.69 9.64a1.998 1.998 0 0 0 2.83 0l1.25-1.25a.751.751 0 0 1 1.042.018.751.751 0 0 1 .018 1.042l-1.25 1.25a3.5 3.5 0 1 1-4.95-4.95l2.5-2.5a3.5 3.5 0 0 1 4.95 0 .751.751 0 0 1-.018 1.042.751.751 0 0 1-1.042.018 1.998 1.998 0 0 0-2.83 0l-2.5 2.5a1.998 1.998 0 0 0 0 2.83Z"></path></svg></a></div> <p dir="auto">Historically, new language features have been implemented by naive compilation of the AST into new, complex bytecode instructions. Those bytecodes have often had their own internal flow-control, performing operations that could, and should, have been done in the compiler.</p> <p dir="auto">For example, until recently flow control within the <code>try</code>-<code>finally</code> and <code>with</code> statements was managed by complicated bytecodes with context-dependent semantics. The control flow within those statements is now implemented in the compiler, making the interpreter simpler and faster.</p> <p dir="auto">By implementing new features as AST transformations, the existing compiler can generate the bytecode for a feature without having to modify the interpreter.</p> <p dir="auto">A stable interpreter is necessary if we are to improve the performance and portability of the CPython VM.</p> <a name="user-content-rationale"></a> <div class="markdown-heading" dir="auto"><h2 tabindex="-1" class="heading-element" dir="auto">Rationale</h2><a id="user-content-rationale" class="anchor" aria-label="Permalink: Rationale" href="#rationale"><svg class="octicon octicon-link" viewBox="0 0 16 16" version="1.1" width="16" height="16" aria-hidden="true"><path d="m7.775 3.275 1.25-1.25a3.5 3.5 0 1 1 4.95 4.95l-2.5 2.5a3.5 3.5 0 0 1-4.95 0 .751.751 0 0 1 .018-1.042.751.751 0 0 1 1.042-.018 1.998 1.998 0 0 0 2.83 0l2.5-2.5a2.002 2.002 0 0 0-2.83-2.83l-1.25 1.25a.751.751 0 0 1-1.042-.018.751.751 0 0 1-.018-1.042Zm-4.69 9.64a1.998 1.998 0 0 0 2.83 0l1.25-1.25a.751.751 0 0 1 1.042.018.751.751 0 0 1 .018 1.042l-1.25 1.25a3.5 3.5 0 1 1-4.95-4.95l2.5-2.5a3.5 3.5 0 0 1 4.95 0 .751.751 0 0 1-.018 1.042.751.751 0 0 1-1.042.018 1.998 1.998 0 0 0-2.83 0l-2.5 2.5a1.998 1.998 0 0 0 0 2.83Z"></path></svg></a></div> <p dir="auto">Python is both expressive and easy to learn; it is widely recognized as the easiest-to-learn, widely used programming language. However, it is not the most flexible. That title belongs to lisp.</p> <p dir="auto">Because lisp is homoiconic, meaning that lisp programs are lisp data structures, lisp programs can be manipulated by lisp programs. Thus much of the language can be defined in itself.</p> <p dir="auto">We would like that ability in Python, without the many parentheses that characterize lisp. Fortunately, homoiconicity is not needed for a language to be able to manipulate itself, all that is needed is the ability to manipulate programs after parsing, but before translation to an executable form.</p> <p dir="auto">Python already has the components needed. The syntax tree of Python is available through the <code>ast</code> module. All that is needed is a marker to tell the compiler that a macro is present, and the ability for the compiler to callback into user code to manipulate the AST.</p> <a name="user-content-specification"></a> <div class="markdown-heading" dir="auto"><h2 tabindex="-1" class="heading-element" dir="auto">Specification</h2><a id="user-content-specification" class="anchor" aria-label="Permalink: Specification" href="#specification"><svg class="octicon octicon-link" viewBox="0 0 16 16" version="1.1" width="16" height="16" aria-hidden="true"><path d="m7.775 3.275 1.25-1.25a3.5 3.5 0 1 1 4.95 4.95l-2.5 2.5a3.5 3.5 0 0 1-4.95 0 .751.751 0 0 1 .018-1.042.751.751 0 0 1 1.042-.018 1.998 1.998 0 0 0 2.83 0l2.5-2.5a2.002 2.002 0 0 0-2.83-2.83l-1.25 1.25a.751.751 0 0 1-1.042-.018.751.751 0 0 1-.018-1.042Zm-4.69 9.64a1.998 1.998 0 0 0 2.83 0l1.25-1.25a.751.751 0 0 1 1.042.018.751.751 0 0 1 .018 1.042l-1.25 1.25a3.5 3.5 0 1 1-4.95-4.95l2.5-2.5a3.5 3.5 0 0 1 4.95 0 .751.751 0 0 1-.018 1.042.751.751 0 0 1-1.042.018 1.998 1.998 0 0 0-2.83 0l-2.5 2.5a1.998 1.998 0 0 0 0 2.83Z"></path></svg></a></div> <a name="user-content-syntax"></a> <div class="markdown-heading" dir="auto"><h3 tabindex="-1" class="heading-element" dir="auto">Syntax</h3><a id="user-content-syntax" class="anchor" aria-label="Permalink: Syntax" href="#syntax"><svg class="octicon octicon-link" viewBox="0 0 16 16" version="1.1" width="16" height="16" aria-hidden="true"><path d="m7.775 3.275 1.25-1.25a3.5 3.5 0 1 1 4.95 4.95l-2.5 2.5a3.5 3.5 0 0 1-4.95 0 .751.751 0 0 1 .018-1.042.751.751 0 0 1 1.042-.018 1.998 1.998 0 0 0 2.83 0l2.5-2.5a2.002 2.002 0 0 0-2.83-2.83l-1.25 1.25a.751.751 0 0 1-1.042-.018.751.751 0 0 1-.018-1.042Zm-4.69 9.64a1.998 1.998 0 0 0 2.83 0l1.25-1.25a.751.751 0 0 1 1.042.018.751.751 0 0 1 .018 1.042l-1.25 1.25a3.5 3.5 0 1 1-4.95-4.95l2.5-2.5a3.5 3.5 0 0 1 4.95 0 .751.751 0 0 1-.018 1.042.751.751 0 0 1-1.042.018 1.998 1.998 0 0 0-2.83 0l-2.5 2.5a1.998 1.998 0 0 0 0 2.83Z"></path></svg></a></div> <a name="user-content-lexical-analysis"></a> <div class="markdown-heading" dir="auto"><h4 tabindex="-1" class="heading-element" dir="auto">Lexical analysis</h4><a id="user-content-lexical-analysis" class="anchor" aria-label="Permalink: Lexical analysis" href="#lexical-analysis"><svg class="octicon octicon-link" viewBox="0 0 16 16" version="1.1" width="16" height="16" aria-hidden="true"><path d="m7.775 3.275 1.25-1.25a3.5 3.5 0 1 1 4.95 4.95l-2.5 2.5a3.5 3.5 0 0 1-4.95 0 .751.751 0 0 1 .018-1.042.751.751 0 0 1 1.042-.018 1.998 1.998 0 0 0 2.83 0l2.5-2.5a2.002 2.002 0 0 0-2.83-2.83l-1.25 1.25a.751.751 0 0 1-1.042-.018.751.751 0 0 1-.018-1.042Zm-4.69 9.64a1.998 1.998 0 0 0 2.83 0l1.25-1.25a.751.751 0 0 1 1.042.018.751.751 0 0 1 .018 1.042l-1.25 1.25a3.5 3.5 0 1 1-4.95-4.95l2.5-2.5a3.5 3.5 0 0 1 4.95 0 .751.751 0 0 1-.018 1.042.751.751 0 0 1-1.042.018 1.998 1.998 0 0 0-2.83 0l-2.5 2.5a1.998 1.998 0 0 0 0 2.83Z"></path></svg></a></div> <p dir="auto">Any sequence of identifier characters followed by an exclamation point (exclamation mark, UK English) will be tokenized as a <code>MACRO_NAME</code>.</p> <a name="user-content-statement-form"></a> <div class="markdown-heading" dir="auto"><h4 tabindex="-1" class="heading-element" dir="auto">Statement form</h4><a id="user-content-statement-form" class="anchor" aria-label="Permalink: Statement form" href="#statement-form"><svg class="octicon octicon-link" viewBox="0 0 16 16" version="1.1" width="16" height="16" aria-hidden="true"><path d="m7.775 3.275 1.25-1.25a3.5 3.5 0 1 1 4.95 4.95l-2.5 2.5a3.5 3.5 0 0 1-4.95 0 .751.751 0 0 1 .018-1.042.751.751 0 0 1 1.042-.018 1.998 1.998 0 0 0 2.83 0l2.5-2.5a2.002 2.002 0 0 0-2.83-2.83l-1.25 1.25a.751.751 0 0 1-1.042-.018.751.751 0 0 1-.018-1.042Zm-4.69 9.64a1.998 1.998 0 0 0 2.83 0l1.25-1.25a.751.751 0 0 1 1.042.018.751.751 0 0 1 .018 1.042l-1.25 1.25a3.5 3.5 0 1 1-4.95-4.95l2.5-2.5a3.5 3.5 0 0 1 4.95 0 .751.751 0 0 1-.018 1.042.751.751 0 0 1-1.042.018 1.998 1.998 0 0 0-2.83 0l-2.5 2.5a1.998 1.998 0 0 0 0 2.83Z"></path></svg></a></div> <pre>macro_stmt = MACRO_NAME testlist [ "import" NAME ] [ "as" NAME ] [ ":" NEWLINE suite ] </pre> <a name="user-content-expression-form"></a> <div class="markdown-heading" dir="auto"><h4 tabindex="-1" class="heading-element" dir="auto">Expression form</h4><a id="user-content-expression-form" class="anchor" aria-label="Permalink: Expression form" href="#expression-form"><svg class="octicon octicon-link" viewBox="0 0 16 16" version="1.1" width="16" height="16" aria-hidden="true"><path d="m7.775 3.275 1.25-1.25a3.5 3.5 0 1 1 4.95 4.95l-2.5 2.5a3.5 3.5 0 0 1-4.95 0 .751.751 0 0 1 .018-1.042.751.751 0 0 1 1.042-.018 1.998 1.998 0 0 0 2.83 0l2.5-2.5a2.002 2.002 0 0 0-2.83-2.83l-1.25 1.25a.751.751 0 0 1-1.042-.018.751.751 0 0 1-.018-1.042Zm-4.69 9.64a1.998 1.998 0 0 0 2.83 0l1.25-1.25a.751.751 0 0 1 1.042.018.751.751 0 0 1 .018 1.042l-1.25 1.25a3.5 3.5 0 1 1-4.95-4.95l2.5-2.5a3.5 3.5 0 0 1 4.95 0 .751.751 0 0 1-.018 1.042.751.751 0 0 1-1.042.018 1.998 1.998 0 0 0-2.83 0l-2.5 2.5a1.998 1.998 0 0 0 0 2.83Z"></path></svg></a></div> <pre>macro_expr = MACRO_NAME "(" testlist ")" </pre> <a name="user-content-resolving-ambiguity"></a> <div class="markdown-heading" dir="auto"><h4 tabindex="-1" class="heading-element" dir="auto">Resolving ambiguity</h4><a id="user-content-resolving-ambiguity" class="anchor" aria-label="Permalink: Resolving ambiguity" href="#resolving-ambiguity"><svg class="octicon octicon-link" viewBox="0 0 16 16" version="1.1" width="16" height="16" aria-hidden="true"><path d="m7.775 3.275 1.25-1.25a3.5 3.5 0 1 1 4.95 4.95l-2.5 2.5a3.5 3.5 0 0 1-4.95 0 .751.751 0 0 1 .018-1.042.751.751 0 0 1 1.042-.018 1.998 1.998 0 0 0 2.83 0l2.5-2.5a2.002 2.002 0 0 0-2.83-2.83l-1.25 1.25a.751.751 0 0 1-1.042-.018.751.751 0 0 1-.018-1.042Zm-4.69 9.64a1.998 1.998 0 0 0 2.83 0l1.25-1.25a.751.751 0 0 1 1.042.018.751.751 0 0 1 .018 1.042l-1.25 1.25a3.5 3.5 0 1 1-4.95-4.95l2.5-2.5a3.5 3.5 0 0 1 4.95 0 .751.751 0 0 1-.018 1.042.751.751 0 0 1-1.042.018 1.998 1.998 0 0 0-2.83 0l-2.5 2.5a1.998 1.998 0 0 0 0 2.83Z"></path></svg></a></div> <p dir="auto">The statement form of a macro takes precedence, so that the code <code>macro_name!(x)</code> will be parsed as a macro statement, not as an expression statement containing a macro expression.</p> <a name="user-content-semantics"></a> <div class="markdown-heading" dir="auto"><h3 tabindex="-1" class="heading-element" dir="auto">Semantics</h3><a id="user-content-semantics" class="anchor" aria-label="Permalink: Semantics" href="#semantics"><svg class="octicon octicon-link" viewBox="0 0 16 16" version="1.1" width="16" height="16" aria-hidden="true"><path d="m7.775 3.275 1.25-1.25a3.5 3.5 0 1 1 4.95 4.95l-2.5 2.5a3.5 3.5 0 0 1-4.95 0 .751.751 0 0 1 .018-1.042.751.751 0 0 1 1.042-.018 1.998 1.998 0 0 0 2.83 0l2.5-2.5a2.002 2.002 0 0 0-2.83-2.83l-1.25 1.25a.751.751 0 0 1-1.042-.018.751.751 0 0 1-.018-1.042Zm-4.69 9.64a1.998 1.998 0 0 0 2.83 0l1.25-1.25a.751.751 0 0 1 1.042.018.751.751 0 0 1 .018 1.042l-1.25 1.25a3.5 3.5 0 1 1-4.95-4.95l2.5-2.5a3.5 3.5 0 0 1 4.95 0 .751.751 0 0 1-.018 1.042.751.751 0 0 1-1.042.018 1.998 1.998 0 0 0-2.83 0l-2.5 2.5a1.998 1.998 0 0 0 0 2.83Z"></path></svg></a></div> <a name="user-content-compilation"></a> <div class="markdown-heading" dir="auto"><h4 tabindex="-1" class="heading-element" dir="auto">Compilation</h4><a id="user-content-compilation" class="anchor" aria-label="Permalink: Compilation" href="#compilation"><svg class="octicon octicon-link" viewBox="0 0 16 16" version="1.1" width="16" height="16" aria-hidden="true"><path d="m7.775 3.275 1.25-1.25a3.5 3.5 0 1 1 4.95 4.95l-2.5 2.5a3.5 3.5 0 0 1-4.95 0 .751.751 0 0 1 .018-1.042.751.751 0 0 1 1.042-.018 1.998 1.998 0 0 0 2.83 0l2.5-2.5a2.002 2.002 0 0 0-2.83-2.83l-1.25 1.25a.751.751 0 0 1-1.042-.018.751.751 0 0 1-.018-1.042Zm-4.69 9.64a1.998 1.998 0 0 0 2.83 0l1.25-1.25a.751.751 0 0 1 1.042.018.751.751 0 0 1 .018 1.042l-1.25 1.25a3.5 3.5 0 1 1-4.95-4.95l2.5-2.5a3.5 3.5 0 0 1 4.95 0 .751.751 0 0 1-.018 1.042.751.751 0 0 1-1.042.018 1.998 1.998 0 0 0-2.83 0l-2.5 2.5a1.998 1.998 0 0 0 0 2.83Z"></path></svg></a></div> <p dir="auto">Upon encountering a <code>macro</code> during translation to bytecode, the code generator will look up the macro processor registered for the macro, and pass the AST rooted at the macro to the processor function. The returned AST will then be substituted for the original tree.</p> <p dir="auto">For macros with multiple names, several trees will be passed to the macro processor, but only one will be returned and substituted, shorting the enclosing block of statements.</p> <p dir="auto">This process can be repeated, to enable macros to return AST nodes including other macros.</p> <p dir="auto">The compiler will not look up a macro processor until that macro is reached, so that inner macros do not need to have processors registered. For example, in a <code>switch</code> macro, the <code>case</code> and <code>default</code> macros wouldn't need processors registered as they would be eliminated by the <code>switch</code> processor.</p> <p dir="auto">To enable definition of macros to be imported, the macros <code>import!</code> and <code>from!</code> are predefined. They support the following syntax:</p> <pre>"import!" dotted_name "as" name "from!" dotted_name "import" name [ "as" name ] </pre> <p dir="auto">The <code>import!</code> macro performs a compile-time import of <code>dotted_name</code> to find the macro processor, then registers it under <code>name</code> for the scope currently being compiled.</p> <p dir="auto">The <code>from!</code> macro performs a compile-time import of <code>dotted_name.name</code> to find the macro processor, then registers it under <code>name</code> (using the <code>name</code> following "as", if present) for the scope currently being compiled.</p> <p dir="auto">Note that, since <code>import!</code> and <code>from!</code> only define the macro for the scope in which the import is present, all uses of a macro must be preceded by an explicit <code>import!</code> or <code>from!</code> to improve clarity.</p> <p dir="auto">For example, to import the macro "compile" from "my.compiler":</p> <pre>from! my.compiler import compile </pre> <a name="user-content-defining-macro-processors"></a> <div class="markdown-heading" dir="auto"><h4 tabindex="-1" class="heading-element" dir="auto">Defining macro processors</h4><a id="user-content-defining-macro-processors" class="anchor" aria-label="Permalink: Defining macro processors" href="#defining-macro-processors"><svg class="octicon octicon-link" viewBox="0 0 16 16" version="1.1" width="16" height="16" aria-hidden="true"><path d="m7.775 3.275 1.25-1.25a3.5 3.5 0 1 1 4.95 4.95l-2.5 2.5a3.5 3.5 0 0 1-4.95 0 .751.751 0 0 1 .018-1.042.751.751 0 0 1 1.042-.018 1.998 1.998 0 0 0 2.83 0l2.5-2.5a2.002 2.002 0 0 0-2.83-2.83l-1.25 1.25a.751.751 0 0 1-1.042-.018.751.751 0 0 1-.018-1.042Zm-4.69 9.64a1.998 1.998 0 0 0 2.83 0l1.25-1.25a.751.751 0 0 1 1.042.018.751.751 0 0 1 .018 1.042l-1.25 1.25a3.5 3.5 0 1 1-4.95-4.95l2.5-2.5a3.5 3.5 0 0 1 4.95 0 .751.751 0 0 1-.018 1.042.751.751 0 0 1-1.042.018 1.998 1.998 0 0 0-2.83 0l-2.5 2.5a1.998 1.998 0 0 0 0 2.83Z"></path></svg></a></div> <p dir="auto">A macro processor is defined by a four-tuple, consisting of <code>(func, kind, version, additional_names)</code>:</p> <ul dir="auto"> <li><code>func</code> must be a callable that takes <code>len(additional_names)+1</code> arguments, all of which are abstract syntax trees, and returns a single abstract syntax tree.</li> <li><code>kind</code> must be one of the following:<ul dir="auto"> <li><code>macros.STMT_MACRO</code>: A statement macro where the body of the macro is indented. This is the only form allowed to have additional names.</li> <li><code>macros.SIBLING_MACRO</code>: A statement macro where the body of the macro is the next statement in the same block. The following statement is moved into the macro as its body.</li> <li><code>macros.EXPR_MACRO</code>: An expression macro.</li> </ul> </li> <li><code>version</code> is used to track versions of macros, so that generated bytecodes can be correctly cached. It must be an integer.</li> <li><code>additional_names</code> are the names of the additional parts of the macro, and must be a tuple of strings.</li> </ul> <pre># (func, _ast.STMT_MACRO, VERSION, ()) stmt_macro!: multi_statement_body # (func, _ast.SIBLING_MACRO, VERSION, ()) sibling_macro! single_statement_body # (func, _ast.EXPR_MACRO, VERSION, ()) x = expr_macro!(...) # (func, _ast.STMT_MACRO, VERSION, ("subsequent_macro_part",)) multi_part_macro!: multi_statement_body subsequent_macro_part!: multi_statement_body </pre> <p dir="auto">The compiler will check that the syntax used matches the declared kind.</p> <p dir="auto">For convenience, the decorator <code>macro_processor</code> is provided in the <code>macros</code> module to mark a function as a macro processor:</p> <pre>def macro_processor(kind, version, *additional_names): def deco(func): return func, kind, version, additional_names return deco </pre> <p dir="auto">Which can be used to help declare macro processors, for example:</p> <pre>@macros.macro_processor(macros.STMT_MACRO, 1_08) def switch(astnode): ... </pre> <a name="user-content-ast-extensions"></a> <div class="markdown-heading" dir="auto"><h4 tabindex="-1" class="heading-element" dir="auto">AST extensions</h4><a id="user-content-ast-extensions" class="anchor" aria-label="Permalink: AST extensions" href="#ast-extensions"><svg class="octicon octicon-link" viewBox="0 0 16 16" version="1.1" width="16" height="16" aria-hidden="true"><path d="m7.775 3.275 1.25-1.25a3.5 3.5 0 1 1 4.95 4.95l-2.5 2.5a3.5 3.5 0 0 1-4.95 0 .751.751 0 0 1 .018-1.042.751.751 0 0 1 1.042-.018 1.998 1.998 0 0 0 2.83 0l2.5-2.5a2.002 2.002 0 0 0-2.83-2.83l-1.25 1.25a.751.751 0 0 1-1.042-.018.751.751 0 0 1-.018-1.042Zm-4.69 9.64a1.998 1.998 0 0 0 2.83 0l1.25-1.25a.751.751 0 0 1 1.042.018.751.751 0 0 1 .018 1.042l-1.25 1.25a3.5 3.5 0 1 1-4.95-4.95l2.5-2.5a3.5 3.5 0 0 1 4.95 0 .751.751 0 0 1-.018 1.042.751.751 0 0 1-1.042.018 1.998 1.998 0 0 0-2.83 0l-2.5 2.5a1.998 1.998 0 0 0 0 2.83Z"></path></svg></a></div> <p dir="auto">Two new AST nodes will be needed to express macros, <code>macro_stmt</code> and <code>macro_expr</code>.</p> <pre>class macro_stmt(_ast.stmt): _fields = "name", "args", "importname", "asname", "body" class macro_expr(_ast.expr): _fields = "name", "args" </pre> <p dir="auto">In addition, macro processors will need a means to express control flow or side-effecting code, that produces a value. A new AST node called <code>stmt_expr</code> will be added, combining a statement and an expression. This new ast node will be a subtype of <code>expr</code>, but include a statement to allow side effects. It will be compiled to bytecode by compiling the statement, then compiling the value.</p> <pre>class stmt_expr(_ast.expr): _fields = "stmt", "value" </pre> <a name="user-content-hygiene-and-debugging"></a> <div class="markdown-heading" dir="auto"><h4 tabindex="-1" class="heading-element" dir="auto">Hygiene and debugging</h4><a id="user-content-hygiene-and-debugging" class="anchor" aria-label="Permalink: Hygiene and debugging" href="#hygiene-and-debugging"><svg class="octicon octicon-link" viewBox="0 0 16 16" version="1.1" width="16" height="16" aria-hidden="true"><path d="m7.775 3.275 1.25-1.25a3.5 3.5 0 1 1 4.95 4.95l-2.5 2.5a3.5 3.5 0 0 1-4.95 0 .751.751 0 0 1 .018-1.042.751.751 0 0 1 1.042-.018 1.998 1.998 0 0 0 2.83 0l2.5-2.5a2.002 2.002 0 0 0-2.83-2.83l-1.25 1.25a.751.751 0 0 1-1.042-.018.751.751 0 0 1-.018-1.042Zm-4.69 9.64a1.998 1.998 0 0 0 2.83 0l1.25-1.25a.751.751 0 0 1 1.042.018.751.751 0 0 1 .018 1.042l-1.25 1.25a3.5 3.5 0 1 1-4.95-4.95l2.5-2.5a3.5 3.5 0 0 1 4.95 0 .751.751 0 0 1-.018 1.042.751.751 0 0 1-1.042.018 1.998 1.998 0 0 0-2.83 0l-2.5 2.5a1.998 1.998 0 0 0 0 2.83Z"></path></svg></a></div> <p dir="auto">Macro processors will often need to create new variables. Those variables need to named in such as way as to avoid contaminating the original code and other macros. No rules for naming will be enforced, but to ensure hygiene and help debugging, the following naming scheme is recommended:</p> <ul dir="auto"> <li>All generated variable names should start with a <code>$</code></li> <li>Purely artificial variable names should start <code>$$mname</code> where <code>mname</code> is the name of the macro.</li> <li>Variables derived from real variables should start <code>$vname</code> where <code>vname</code> is the name of the variable.</li> <li>All variable names should include the line number and the column offset, separated by an underscore.</li> </ul> <p dir="auto">Examples:</p> <ul dir="auto"> <li>Purely generated name: <code>$$macro_17_0</code></li> <li>Name derived from a variable for an expression macro: <code>$var_12_5</code></li> </ul> <a name="user-content-examples"></a> <div class="markdown-heading" dir="auto"><h3 tabindex="-1" class="heading-element" dir="auto">Examples</h3><a id="user-content-examples" class="anchor" aria-label="Permalink: Examples" href="#examples"><svg class="octicon octicon-link" viewBox="0 0 16 16" version="1.1" width="16" height="16" aria-hidden="true"><path d="m7.775 3.275 1.25-1.25a3.5 3.5 0 1 1 4.95 4.95l-2.5 2.5a3.5 3.5 0 0 1-4.95 0 .751.751 0 0 1 .018-1.042.751.751 0 0 1 1.042-.018 1.998 1.998 0 0 0 2.83 0l2.5-2.5a2.002 2.002 0 0 0-2.83-2.83l-1.25 1.25a.751.751 0 0 1-1.042-.018.751.751 0 0 1-.018-1.042Zm-4.69 9.64a1.998 1.998 0 0 0 2.83 0l1.25-1.25a.751.751 0 0 1 1.042.018.751.751 0 0 1 .018 1.042l-1.25 1.25a3.5 3.5 0 1 1-4.95-4.95l2.5-2.5a3.5 3.5 0 0 1 4.95 0 .751.751 0 0 1-.018 1.042.751.751 0 0 1-1.042.018 1.998 1.998 0 0 0-2.83 0l-2.5 2.5a1.998 1.998 0 0 0 0 2.83Z"></path></svg></a></div> <a name="user-content-compile-time-checked-data-structures"></a> <div class="markdown-heading" dir="auto"><h4 tabindex="-1" class="heading-element" dir="auto">Compile-time-checked data structures</h4><a id="user-content-compile-time-checked-data-structures" class="anchor" aria-label="Permalink: Compile-time-checked data structures" href="#compile-time-checked-data-structures"><svg class="octicon octicon-link" viewBox="0 0 16 16" version="1.1" width="16" height="16" aria-hidden="true"><path d="m7.775 3.275 1.25-1.25a3.5 3.5 0 1 1 4.95 4.95l-2.5 2.5a3.5 3.5 0 0 1-4.95 0 .751.751 0 0 1 .018-1.042.751.751 0 0 1 1.042-.018 1.998 1.998 0 0 0 2.83 0l2.5-2.5a2.002 2.002 0 0 0-2.83-2.83l-1.25 1.25a.751.751 0 0 1-1.042-.018.751.751 0 0 1-.018-1.042Zm-4.69 9.64a1.998 1.998 0 0 0 2.83 0l1.25-1.25a.751.751 0 0 1 1.042.018.751.751 0 0 1 .018 1.042l-1.25 1.25a3.5 3.5 0 1 1-4.95-4.95l2.5-2.5a3.5 3.5 0 0 1 4.95 0 .751.751 0 0 1-.018 1.042.751.751 0 0 1-1.042.018 1.998 1.998 0 0 0-2.83 0l-2.5 2.5a1.998 1.998 0 0 0 0 2.83Z"></path></svg></a></div> <p dir="auto">It is common to encode tables of data in Python as large dictionaries. However, these can be hard to maintain and error prone. Macros allow such data to be written in a more readable format. Then, at compile time, the data can be verified and converted to an efficient format.</p> <p dir="auto">For example, suppose we have a two dictionary literals mapping codes to names, and vice versa. This is error prone, as the dictionaries may have duplicate keys, or one table may not be the inverse of the other. A macro could generate the two mappings from a single table and, at the same time, verify that no duplicates are present.</p> <pre>color_to_code = { "red": 1, "blue": 2, "green": 3, } code_to_color = { 1: "red", 2: "blue", 3: "yellow", # error } </pre> <p dir="auto">would become:</p> <pre>bijection! color_to_code, code_to_color: "red" = 1 "blue" = 2 "green" = 3 </pre> <a name="user-content-domain-specific-extensions"></a> <div class="markdown-heading" dir="auto"><h4 tabindex="-1" class="heading-element" dir="auto">Domain-specific extensions</h4><a id="user-content-domain-specific-extensions" class="anchor" aria-label="Permalink: Domain-specific extensions" href="#domain-specific-extensions"><svg class="octicon octicon-link" viewBox="0 0 16 16" version="1.1" width="16" height="16" aria-hidden="true"><path d="m7.775 3.275 1.25-1.25a3.5 3.5 0 1 1 4.95 4.95l-2.5 2.5a3.5 3.5 0 0 1-4.95 0 .751.751 0 0 1 .018-1.042.751.751 0 0 1 1.042-.018 1.998 1.998 0 0 0 2.83 0l2.5-2.5a2.002 2.002 0 0 0-2.83-2.83l-1.25 1.25a.751.751 0 0 1-1.042-.018.751.751 0 0 1-.018-1.042Zm-4.69 9.64a1.998 1.998 0 0 0 2.83 0l1.25-1.25a.751.751 0 0 1 1.042.018.751.751 0 0 1 .018 1.042l-1.25 1.25a3.5 3.5 0 1 1-4.95-4.95l2.5-2.5a3.5 3.5 0 0 1 4.95 0 .751.751 0 0 1-.018 1.042.751.751 0 0 1-1.042.018 1.998 1.998 0 0 0-2.83 0l-2.5 2.5a1.998 1.998 0 0 0 0 2.83Z"></path></svg></a></div> <p dir="auto">Where I see macros having real value is in specific domains, not in general-purpose language features.</p> <p dir="auto">For example, parsers. Here's part of a parser definition for Python, using macros:</p> <pre>choice! single_input: NEWLINE simple_stmt sequence!: compound_stmt NEWLINE </pre> <a name="user-content-compilers"></a> <div class="markdown-heading" dir="auto"><h4 tabindex="-1" class="heading-element" dir="auto">Compilers</h4><a id="user-content-compilers" class="anchor" aria-label="Permalink: Compilers" href="#compilers"><svg class="octicon octicon-link" viewBox="0 0 16 16" version="1.1" width="16" height="16" aria-hidden="true"><path d="m7.775 3.275 1.25-1.25a3.5 3.5 0 1 1 4.95 4.95l-2.5 2.5a3.5 3.5 0 0 1-4.95 0 .751.751 0 0 1 .018-1.042.751.751 0 0 1 1.042-.018 1.998 1.998 0 0 0 2.83 0l2.5-2.5a2.002 2.002 0 0 0-2.83-2.83l-1.25 1.25a.751.751 0 0 1-1.042-.018.751.751 0 0 1-.018-1.042Zm-4.69 9.64a1.998 1.998 0 0 0 2.83 0l1.25-1.25a.751.751 0 0 1 1.042.018.751.751 0 0 1 .018 1.042l-1.25 1.25a3.5 3.5 0 1 1-4.95-4.95l2.5-2.5a3.5 3.5 0 0 1 4.95 0 .751.751 0 0 1-.018 1.042.751.751 0 0 1-1.042.018 1.998 1.998 0 0 0-2.83 0l-2.5 2.5a1.998 1.998 0 0 0 0 2.83Z"></path></svg></a></div> <p dir="auto">Runtime compilers, such as <code>numba</code> have to reconstitute the Python source, or attempt to analyze the bytecode. It would be simpler and more reliable for them to get the AST directly:</p> <pre>from! my.jit.library import jit jit! def func(): ... </pre> <a name="user-content-matching-symbolic-expressions"></a> <div class="markdown-heading" dir="auto"><h4 tabindex="-1" class="heading-element" dir="auto">Matching symbolic expressions</h4><a id="user-content-matching-symbolic-expressions" class="anchor" aria-label="Permalink: Matching symbolic expressions" href="#matching-symbolic-expressions"><svg class="octicon octicon-link" viewBox="0 0 16 16" version="1.1" width="16" height="16" aria-hidden="true"><path d="m7.775 3.275 1.25-1.25a3.5 3.5 0 1 1 4.95 4.95l-2.5 2.5a3.5 3.5 0 0 1-4.95 0 .751.751 0 0 1 .018-1.042.751.751 0 0 1 1.042-.018 1.998 1.998 0 0 0 2.83 0l2.5-2.5a2.002 2.002 0 0 0-2.83-2.83l-1.25 1.25a.751.751 0 0 1-1.042-.018.751.751 0 0 1-.018-1.042Zm-4.69 9.64a1.998 1.998 0 0 0 2.83 0l1.25-1.25a.751.751 0 0 1 1.042.018.751.751 0 0 1 .018 1.042l-1.25 1.25a3.5 3.5 0 1 1-4.95-4.95l2.5-2.5a3.5 3.5 0 0 1 4.95 0 .751.751 0 0 1-.018 1.042.751.751 0 0 1-1.042.018 1.998 1.998 0 0 0-2.83 0l-2.5 2.5a1.998 1.998 0 0 0 0 2.83Z"></path></svg></a></div> <p dir="auto">When matching something representing syntax, such a Python <code>ast</code> node, or a <code>sympy</code> expression, it is convenient to match against the actual syntax, not the data structure representing it. For example, a calculator could be implemented using a domain-specific macro for matching syntax:</p> <pre>from! ast_matcher import match def calculate(node): if isinstance(node, Num): return node.n match! node: case! a + b: return calculate(a) + calculate(b) case! a - b: return calculate(a) - calculate(b) case! a * b: return calculate(a) * calculate(b) case! a / b: return calculate(a) / calculate(b) </pre> <p dir="auto">Which could be converted to:</p> <pre>def calculate(node): if isinstance(node, Num): return node.n $$match_4_0 = node if isinstance($$match_4_0, _ast.Add): a, b = $$match_4_0.left, $$match_4_0.right return calculate(a) + calculate(b) elif isinstance($$match_4_0, _ast.Sub): a, b = $$match_4_0.left, $$match_4_0.right return calculate(a) - calculate(b) elif isinstance($$match_4_0, _ast.Mul): a, b = $$match_4_0.left, $$match_4_0.right return calculate(a) * calculate(b) elif isinstance($$match_4_0, _ast.Div): a, b = $$match_4_0.left, $$match_4_0.right return calculate(a) / calculate(b) </pre> <a name="user-content-zero-cost-markers-and-annotations"></a> <div class="markdown-heading" dir="auto"><h4 tabindex="-1" class="heading-element" dir="auto">Zero-cost markers and annotations</h4><a id="user-content-zero-cost-markers-and-annotations" class="anchor" aria-label="Permalink: Zero-cost markers and annotations" href="#zero-cost-markers-and-annotations"><svg class="octicon octicon-link" viewBox="0 0 16 16" version="1.1" width="16" height="16" aria-hidden="true"><path d="m7.775 3.275 1.25-1.25a3.5 3.5 0 1 1 4.95 4.95l-2.5 2.5a3.5 3.5 0 0 1-4.95 0 .751.751 0 0 1 .018-1.042.751.751 0 0 1 1.042-.018 1.998 1.998 0 0 0 2.83 0l2.5-2.5a2.002 2.002 0 0 0-2.83-2.83l-1.25 1.25a.751.751 0 0 1-1.042-.018.751.751 0 0 1-.018-1.042Zm-4.69 9.64a1.998 1.998 0 0 0 2.83 0l1.25-1.25a.751.751 0 0 1 1.042.018.751.751 0 0 1 .018 1.042l-1.25 1.25a3.5 3.5 0 1 1-4.95-4.95l2.5-2.5a3.5 3.5 0 0 1 4.95 0 .751.751 0 0 1-.018 1.042.751.751 0 0 1-1.042.018 1.998 1.998 0 0 0-2.83 0l-2.5 2.5a1.998 1.998 0 0 0 0 2.83Z"></path></svg></a></div> <p dir="auto">Annotations, either decorators or <a href="http://www.python.org/dev/peps/pep-3107" rel="nofollow">PEP 3107</a> function annotations, have a runtime cost even if they serve only as markers for checkers or as documentation.</p> <pre>@do_nothing_marker def foo(...): ... </pre> <p dir="auto">can be replaced with the zero-cost macro:</p> <pre>do_nothing_marker!: def foo(...): ... </pre> <a name="user-content-prototyping-language-extensions"></a> <div class="markdown-heading" dir="auto"><h4 tabindex="-1" class="heading-element" dir="auto">Prototyping language extensions</h4><a id="user-content-prototyping-language-extensions" class="anchor" aria-label="Permalink: Prototyping language extensions" href="#prototyping-language-extensions"><svg class="octicon octicon-link" viewBox="0 0 16 16" version="1.1" width="16" height="16" aria-hidden="true"><path d="m7.775 3.275 1.25-1.25a3.5 3.5 0 1 1 4.95 4.95l-2.5 2.5a3.5 3.5 0 0 1-4.95 0 .751.751 0 0 1 .018-1.042.751.751 0 0 1 1.042-.018 1.998 1.998 0 0 0 2.83 0l2.5-2.5a2.002 2.002 0 0 0-2.83-2.83l-1.25 1.25a.751.751 0 0 1-1.042-.018.751.751 0 0 1-.018-1.042Zm-4.69 9.64a1.998 1.998 0 0 0 2.83 0l1.25-1.25a.751.751 0 0 1 1.042.018.751.751 0 0 1 .018 1.042l-1.25 1.25a3.5 3.5 0 1 1-4.95-4.95l2.5-2.5a3.5 3.5 0 0 1 4.95 0 .751.751 0 0 1-.018 1.042.751.751 0 0 1-1.042.018 1.998 1.998 0 0 0-2.83 0l-2.5 2.5a1.998 1.998 0 0 0 0 2.83Z"></path></svg></a></div> <p dir="auto">Although macros would be most valuable for domain-specific extensions, it is possible to demonstrate possible language extensions using macros.</p> <a name="user-content-f-strings"></a> <div class="markdown-heading" dir="auto"><h5 tabindex="-1" class="heading-element" dir="auto">f-strings:</h5><a id="user-content-f-strings" class="anchor" aria-label="Permalink: f-strings:" href="#f-strings"><svg class="octicon octicon-link" viewBox="0 0 16 16" version="1.1" width="16" height="16" aria-hidden="true"><path d="m7.775 3.275 1.25-1.25a3.5 3.5 0 1 1 4.95 4.95l-2.5 2.5a3.5 3.5 0 0 1-4.95 0 .751.751 0 0 1 .018-1.042.751.751 0 0 1 1.042-.018 1.998 1.998 0 0 0 2.83 0l2.5-2.5a2.002 2.002 0 0 0-2.83-2.83l-1.25 1.25a.751.751 0 0 1-1.042-.018.751.751 0 0 1-.018-1.042Zm-4.69 9.64a1.998 1.998 0 0 0 2.83 0l1.25-1.25a.751.751 0 0 1 1.042.018.751.751 0 0 1 .018 1.042l-1.25 1.25a3.5 3.5 0 1 1-4.95-4.95l2.5-2.5a3.5 3.5 0 0 1 4.95 0 .751.751 0 0 1-.018 1.042.751.751 0 0 1-1.042.018 1.998 1.998 0 0 0-2.83 0l-2.5 2.5a1.998 1.998 0 0 0 0 2.83Z"></path></svg></a></div> <p dir="auto">The f-string <code>f"..."</code> could be implemented as macro as <code>f!("...")</code>. Not quite as nice to read, but would still be useful for experimenting with.</p> <a name="user-content-try-finally-statement"></a> <div class="markdown-heading" dir="auto"><h5 tabindex="-1" class="heading-element" dir="auto">Try finally statement:</h5><a id="user-content-try-finally-statement" class="anchor" aria-label="Permalink: Try finally statement:" href="#try-finally-statement"><svg class="octicon octicon-link" viewBox="0 0 16 16" version="1.1" width="16" height="16" aria-hidden="true"><path d="m7.775 3.275 1.25-1.25a3.5 3.5 0 1 1 4.95 4.95l-2.5 2.5a3.5 3.5 0 0 1-4.95 0 .751.751 0 0 1 .018-1.042.751.751 0 0 1 1.042-.018 1.998 1.998 0 0 0 2.83 0l2.5-2.5a2.002 2.002 0 0 0-2.83-2.83l-1.25 1.25a.751.751 0 0 1-1.042-.018.751.751 0 0 1-.018-1.042Zm-4.69 9.64a1.998 1.998 0 0 0 2.83 0l1.25-1.25a.751.751 0 0 1 1.042.018.751.751 0 0 1 .018 1.042l-1.25 1.25a3.5 3.5 0 1 1-4.95-4.95l2.5-2.5a3.5 3.5 0 0 1 4.95 0 .751.751 0 0 1-.018 1.042.751.751 0 0 1-1.042.018 1.998 1.998 0 0 0-2.83 0l-2.5 2.5a1.998 1.998 0 0 0 0 2.83Z"></path></svg></a></div> <pre>try_!: body finally!: closing </pre> <p dir="auto">Would be translated roughly as:</p> <pre>try: body except: closing else: closing </pre> <dl> <dt>Note:</dt> <dd>Care must be taken to handle returns, breaks and continues correctly. The above code is merely illustrative.</dd> </dl> <a name="user-content-with-statement"></a> <div class="markdown-heading" dir="auto"><h5 tabindex="-1" class="heading-element" dir="auto">With statement:</h5><a id="user-content-with-statement" class="anchor" aria-label="Permalink: With statement:" href="#with-statement"><svg class="octicon octicon-link" viewBox="0 0 16 16" version="1.1" width="16" height="16" aria-hidden="true"><path d="m7.775 3.275 1.25-1.25a3.5 3.5 0 1 1 4.95 4.95l-2.5 2.5a3.5 3.5 0 0 1-4.95 0 .751.751 0 0 1 .018-1.042.751.751 0 0 1 1.042-.018 1.998 1.998 0 0 0 2.83 0l2.5-2.5a2.002 2.002 0 0 0-2.83-2.83l-1.25 1.25a.751.751 0 0 1-1.042-.018.751.751 0 0 1-.018-1.042Zm-4.69 9.64a1.998 1.998 0 0 0 2.83 0l1.25-1.25a.751.751 0 0 1 1.042.018.751.751 0 0 1 .018 1.042l-1.25 1.25a3.5 3.5 0 1 1-4.95-4.95l2.5-2.5a3.5 3.5 0 0 1 4.95 0 .751.751 0 0 1-.018 1.042.751.751 0 0 1-1.042.018 1.998 1.998 0 0 0-2.83 0l-2.5 2.5a1.998 1.998 0 0 0 0 2.83Z"></path></svg></a></div> <pre>with! open(filename) as fd: return fd.read() </pre> <p dir="auto">The above would require handling <code>open</code> specially. An alternative that would be more explicit, would be:</p> <pre>with! open!(filename) as fd: return fd.read() </pre> <a name="user-content-macro-definition-macros"></a> <div class="markdown-heading" dir="auto"><h4 tabindex="-1" class="heading-element" dir="auto">Macro definition macros</h4><a id="user-content-macro-definition-macros" class="anchor" aria-label="Permalink: Macro definition macros" href="#macro-definition-macros"><svg class="octicon octicon-link" viewBox="0 0 16 16" version="1.1" width="16" height="16" aria-hidden="true"><path d="m7.775 3.275 1.25-1.25a3.5 3.5 0 1 1 4.95 4.95l-2.5 2.5a3.5 3.5 0 0 1-4.95 0 .751.751 0 0 1 .018-1.042.751.751 0 0 1 1.042-.018 1.998 1.998 0 0 0 2.83 0l2.5-2.5a2.002 2.002 0 0 0-2.83-2.83l-1.25 1.25a.751.751 0 0 1-1.042-.018.751.751 0 0 1-.018-1.042Zm-4.69 9.64a1.998 1.998 0 0 0 2.83 0l1.25-1.25a.751.751 0 0 1 1.042.018.751.751 0 0 1 .018 1.042l-1.25 1.25a3.5 3.5 0 1 1-4.95-4.95l2.5-2.5a3.5 3.5 0 0 1 4.95 0 .751.751 0 0 1-.018 1.042.751.751 0 0 1-1.042.018 1.998 1.998 0 0 0-2.83 0l-2.5 2.5a1.998 1.998 0 0 0 0 2.83Z"></path></svg></a></div> <p dir="auto">Languages that have syntactic macros usually provide a macro for defining macros. This PEP intentionally does not do that, as it is not yet clear what a good design would be, and we want to allow the community to define their own macros.</p> <p dir="auto">One possible form could be:</p> <pre>macro_def! name: input: ... # input pattern, defining meta-variables output: ... # output pattern, using meta-variables </pre> <a name="user-content-backwards-compatibility"></a> <div class="markdown-heading" dir="auto"><h2 tabindex="-1" class="heading-element" dir="auto">Backwards Compatibility</h2><a id="user-content-backwards-compatibility" class="anchor" aria-label="Permalink: Backwards Compatibility" href="#backwards-compatibility"><svg class="octicon octicon-link" viewBox="0 0 16 16" version="1.1" width="16" height="16" aria-hidden="true"><path d="m7.775 3.275 1.25-1.25a3.5 3.5 0 1 1 4.95 4.95l-2.5 2.5a3.5 3.5 0 0 1-4.95 0 .751.751 0 0 1 .018-1.042.751.751 0 0 1 1.042-.018 1.998 1.998 0 0 0 2.83 0l2.5-2.5a2.002 2.002 0 0 0-2.83-2.83l-1.25 1.25a.751.751 0 0 1-1.042-.018.751.751 0 0 1-.018-1.042Zm-4.69 9.64a1.998 1.998 0 0 0 2.83 0l1.25-1.25a.751.751 0 0 1 1.042.018.751.751 0 0 1 .018 1.042l-1.25 1.25a3.5 3.5 0 1 1-4.95-4.95l2.5-2.5a3.5 3.5 0 0 1 4.95 0 .751.751 0 0 1-.018 1.042.751.751 0 0 1-1.042.018 1.998 1.998 0 0 0-2.83 0l-2.5 2.5a1.998 1.998 0 0 0 0 2.83Z"></path></svg></a></div> <p dir="auto">This PEP is fully backwards compatible.</p> <a name="user-content-performance-implications"></a> <div class="markdown-heading" dir="auto"><h2 tabindex="-1" class="heading-element" dir="auto">Performance Implications</h2><a id="user-content-performance-implications" class="anchor" aria-label="Permalink: Performance Implications" href="#performance-implications"><svg class="octicon octicon-link" viewBox="0 0 16 16" version="1.1" width="16" height="16" aria-hidden="true"><path d="m7.775 3.275 1.25-1.25a3.5 3.5 0 1 1 4.95 4.95l-2.5 2.5a3.5 3.5 0 0 1-4.95 0 .751.751 0 0 1 .018-1.042.751.751 0 0 1 1.042-.018 1.998 1.998 0 0 0 2.83 0l2.5-2.5a2.002 2.002 0 0 0-2.83-2.83l-1.25 1.25a.751.751 0 0 1-1.042-.018.751.751 0 0 1-.018-1.042Zm-4.69 9.64a1.998 1.998 0 0 0 2.83 0l1.25-1.25a.751.751 0 0 1 1.042.018.751.751 0 0 1 .018 1.042l-1.25 1.25a3.5 3.5 0 1 1-4.95-4.95l2.5-2.5a3.5 3.5 0 0 1 4.95 0 .751.751 0 0 1-.018 1.042.751.751 0 0 1-1.042.018 1.998 1.998 0 0 0-2.83 0l-2.5 2.5a1.998 1.998 0 0 0 0 2.83Z"></path></svg></a></div> <p dir="auto">For code that doesn't use macros, there will be no effect on performance.</p> <p dir="auto">For code that does use macros and has already been compiled to bytecode, there will be some slight overhead to check that the version of macros used to compile the code match the imported macro processors.</p> <p dir="auto">For code that has not been compiled, or compiled with different versions of the macro processors, then there would be the usual overhead of bytecode compilation, plus any additional overhead of macro processing.</p> <p dir="auto">It is worth noting that the speed of source to bytecode compilation is largely irrelevant for Python performance.</p> <a name="user-content-implementation"></a> <div class="markdown-heading" dir="auto"><h2 tabindex="-1" class="heading-element" dir="auto">Implementation</h2><a id="user-content-implementation" class="anchor" aria-label="Permalink: Implementation" href="#implementation"><svg class="octicon octicon-link" viewBox="0 0 16 16" version="1.1" width="16" height="16" aria-hidden="true"><path d="m7.775 3.275 1.25-1.25a3.5 3.5 0 1 1 4.95 4.95l-2.5 2.5a3.5 3.5 0 0 1-4.95 0 .751.751 0 0 1 .018-1.042.751.751 0 0 1 1.042-.018 1.998 1.998 0 0 0 2.83 0l2.5-2.5a2.002 2.002 0 0 0-2.83-2.83l-1.25 1.25a.751.751 0 0 1-1.042-.018.751.751 0 0 1-.018-1.042Zm-4.69 9.64a1.998 1.998 0 0 0 2.83 0l1.25-1.25a.751.751 0 0 1 1.042.018.751.751 0 0 1 .018 1.042l-1.25 1.25a3.5 3.5 0 1 1-4.95-4.95l2.5-2.5a3.5 3.5 0 0 1 4.95 0 .751.751 0 0 1-.018 1.042.751.751 0 0 1-1.042.018 1.998 1.998 0 0 0-2.83 0l-2.5 2.5a1.998 1.998 0 0 0 0 2.83Z"></path></svg></a></div> <p dir="auto">In order to allow transformation of the AST at compile time by Python code, all AST nodes in the compiler will have to be Python objects.</p> <p dir="auto">To do that efficiently, will mean making all the nodes in the <code>_ast</code> module immutable, so as not degrade performance by much. They will need to be immutable to guarantee that the AST remains a <em>tree</em> to avoid having to support cyclic GC. Making them immutable means they will not have a <code>__dict__</code> attribute, making them compact.</p> <p dir="auto">AST nodes in the <code>ast</code> module will remain mutable.</p> <p dir="auto">Currently, all AST nodes are allocated using an arena allocator. Changing to use the standard allocator might slow compilation down a little, but has advantages in terms of maintenance, as much code can be deleted.</p> <a name="user-content-reference-implementation"></a> <div class="markdown-heading" dir="auto"><h3 tabindex="-1" class="heading-element" dir="auto">Reference Implementation</h3><a id="user-content-reference-implementation" class="anchor" aria-label="Permalink: Reference Implementation" href="#reference-implementation"><svg class="octicon octicon-link" viewBox="0 0 16 16" version="1.1" width="16" height="16" aria-hidden="true"><path d="m7.775 3.275 1.25-1.25a3.5 3.5 0 1 1 4.95 4.95l-2.5 2.5a3.5 3.5 0 0 1-4.95 0 .751.751 0 0 1 .018-1.042.751.751 0 0 1 1.042-.018 1.998 1.998 0 0 0 2.83 0l2.5-2.5a2.002 2.002 0 0 0-2.83-2.83l-1.25 1.25a.751.751 0 0 1-1.042-.018.751.751 0 0 1-.018-1.042Zm-4.69 9.64a1.998 1.998 0 0 0 2.83 0l1.25-1.25a.751.751 0 0 1 1.042.018.751.751 0 0 1 .018 1.042l-1.25 1.25a3.5 3.5 0 1 1-4.95-4.95l2.5-2.5a3.5 3.5 0 0 1 4.95 0 .751.751 0 0 1-.018 1.042.751.751 0 0 1-1.042.018 1.998 1.998 0 0 0-2.83 0l-2.5 2.5a1.998 1.998 0 0 0 0 2.83Z"></path></svg></a></div> <p dir="auto">None as yet.</p> <a name="user-content-copyright"></a> <div class="markdown-heading" dir="auto"><h2 tabindex="-1" class="heading-element" dir="auto">Copyright</h2><a id="user-content-copyright" class="anchor" aria-label="Permalink: Copyright" href="#copyright"><svg class="octicon octicon-link" viewBox="0 0 16 16" version="1.1" width="16" height="16" aria-hidden="true"><path d="m7.775 3.275 1.25-1.25a3.5 3.5 0 1 1 4.95 4.95l-2.5 2.5a3.5 3.5 0 0 1-4.95 0 .751.751 0 0 1 .018-1.042.751.751 0 0 1 1.042-.018 1.998 1.998 0 0 0 2.83 0l2.5-2.5a2.002 2.002 0 0 0-2.83-2.83l-1.25 1.25a.751.751 0 0 1-1.042-.018.751.751 0 0 1-.018-1.042Zm-4.69 9.64a1.998 1.998 0 0 0 2.83 0l1.25-1.25a.751.751 0 0 1 1.042.018.751.751 0 0 1 .018 1.042l-1.25 1.25a3.5 3.5 0 1 1-4.95-4.95l2.5-2.5a3.5 3.5 0 0 1 4.95 0 .751.751 0 0 1-.018 1.042.751.751 0 0 1-1.042.018 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