Control flow | Raku Documentation

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Exit this popup by pressing <Escape>, or clicking on X or on the background. </div> </div> <button class="modal-close is-large" aria-label="close"></button> </div> </div> </nav> <div class="tile is-ancestor section"> <div class="page-edit"> <a class="button page-edit-button" href="https://github.com/Raku/doc/edit/main/doc/Language/control.rakudoc" title="Edit this page.Commit: 8e397fdce 2024-12-26"> </a> </div> <div id="left-column" class="tile is-parent is-2 is-hidden"> <div id="left-col-inner"> <div class="tabs" id="tabs"> <ul> <li class="is-active" id="toc-tab"> <a>Table of Contents</a> </li> <li id="index-tab"> <a>Index</a> </li> </ul> </div> <div class="field"> <div class="control has-icons-right"> <input id="toc-filter" class="input" type="text" placeholder="Filter"> </div> </div> <div class="raku-sidebar"> <aside id="toc-menu" class="menu"> <ul class="menu-list"> <li><a href="#Statements">Statements</a></li> <li><a href="#Blocks">Blocks</a></li> <li><a href="#Phasers">Phasers</a></li> <li><a href="#do">do</a></li> <li><a href="#start">start</a></li> <li><a href="#if">if</a></li> <ul> <li><a href="#else/elsif">else/elsif</a></li> </ul> <li><a href="#unless">unless</a></li> <li><a href="#with_orwith_without">with orwith without</a></li> <li><a href="#when">when</a></li> <li><a href="#for">for</a></li> <li><a href="#gather/take">gather/take</a></li> <li><a href="#supply/emit">supply/emit</a></li> <li><a href="#given">given</a></li> <ul> <li><a href="#default_and_when">default and when</a></li> <li><a href="#proceed_and_succeed">proceed and succeed</a></li> <li><a href="#given_as_a_statement">given as a statement</a></li> </ul> <li><a href="#loop">loop</a></li> <li><a href="#while,_until">while, until</a></li> <li><a href="#repeat/while,_repeat/until">repeat/while, repeat/until</a></li> <li><a href="#return">return</a></li> <li><a href="#return-rw">return-rw</a></li> <li><a href="#fail">fail</a></li> <li><a href="#once">once</a></li> <li><a href="#LABELs">LABELs</a></li> <li><a href="#next">next</a></li> <li><a href="#last">last</a></li> <li><a href="#redo">redo</a></li> </ul> </aside> <aside id="index-menu" class="menu is-hidden"> <ul class="menu-list"> <li>Control flow<ul> </ul></li> <li>Other languages<ul> <li><a href="#index-entry-case_statements_(given)">case statements </a></li> <li><a href="#index-entry-switch_(given)">switch </a></li> </ul></li> </ul> </aside> </div> </div> </div> <div id="main-column" class="tile is-parent" style="overflow-x: hidden;"> <div id="main-col-inner"> <section class="raku page-header"> <div class="container px-4"> <div class="raku page-title has-text-centered"> Control flow </div> <div class="raku page-subtitle has-text-centered"> Statements used to control the flow of execution </div> </div> </section> <section class="raku page-content"><div class="container px-4"><div class="columns one-col"> <h1 id="Statements" class="raku-h1"><a name="index-entry-statements-Statements" data-indexedheader="Control flow;statements"></a><a href="#Control_flow" title="go to top of document">Statements<a class="raku-anchor" title="direct link" href="#Statements">§</a></a></h1>  Raku programs consist of one or more statements. Simple statements are separated by semicolons. The following program will print "Hello" and then "World" on the next line. <div class="raku-code raku-lang"> <button class="copy-code" title="Copy code"></button> <label>Raku highlighting</label> <div> <pre class="nohighlights">say "Hello"; say "World";</pre> </div> </div> In most places where spaces appear in a statement, and before the semicolon, they may be split up over many lines. Also, multiple statements may appear on the same line. It would be awkward, but the above example could also be written as: <div class="raku-code raku-lang"> <button class="copy-code" title="Copy code"></button> <label>Raku highlighting</label> <div> <pre class="nohighlights">say "Hello"; say "World";</pre> </div> </div> <h1 id="Blocks" class="raku-h1"><a name="index-entry-blocks-Blocks" data-indexedheader="Control flow;blocks"></a><a href="#Control_flow" title="go to top of document">Blocks<a class="raku-anchor" title="direct link" href="#Blocks">§</a></a></h1>  Like many other languages, Raku uses <code>blocks</code> enclosed by <code>{</code> and <code>}</code> to turn a sequence of statements into a <a href="/type/Block"><code>Block</code></a> that acts as a single one. It is OK to omit the semicolon between the last statement in a block and the closing <code>}</code>. <div class="raku-code raku-lang"> <button class="copy-code" title="Copy code"></button> <label>Raku highlighting</label> <div> <pre class="nohighlights">{ say "Hello"; say "World" }</pre> </div> </div> When a block stands alone as a statement, it will be entered immediately after the previous statement finishes, and the statements inside it will be executed. <div class="raku-code raku-lang"> <button class="copy-code" title="Copy code"></button> <label>Raku highlighting</label> <div> <pre class="nohighlights">say 1; # OUTPUT: «1␤» { say 2; say 3 }; # OUTPUT: «2␤3␤» say 4; # OUTPUT: «4␤»</pre> </div> </div> Unless it stands alone as a statement, a block simply creates a closure. The statements inside are not executed immediately. Closures are another topic and how they are used is explained <a href="/language/functions#Blocks_and_lambdas">elsewhere</a>. For now it is just important to understand when blocks run and when they do not: <div class="raku-code raku-lang"> <button class="copy-code" title="Copy code"></button> <label>Raku highlighting</label> <div> <pre class="nohighlights">say "We get here"; { say "then here." }; { say "not here"; 0; } or die; </pre> </div> </div> In the above example, after running the first statement, the first block stands alone as a second statement, so we run the statement inside it. The second block is a closure, so instead, it makes an object of type <a href="/type/Block"><code>Block</code></a> but does not run it. Object instances are usually considered to be true, so the code does not die, even though that block would evaluate to 0, were it to be executed. The example does not say what to do with the <a href="/type/Block"><code>Block</code></a> object, so it just gets thrown away.Most of the flow control constructs covered below are just ways to tell Raku when, how, and how many times, to enter blocks like that second block.Before we go into those, an important side-note on syntax: If there is nothing (or nothing but comments) on a line after a closing curly brace where you would normally put semicolon, then you do not need the semicolon: <div class="raku-code raku-lang"> <button class="copy-code" title="Copy code"></button> <label>Raku highlighting</label> <div> <pre class="nohighlights"># All three of these lines can appear as a group, as is, in a program { 42.say } # OUTPUT: «42␤» { 43.say } # OUTPUT: «43␤» { 42.say }; { 43.say } # OUTPUT: «42␤43␤»</pre> </div> </div> ...but: <div class="raku-code raku-lang"> <button class="copy-code" title="Copy code"></button> <label>Raku highlighting</label> <div> <pre class="nohighlights">{ 42.say } { 43.say } # Syntax error { 42.say; } { 43.say } # Also a syntax error, of course </pre> </div> </div> So, be careful when you backspace in a line-wrapping editor: <div class="raku-code raku-lang"> <button class="copy-code" title="Copy code"></button> <label>Raku highlighting</label> <div> <pre class="nohighlights">{ "Without semicolons line-wrapping can be a bit treacherous.".say } \ { 43.say } # Syntax error </pre> </div> </div> You have to watch out for this in most languages anyway to prevent things from getting accidentally commented out. Many of the examples below may have unnecessary semicolons for clarity.Class bodies behave like simple blocks for any top level expression; same goes to roles and other packages, like grammars (which are actually classes) or modules. <div class="raku-code raku-lang"> <button class="copy-code" title="Copy code"></button> <label>Raku highlighting</label> <div> <pre class="nohighlights">class C { say "I live"; die "I will never live!" }; my $c = C.new; │ # OUTPUT: Fails and writes «I live␤I will never live!␤ </pre> </div> </div> This block will first run the first statement, and then <code>die</code> printing the second statement. <code>$c</code> will never get a value. <h1 id="Phasers" class="raku-h1"><a name="index-entry-Phasers-Phasers" data-indexedheader="Control flow;Phasers"></a><a href="#Control_flow" title="go to top of document">Phasers<a class="raku-anchor" title="direct link" href="#Phasers">§</a></a></h1>  Blocks may have phasers: special labeled blocks that break their execution into phases that run in particular phases. See the page <a href="/language/phasers">phasers</a> for the details. <h1 id="do" class="raku-h1"><a name="index-entry-do-do" data-indexedheader="Control flow;do"></a><a href="#Control_flow" title="go to top of document">do<a class="raku-anchor" title="direct link" href="#do">§</a></a></h1>  The simplest way to run a block where it cannot be a stand-alone statement is by writing <code>do</code> before it: <div class="raku-code raku-lang"> <button class="copy-code" title="Copy code"></button> <label>Raku highlighting</label> <div> <pre class="nohighlights"># This dies half of the time do { say "Heads I win, tails I die."; Bool.pick } or die; say "I win."; </pre> </div> </div> Note that you need a space between the <code>do</code> and the block.The whole <code>do {...}</code> evaluates to the final value of the block. The block will be run when that value is needed in order to evaluate the rest of the expression. So: <div class="raku-code raku-lang"> <button class="copy-code" title="Copy code"></button> <label>Raku highlighting</label> <div> <pre class="nohighlights">False and do { 42.say };</pre> </div> </div> ...will not say 42. However, the block is only evaluated once each time the expression it is contained in is evaluated: <div class="raku-code raku-lang"> <button class="copy-code" title="Copy code"></button> <label>Raku highlighting</label> <div> <pre class="nohighlights"># This says "(..1 ..2 ..3)" not "(..1 ...2 ....3)" my $f = "."; say do { $f ~= "." } X~ 1, 2, 3;</pre> </div> </div> In other words, it follows the same <a href="/language/glossary#Reify">reification</a> rules as everything else.Technically, <code>do</code> is a loop which runs exactly one iteration.A <code>do</code> may also be used on a bare statement (without curly braces) but this is mainly just useful for avoiding the syntactical need to parenthesize a statement if it is the last thing in an expression: <div class="raku-code raku-lang"> <button class="copy-code" title="Copy code"></button> <label>Raku highlighting</label> <div> <pre class="nohighlights">3, do if 1 { 2 } ; # OUTPUT: «(3, 2)␤» 3, (if 1 { 2 }) ; # OUTPUT: «(3, 2)␤» </pre> </div> </div> <div class="raku-code raku-lang"> <button class="copy-code" title="Copy code"></button> <label>Raku highlighting</label> <div> <pre class="nohighlights">3, if 1 { 2 } ; # Syntax error </pre> </div> </div> As a consequence, <code>do</code> does not run blocks that, by their syntax, must be functions. For example, if <code>-></code> is used to specify a <a href="/language/syntax#Block_declaration">signature</a>, <code>do</code> treats these as single-expression statements.Thus, adding <code>-></code> to our first example prevents the closure from being evaluated: <div class="raku-code raku-lang"> <button class="copy-code" title="Copy code"></button> <label>Raku highlighting</label> <div> <pre class="nohighlights"># This never dies and never prints "Heads I win, tails I die." do -> { say "Heads I win, tails I die."; Bool.pick } or die; say "I win."; </pre> </div> </div> <h1 id="start" class="raku-h1"><a name="index-entry-start-start" data-indexedheader="Control flow;start"></a><a href="#Control_flow" title="go to top of document">start<a class="raku-anchor" title="direct link" href="#start">§</a></a></h1>  The simplest way to run a statement or block asynchronously is by writing <code>start</code> before it: <div class="raku-code raku-lang"> <button class="copy-code" title="Copy code"></button> <label>Raku highlighting</label> <div> <pre class="nohighlights">start { sleep 1; say "done" } say "working"; # working, done </pre> </div> </div> Note that you need a space between the <code>start</code> and the block. In the example above, the <code>start</code> block is in sink context since it's not assigned to a variable. From version 6.d, these sunk blocks have an exception handler attached: <div class="raku-code raku-lang"> <button class="copy-code" title="Copy code"></button> <label>Raku highlighting</label> <div> <pre class="nohighlights">start { die "We're dead"; } say "working"; sleep 10; </pre> </div> </div> This code will print <code>Unhandled exception in code scheduled on thread 4 We're dead</code> in version 6.d, while it will simply get out after waiting for 10 seconds in version 6.c.The <code>start {...}</code> immediately returns a <a href="/type/Promise"><code>Promise</code></a> that can be safely ignored if you are not interested in the result of the block. If you are interested in the final value of the block, you can call the <code>.result</code> method on the returned promise. So: <div class="raku-code raku-lang"> <button class="copy-code" title="Copy code"></button> <label>Raku highlighting</label> <div> <pre class="nohighlights">my $promise = start { sleep 10; 42 } # ... do other stuff say "The result is $promise.result()";</pre> </div> </div> If the code inside the block has not finished, the call to <code>.result</code> will wait until it is done.A <code>start</code> used on a bare statement is useful when the only thing to do asynchronously is a subroutine or method: <div class="raku-code raku-lang"> <button class="copy-code" title="Copy code"></button> <label>Raku highlighting</label> <div> <pre class="nohighlights">sub get42 { 42 } my $promise = start get42; say $promise.result; # OUTPUT: «42␤»</pre> </div> </div> Note that start code does not have access to the special variables <a href="/syntax/$!"><code>$!</code></a> and <a href="/syntax/$$SOLIDUS"><code>$/</code></a> of its outer block, but receives new ones, so every asynchronous task has its per-task state.Thus, <code>try</code> expressions and regex matches executed in the asynchronous task have their per-task state. <div class="raku-code raku-lang"> <button class="copy-code" title="Copy code"></button> <label>Raku highlighting</label> <div> <pre class="nohighlights">'a' ~~ /a/; # $/ is set to ｢a｣ try die; # $! is defined now with an anonymous AdHoc exception # as a code block await start { say $! }; # OUTPUT: «Nil␤» await start { say $/ }; # OUTPUT: «Nil␤» # as a single statement await start $!.say; # OUTPUT: «Nil␤» await start $/.say; # OUTPUT: «Nil␤»</pre> </div> </div> <h1 id="if" class="raku-h1"><a name="index-entry-if-if" data-indexedheader="Control flow;if"></a><a href="#Control_flow" title="go to top of document">if<a class="raku-anchor" title="direct link" href="#if">§</a></a></h1>  To conditionally run a block of code, use an <code>if</code> followed by a condition. The condition, an expression, will be evaluated immediately after the statement before the <code>if</code> finishes. The block attached to the condition will only be evaluated if the condition means <code>True</code> when coerced to <a href="/type/Bool"><code>Bool</code></a>. Unlike some languages the condition does not have to be parenthesized, instead the <code>{</code> and <code>}</code> around the block are mandatory: <div class="raku-code raku-lang"> <button class="copy-code" title="Copy code"></button> <label>Raku highlighting</label> <div> <pre class="nohighlights">if 1 { "1 is true".say } ; # says "1 is true" </pre> </div> </div> <div class="raku-code raku-lang"> <button class="copy-code" title="Copy code"></button> <label>Raku highlighting</label> <div> <pre class="nohighlights">if 1 "1 is true".say ; # syntax error, missing block </pre> </div> </div> <div class="raku-code raku-lang"> <button class="copy-code" title="Copy code"></button> <label>Raku highlighting</label> <div> <pre class="nohighlights">if 0 { "0 is true".say } ; # does not say anything, because 0 is false </pre> </div> </div> <div class="raku-code raku-lang"> <button class="copy-code" title="Copy code"></button> <label>Raku highlighting</label> <div> <pre class="nohighlights">if 42.say and 0 { 43.say }; # says "42" but does not say "43" </pre> </div> </div> There is also a form of <code>if</code> called a "statement modifier" form. In this case, the <code>if</code> and the condition come after the code you want to run conditionally. Do note that the condition is still always evaluated first: <div class="raku-code raku-lang"> <button class="copy-code" title="Copy code"></button> <label>Raku highlighting</label> <div> <pre class="nohighlights">43.say if 42.say and 0; # says "42" but does not say "43" 43.say if 42.say and 1; # says "42" and then says "43" say "It is easier to read code when 'if's are kept on left of screen" if True; # says the above, because it is true { 43.say } if True; # says "43" as well</pre> </div> </div> The statement modifier form is probably best used sparingly.The <code>if</code> statement itself will either <a href="/type/Slip"><code>Slip</code></a> us an empty list, if it does not run the block, or it will return the value which the block produces: <div class="raku-code raku-lang"> <button class="copy-code" title="Copy code"></button> <label>Raku highlighting</label> <div> <pre class="nohighlights">my $d = 0; say (1, (if 0 { $d += 42; 2; }), 3, $d); # says "(1 3 0)" my $c = 0; say (1, (if 1 { $c += 42; 2; }), 3, $c); # says "(1 2 3 42)" say (1, (if 1 { 2, 2 }), 3); # does not slip, says "(1 (2 2) 3)"</pre> </div> </div> For the statement modifier it is the same, except you have the value of the statement instead of a block: <div class="raku-code raku-lang"> <button class="copy-code" title="Copy code"></button> <label>Raku highlighting</label> <div> <pre class="nohighlights">say (1, (42 if True) , 2); # says "(1 42 2)" say (1, (42 if False), 2); # says "(1 2)" say (1, 42 if False , 2); # says "(1 42)" because "if False, 2" is true</pre> </div> </div> The <code>if</code> does not change the topic (<code>$_</code>) by default. In order to access the value which the conditional expression produced, you have to ask for it more strongly: <div class="raku-code raku-lang"> <button class="copy-code" title="Copy code"></button> <label>Raku highlighting</label> <div> <pre class="nohighlights">$_ = 1; if 42 { $_.say } ; # says "1" $_ = 1; if 42 -> $_ { $_.say } ; # says "42" $_ = 1; if 42 -> $a { $_.say; $a.say } ; # says "1" then says "42" $_ = 1; if 42 { $_.say; $^a.say } ; # says "1" then says "42"</pre> </div> </div> <h2 id="else/elsif" class="raku-h2"><a name="index-entry-else_elsif-else/elsif" data-indexedheader="Control flow;else elsif"></a><a href="#Control_flow" title="go to top of document"><code>else/elsif</code><a class="raku-anchor" title="direct link" href="#else/elsif">§</a></a></h2>  A compound conditional may be produced by following an <code>if</code> conditional with <code>else</code> to provide an alternative block to run when the conditional expression is false: <div class="raku-code raku-lang"> <button class="copy-code" title="Copy code"></button> <label>Raku highlighting</label> <div> <pre class="nohighlights">if 0 { say "no" } else { say "yes" } ; # says "yes" if 0 { say "no" } else{ say "yes" } ; # says "yes", space is not required </pre> </div> </div> The <code>else</code> cannot be separated from the conditional statement by a semicolon, but as a special case, it is OK to have a newline. <div class="raku-code raku-lang"> <button class="copy-code" title="Copy code"></button> <label>Raku highlighting</label> <div> <pre class="nohighlights">if 0 { say "no" }; else { say "yes" } ; # syntax error </pre> </div> </div> <div class="raku-code raku-lang"> <button class="copy-code" title="Copy code"></button> <label>Raku highlighting</label> <div> <pre class="nohighlights">if 0 { say "no" } else { say "yes" } ; # says "yes" </pre> </div> </div> Additional conditions may be sandwiched between the <code>if</code> and the <code>else</code> using <code>elsif</code>. An extra condition will only be evaluated if all the conditions before it were false, and only the block next to the first true condition will be run. You can end with an <code>elsif</code> instead of an <code>else</code> if you want. <div class="raku-code raku-lang"> <button class="copy-code" title="Copy code"></button> <label>Raku highlighting</label> <div> <pre class="nohighlights">if 0 { say "no" } elsif False { say "NO" } else { say "yes" } # says "yes" if 0 { say "no" } elsif True { say "YES" } else { say "yes" } # says "YES" if 0 { say "no" } elsif False { say "NO" } # does not say anything sub right { "Right!".say; True } sub wrong { "Wrong!".say; False } if wrong() { say "no" } elsif right() { say "yes" } else { say "maybe" } # The above says "Wrong!" then says "Right!" then says "yes"</pre> </div> </div> You cannot use the statement modifier form with <code>else</code> or <code>elsif</code>: <div class="raku-code raku-lang"> <button class="copy-code" title="Copy code"></button> <label>Raku highlighting</label> <div> <pre class="nohighlights">42.say if 0 else { 43.say } # syntax error </pre> </div> </div> All the same rules for semicolons and newlines apply, consistently <div class="raku-code raku-lang"> <button class="copy-code" title="Copy code"></button> <label>Raku highlighting</label> <div> <pre class="nohighlights">if 0 { say 0 }; elsif 1 { say 1 } else { say "how?" } ; # syntax error if 0 { say 0 } elsif 1 { say 1 }; else { say "how?" } ; # syntax error </pre> </div> </div> <div class="raku-code raku-lang"> <button class="copy-code" title="Copy code"></button> <label>Raku highlighting</label> <div> <pre class="nohighlights">if 0 { say 0 } elsif 1 { say 1 } else { say "how?" } ; # says "1" </pre> </div> </div> <div class="raku-code raku-lang"> <button class="copy-code" title="Copy code"></button> <label>Raku highlighting</label> <div> <pre class="nohighlights">if 0 { say 0 } elsif 1 { say 1 } else { say "how?" } ; # says "1" if 0 { say 0 } elsif 1 { say 1 } else { say "how?" } ; # says "1" if 0 { say "no" } elsif False { say "NO" } else { say "yes" } ; # says "yes"</pre> </div> </div> The whole thing either <a href="/type/Slip"><code>Slip</code></a>s us an empty list (if no blocks were run) or returns the value produced by the block that did run: <div class="raku-code raku-lang"> <button class="copy-code" title="Copy code"></button> <label>Raku highlighting</label> <div> <pre class="nohighlights">my $d = 0; say (1, (if 0 { $d += 42; "two"; } elsif False { $d += 43; 2; }), 3, $d); # says "(1 3 0)" my $c = 0; say (1, (if 0 { $c += 42; "two"; } else { $c += 43; 2; }), 3, $c); # says "(1 2 3 43)"</pre> </div> </div> It's possible to obtain the value of the previous expression inside an <code>else</code>, which could be from <code>if</code> or the last <code>elsif</code> if any are present: <div class="raku-code raku-lang"> <button class="copy-code" title="Copy code"></button> <label>Raku highlighting</label> <div> <pre class="nohighlights">$_ = 1; if 0 { } else -> $a { "$_ $a".say } ; # says "1 0" $_ = 1; if False { } else -> $a { "$_ $a".say } ; # says "1 False" if False { } elsif 0 { } else -> $a { $a.say } ; # says "0"</pre> </div> </div> <h1 id="unless" class="raku-h1"><a name="index-entry-unless-unless" data-indexedheader="Control flow;unless"></a><a href="#Control_flow" title="go to top of document"><code>unless</code><a class="raku-anchor" title="direct link" href="#unless">§</a></a></h1>  When you get sick of typing "if not (X)" you may use <code>unless</code> to invert the sense of a conditional statement. You cannot use <code>else</code> or <code>elsif</code> with <code>unless</code> because that ends up getting confusing. Other than those two differences <code>unless</code> works the same as <a href="#if">if</a>: <div class="raku-code raku-lang"> <button class="copy-code" title="Copy code"></button> <label>Raku highlighting</label> <div> <pre class="nohighlights">unless 1 { "1 is false".say } ; # does not say anything, since 1 is true </pre> </div> </div> <div class="raku-code raku-lang"> <button class="copy-code" title="Copy code"></button> <label>Raku highlighting</label> <div> <pre class="nohighlights">unless 1 "1 is false".say ; # syntax error, missing block </pre> </div> </div> <div class="raku-code raku-lang"> <button class="copy-code" title="Copy code"></button> <label>Raku highlighting</label> <div> <pre class="nohighlights">unless 0 { "0 is false".say } ; # says "0 is false" </pre> </div> </div> <div class="raku-code raku-lang"> <button class="copy-code" title="Copy code"></button> <label>Raku highlighting</label> <div> <pre class="nohighlights">unless 42.say and 1 { 43.say } ; # says "42" but does not say "43" 43.say unless 42.say and 0; # says "42" and then says "43" 43.say unless 42.say and 1; # says "42" but does not say "43" $_ = 1; unless 0 { $_.say } ; # says "1" $_ = 1; unless 0 -> $_ { $_.say } ; # says "0" $_ = 1; unless False -> $a { $a.say } ; # says "False" my $c = 0; say (1, (unless 0 { $c += 42; 2; }), 3, $c); # says "(1 2 3 42)" my $d = 0; say (1, (unless 1 { $d += 42; 2; }), 3, $d); # says "(1 3 0)"</pre> </div> </div> <h1 id="with_orwith_without" class="raku-h1"><a name="index-entry-with_orwith_without-with_orwith_without" data-indexedheader="Control flow;with orwith without"></a><a href="#Control_flow" title="go to top of document"><code>with orwith without</code><a class="raku-anchor" title="direct link" href="#with_orwith_without">§</a></a></h1>  The <code>with</code> statement is like <code>if</code>, but tests for definedness rather than truth, and it topicalizes on the condition, much like <code>given</code>: <div class="raku-code raku-lang"> <button class="copy-code" title="Copy code"></button> <label>Raku highlighting</label> <div> <pre class="nohighlights">with "abc".index("a") { .say } # prints 0</pre> </div> </div> Similarly to <code>elsif</code>, <code>orwith</code> may be used to chain definedness tests: <div class="raku-code raku-lang"> <button class="copy-code" title="Copy code"></button> <label>Raku highlighting</label> <div> <pre class="nohighlights"># The below code says "Found 'a' at 0" my $s = "abc"; with $s.index("a") { say "Found 'a' at $_" } orwith $s.index("b") { say "Found 'b' at $_" } orwith $s.index("c") { say "Found 'c' at $_" } else { say "Didn't find 'a', 'b' or 'c'" }</pre> </div> </div> You may intermix <code>if</code>-based and <code>with</code>-based clauses. <div class="raku-code raku-lang"> <button class="copy-code" title="Copy code"></button> <label>Raku highlighting</label> <div> <pre class="nohighlights"># This says "Yes" if 0 { say "No" } orwith Nil { say "No" } orwith 0 { say "Yes" };</pre> </div> </div> As with <code>unless</code>, you may use <code>without</code> to check for undefinedness, but you may not add an <code>else</code> clause: <div class="raku-code raku-lang"> <button class="copy-code" title="Copy code"></button> <label>Raku highlighting</label> <div> <pre class="nohighlights">my $answer = Any; without $answer { warn "Got: {$_.raku}" }</pre> </div> </div> There are also <code>with</code> and <code>without</code> statement modifiers: <div class="raku-code raku-lang"> <button class="copy-code" title="Copy code"></button> <label>Raku highlighting</label> <div> <pre class="nohighlights">my $answer = (Any, True).roll; say 42 with $answer; warn "undefined answer" without $answer;</pre> </div> </div> As with the other chainable constructs, an <code>else</code> completing a <code>with/if</code>..<code>orwith/elsif</code> chain will itself topicalize to the value of the prior (failed) condition's topic (either the topic of <code>with</code> or the final <code>orwith</code> or <code>elsif</code>).In the case of an <code>else</code> following a <code>with</code> or <code>orwith</code>, topicalizing a value guaranteed to be undefined may seem useless. But it makes for a useful idiom when used in conjunction with operations that may fail, because <a href="/type/Failure"><code>Failure</code></a> values are always undefined: <div class="raku-code raku-lang"> <button class="copy-code" title="Copy code"></button> <label>Raku highlighting</label> <div> <pre class="nohighlights">sub may_fail( --> Numeric:D ) { my $value = (^10).pick || fail "Zero is unacceptable"; fail "Odd is also not okay" if $value % 2; return $value; } with may_fail() -> $value { # defined, so didn't fail say "I know $value isn't zero or odd." } else { # undefined, so failed, and the Failure is the topic say "Uh-oh: {.exception.message}." } </pre> </div> </div> Note that while topicalizing a <a href="/type/Failure"><code>Failure</code></a> marks it <a href="/type/Failure#method_handled"><code>handled</code></a>—so you can use the <code>with</code>/<code>else</code> to proceed safely with execution—it doesn't make the Failure value itself safe. Even within the <code>else</code> clause, if you try to use the value directly, it will result in your <code>else</code> clause itself failing (or, in Rakudo, "promoting" the Failure into a thrown exception).But as seen above, you can use the methods of a handled <a href="/type/Failure"><code>Failure</code></a> object the <code>else</code> topicalizes, such as <a href="/type/Failure#method_exception"><code>exception</code></a>, if you wish to provide diagnostics or interrogate the underlying <a href="/type/Exception"><code>Exception</code></a>. <h1 id="when" class="raku-h1"><a name="index-entry-when-when" data-indexedheader="Control flow;when"></a><a href="#Control_flow" title="go to top of document">when<a class="raku-anchor" title="direct link" href="#when">§</a></a></h1>  The <code>when</code> block is similar to an <code>if</code> block and either or both can be used in an outer block; they also both have a "statement modifier" form. But there is a difference in how following code in the same, outer block is handled: When the <code>when</code> block is executed, control is passed to the enclosing block and following statements are ignored; but when the <code>if</code> block is executed, following statements are executed. <a name="fnret1" href="#fn1">[1]</a> The following examples should illustrate the <code>if</code> or <code>when</code> block's default behavior assuming no special exit or other side effect statements are included in the <code>if</code> or <code>when</code> blocks: <div class="raku-code raku-lang"> <button class="copy-code" title="Copy code"></button> <label>Raku highlighting</label> <div> <pre class="nohighlights">{ if X {...} # if X is true in Boolean context, block is executed # following statements are executed regardless } { when X {...} # if X is true in Boolean context, block is executed # and control passes to the outer block # following statements are NOT executed } </pre> </div> </div> Should the <code>if</code> and <code>when</code> blocks above appear at file scope, following statements would be executed in each case.There is one other feature <code>when</code> has that <code>if</code> doesn't: the <code>when</code>'s Boolean context test defaults to <code>$_ ~~</code> while the <code>if</code>'s does not. That has an effect on how one uses the X in the <code>when</code> block without a value for <code>$_</code> (it's <a href="/type/Any"><code>Any</code></a> in that case and <a href="/type/Any"><code>Any</code></a> smartmatches on <code>True</code>: <code>Any ~~ True</code> yields <code>True</code>). Consider the following: <div class="raku-code raku-lang"> <button class="copy-code" title="Copy code"></button> <label>Raku highlighting</label> <div> <pre class="nohighlights">{ my $a = 1; my $b = True; when $a { say 'a' }; # no output when so $a { say 'a' } # a ("so $a" 'so' coerces $a to Boolean context True # which matches with Any) when $b { say 'b' }; # no output (this statement won't be run) } </pre> </div> </div> Finally, <code>when</code>'s statement modifier form does not affect execution of following statements either inside or outside of another block: <div class="raku-code raku-lang"> <button class="copy-code" title="Copy code"></button> <label>Raku highlighting</label> <div> <pre class="nohighlights">say "foo" when X; # if X is true statement is executed # following statements are not affected </pre> </div> </div> Since a successful match will exit the block, the behavior of this piece of code: <div class="raku-code raku-lang"> <button class="copy-code" title="Copy code"></button> <label>Raku highlighting</label> <div> <pre class="nohighlights">$_ = True; my $a; { $a = do when .so { "foo" } }; say $a; # OUTPUT: «(Any)␤» </pre> </div> </div> is explained since the <code>do</code> block is abandoned before any value is stored or processed. However, in this case: <div class="raku-code raku-lang"> <button class="copy-code" title="Copy code"></button> <label>Raku highlighting</label> <div> <pre class="nohighlights">$_ = False; my $a; { $a = do when .so { "foo" } }; say $a; # OUTPUT: «False␤» </pre> </div> </div> the block is not abandoned since the comparison is false, so <code>$a</code> will actually get a value. <h1 id="for" class="raku-h1"><a name="index-entry-for-for" data-indexedheader="Control flow;for"></a><a href="#Control_flow" title="go to top of document">for<a class="raku-anchor" title="direct link" href="#for">§</a></a></h1>  The <code>for</code> loop iterates over a list, running the statements inside a <a href="/type/Block"><code>Block</code></a> once on each iteration. If the block takes parameters, the elements of the list are provided as arguments. By default, the block takes one parameter, <code>$_</code>: <div class="raku-code raku-lang"> <button class="copy-code" title="Copy code"></button> <label>Raku highlighting</label> <div> <pre class="nohighlights">my @foo = 1..3; for @foo { $_.print } # prints each value contained in @foo for @foo { .print } # same thing, because .print implies a $_ argument for @foo { 42.print } # prints 42 as many times as @foo has elements</pre> </div> </div> Pointy block syntax or a <a href="/language/variables#The_^_twigil">placeholder</a> may be used to name the parameter: <div class="raku-code raku-lang"> <button class="copy-code" title="Copy code"></button> <label>Raku highlighting</label> <div> <pre class="nohighlights">my @foo = 1..3; for @foo -> $item { print $item } for @foo { print $^item } # same thing</pre> </div> </div> Multiple parameters can be declared, in which case the iterator takes as many elements from the list as needed before running the block. <div class="raku-code raku-lang"> <button class="copy-code" title="Copy code"></button> <label>Raku highlighting</label> <div> <pre class="nohighlights">my @foo = 1..3; for @foo.kv -> $idx, $val { say "$idx: $val" } my %hash = <a b c> Z=> 1,2,3; for %hash.kv -> $key, $val { say "$key => $val" } for 1, 1.1, 2, 2.1 { say "$^x < $^y" } # OUTPUT: «1 < 1.1␤2 < 2.1␤»</pre> </div> </div> Parameters of a pointy block can have default values, allowing the code to handle lists with missing elements. <div class="raku-code raku-lang"> <button class="copy-code" title="Copy code"></button> <label>Raku highlighting</label> <div> <pre class="nohighlights">my @list = 1,2,3,4; for @list -> $a, $b = 'N/A', $c = 'N/A' { say "$a $b $c" } # OUTPUT: «1 2 3␤4 N/A N/A␤»</pre> </div> </div> When no parameters are specified for a <code>for</code> loop's block, <code>when</code> can be used within it similarly to how it's used in a <code>given</code> block: <div class="raku-code raku-lang"> <button class="copy-code" title="Copy code"></button> <label>Raku highlighting</label> <div> <pre class="nohighlights"># A solution for FizzBuzz: for 1..100 { when * %% 15 { say 'FizzBuzz' } when * %% 3 { say 'Fizz' } when * %% 5 { say 'Buzz' } default { say $_ } }</pre> </div> </div> If the postfix form of <code>for</code> is used, a block is not required and the topic is set for the statement list. <div class="raku-code raku-lang"> <button class="copy-code" title="Copy code"></button> <label>Raku highlighting</label> <div> <pre class="nohighlights">say „I $_ butterflies!“ for <♥ ♥ ♥>; # OUTPUT: «I ♥ butterflies!␤I ♥ butterflies!␤I ♥ butterflies!␤»</pre> </div> </div> A <code>for</code> may be used on lazy lists – it will only take elements from the list when they are needed, so to read a file line by line, you could use: <div class="raku-code raku-lang"> <button class="copy-code" title="Copy code"></button> <label>Raku highlighting</label> <div> <pre class="nohighlights">for $*IN.lines -> $line { .say } </pre> </div> </div> Iteration variables are always lexical, so you don't need to use <code>my</code> to give them the appropriate scope. Also, they are read-only aliases. If you need them to be writable, use <code><-></code> instead of <code>-></code>. Alternatively, you can add the <a href="/language/signatures#Parameter_traits_and_modifiers"><code>is rw</code></a> trait; this performs a binding operation so assigning to the parameter changes the value of the variable at the caller side. If instead you want to modify copies of the arguments within the block, add <a href="/language/signatures#Parameter_traits_and_modifiers"><code>is copy</code></a>. <div class="raku-code raku-lang"> <button class="copy-code" title="Copy code"></button> <label>Raku highlighting</label> <div> <pre class="nohighlights">my @foo = 1..3; for @foo <-> $value { $value = $value %% 2 ?? "Even" !! "Odd" } say @foo; # OUTPUT: «[Odd Even Odd]␤» @foo = 1..3; for @foo -> $value is rw { $value = $value %% 2 ?? "Even" !! "Odd" } say @foo; # OUTPUT: «[Odd Even Odd]␤» @foo = 1..3; my @bar; for @foo -> $value is copy { $value = $value %% 2 ?? "Even" !! "Odd"; @bar.push: $value } say @foo; # OUTPUT: «[1 2 3]␤» say @bar; # OUTPUT: «[Odd Even Odd]␤» </pre> </div> </div> This rule also applies to the topic variable <code>$_</code>, which by default is a read-write alias; it will become read-only if it's used in a <code>-></code> loop. <div class="raku-code raku-lang"> <button class="copy-code" title="Copy code"></button> <label>Raku highlighting</label> <div> <pre class="nohighlights">my @foo = 1..3; for @foo -> $_ { $_.say } # Error: ...require mutable arguments for @foo -> $_ { $_++ }</pre> </div> </div> A <code>for</code> loop can produce a <a href="/type/List"><code>List</code></a> of the values produced by each run of the attached block. To capture these values, put the for loop in parenthesis or assign them to an array: <div class="raku-code raku-lang"> <button class="copy-code" title="Copy code"></button> <label>Raku highlighting</label> <div> <pre class="nohighlights">(for 1, 2, 3 { $_ * 2 }).say; # OUTPUT: «(2 4 6)␤» my @a = do for 1, 2, 3 { $_ * 2 }; @a.say; # OUTPUT: «[2 4 6]␤» my @b = (for 1, 2, 3 { $_ * 2 }); @b.say; # OUTPUT: «[2 4 6]␤»</pre> </div> </div> This implies that, if the results of the loop are not assigned, they will be in a <a href="/language/contexts#Sink">sink context</a>: <div class="raku-code raku-lang"> <button class="copy-code" title="Copy code"></button> <label>Raku highlighting</label> <div> <pre class="nohighlights">class Sunk { has $.titanic; method sink { say "Sinking $!titanic"; } } Sunk.new( :titanic($_) ) for ^3; for 1 { say "About to sink"; Sunk.new( :titanic($_) ); } # OUTPUT: # Sinking 0 # Sinking 1 # Sinking 2 # About to sink # Sinking 1 </pre> </div> </div> The first loop creates three elements but they are in a sink context, so its <code>sink</code> method is called. In the second loop, its last statement will be in a sink context, so it will be also sunk (from version 6.d).The <code>Empty</code> constant will act as a no-op for a loop: <div class="raku-code raku-lang"> <button class="copy-code" title="Copy code"></button> <label>Raku highlighting</label> <div> <pre class="nohighlights">say "Not here" for Empty; </pre> </div> </div> Will not do anything. This constant is <a href="/syntax/Empty">equivalent to an empty Slip or List</a>.Undefined values will behave in the same way: <div class="raku-code raku-lang"> <button class="copy-code" title="Copy code"></button> <label>Raku highlighting</label> <div> <pre class="nohighlights">my @array := Empty; .say for @array; say @array; # OUTPUT: «()␤» </pre> </div> </div> Assigning <code>Empty</code> will effectively undefine an <a href="/type/Array"><code>Array</code></a>, using <code>for</code> over an undefined array will not even enter the loop, as shown, effectively behaving in the same way as above when <code>Empty</code> was used directly.With <code>hyper</code> and <code>race</code>, the <code>for</code> loop is potentially iterated in parallel. See also the documentation for <code>hyper</code> and <code>race</code> in class <a href="/type/Map"><code>Map</code></a>. <div class="raku-code raku-lang"> <button class="copy-code" title="Copy code"></button> <label>Raku highlighting</label> <div> <pre class="nohighlights">my $primes_h = hyper for ^10_000 -> $number { $number if $number.is-prime }; say $primes_h.elems; # OUTPUT: «1229␤» say $primes_h.tail: 5; # OUTPUT: «(9931 9941 9949 9967 9973)␤» </pre> </div> </div> with <code>hyper</code> the order of elements is preserved. <div class="raku-code raku-lang"> <button class="copy-code" title="Copy code"></button> <label>Raku highlighting</label> <div> <pre class="nohighlights">my $primes_r = race for ^10_000 -> $number { $number if $number.is-prime }; say $primes_r.elems; # OUTPUT: «1229␤» </pre> </div> </div> Unlike <code>hyper</code>, <code>race</code> does not preserve the order of elements. <h1 id="gather/take" class="raku-h1"><a name="index-entry-gather_take-gather/take" data-indexedheader="Control flow;gather take"></a><a href="#Control_flow" title="go to top of document">gather/take<a class="raku-anchor" title="direct link" href="#gather/take">§</a></a></h1>  <code>gather</code> is a statement or block prefix that returns a <a href="/type/Seq">sequence</a> of values. The values come from calls to <a href="/type/Mu#routine_take">take</a> in the dynamic scope of the <code>gather</code> code. In the following example, we implement a subroutine to compute the factors of an integer with <code>gather</code> (note that the factors are not generated in order): <div class="raku-code raku-lang"> <button class="copy-code" title="Copy code"></button> <label>Raku highlighting</label> <div> <pre class="nohighlights">sub factors( Int:D \n ) { my $k = 1; gather { while $k**2 < n { if n %% $k { take $k; take n div $k; } $k++; } take $k if $k**2 == n; } } say factors(36); # OUTPUT: «1, 36, 2, 18, 3, 12, 4, 9, 6␤»</pre> </div> </div> The <code>gather/take</code> combination can generate values lazily, depending on context. Binding to a scalar or sigilless container will force laziness. If you want to force lazy evaluation use the <a href="/type/Iterable#method_lazy">lazy</a> subroutine or method. For example: <div class="raku-code raku-lang"> <button class="copy-code" title="Copy code"></button> <label>Raku highlighting</label> <div> <pre class="nohighlights">my @vals = lazy gather { take 1; say "Produced a value"; take 2; } say @vals[0]; say 'between consumption of two values'; say @vals[1]; # OUTPUT: # 1 # between consumption of two values # Produced a value # 2</pre> </div> </div> <code>gather/take</code> is scoped dynamically, so you can call <code>take</code> from subs or methods that are called from within <code>gather</code>: <div class="raku-code raku-lang"> <button class="copy-code" title="Copy code"></button> <label>Raku highlighting</label> <div> <pre class="nohighlights">sub weird(@elems, :$direction = 'forward') { my %direction = ( forward => sub { take $_ for @elems }, backward => sub { take $_ for @elems.reverse }, random => sub { take $_ for @elems.pick(*) }, ); return gather %direction{$direction}(); } say weird(<a b c>, :direction<backward> ); # OUTPUT: «(c b a)␤»</pre> </div> </div> If values need to be mutable on the caller side, use <a href="/type/Mu#routine_take-rw">take-rw</a>.Note that the <a href="/type/Seq"><code>Seq</code></a> created by <code>gather/take</code> may be coerced to another type. An example with assignment to a hash: <div class="raku-code raku-lang"> <button class="copy-code" title="Copy code"></button> <label>Raku highlighting</label> <div> <pre class="nohighlights">my %h = gather { take "foo" => 1; take "bar" => 2}; say %h; # OUTPUT: «{bar => 2, foo => 1}␤»</pre> </div> </div> Note: <code>gather/take</code> must not be used to collect results from <code>react/whenever</code>. The <code>whenever</code> block is not run from the thread that runs the <code>gather/react</code>, but the thread that runs the <code>emit</code>. On this thread, there is no handler for the control exception thrown by <code>take</code>, causing it to error out. <h1 id="supply/emit" class="raku-h1"><a name="index-entry-supply_emit-supply/emit" data-indexedheader="Control flow;supply emit"></a><a href="#Control_flow" title="go to top of document">supply/emit<a class="raku-anchor" title="direct link" href="#supply/emit">§</a></a></h1>  The keyword <code>supply</code> creates a <a href="/type/Supply">Supply object</a> which is an <a href="/language/concurrency#index-entry-supply_(on-demand)">on-demand supply</a> that you can tap. It pairs with <code>emit</code>, which can be used anywhere from within <code>supply</code> prefixed code.Using the <a href="/type/Mu#method_emit"><code>emit method</code></a> or the <a href="/type/independent-routines#sub_emit"><code>emit routine</code></a> passes the invocant to the enclosing <a href="/language/concurrency#index-entry-supply_(on-demand)">supply</a>: <div class="raku-code raku-lang"> <button class="copy-code" title="Copy code"></button> <label>Raku highlighting</label> <div> <pre class="nohighlights">my $supply = supply { .emit for "foo", 42, .5; } $supply.tap: { say "received {.^name} ($_)"; } # OUTPUT: # received Str (foo) # received Int (42) # received Rat (0.5)</pre> </div> </div> See also: <a href="/routine/tap"><code>tap</code></a> and <a href="/type/Supplier"><code>Supplier</code></a>.<a name="index-entry-switch_(given)" class="index-entry"></a> <a name="index-entry-case_statements_(given)" class="index-entry"></a> <h1 id="given" class="raku-h1"><a name="index-entry-given-given" data-indexedheader="Control flow;given"></a><a href="#Control_flow" title="go to top of document">given<a class="raku-anchor" title="direct link" href="#given">§</a></a></h1>  The <code>given</code> statement is Raku's topicalizing keyword in a similar way that <code>switch</code> topicalizes in languages such as C. In other words, <code>given</code> sets <code>$_</code> inside the following block. The keywords for individual cases are <code>when</code> and <code>default</code>. The usual idiom looks like this: <div class="raku-code raku-lang"> <button class="copy-code" title="Copy code"></button> <label>Raku highlighting</label> <div> <pre class="nohighlights">my $var = (Any, 21, any <answer lie>).pick; given $var { when 21 { say $_ * 2 } when 'lie' { .say } default { say 'default' } }</pre> </div> </div> The <code>given</code> statement is often used alone: <div class="raku-code raku-lang"> <button class="copy-code" title="Copy code"></button> <label>Raku highlighting</label> <div> <pre class="nohighlights">given 42 { .say; .Numeric; }</pre> </div> </div> This is a lot more understandable than: <div class="raku-code raku-lang"> <button class="copy-code" title="Copy code"></button> <label>Raku highlighting</label> <div> <pre class="nohighlights">{ .say; .Numeric; }(42)</pre> </div> </div> <h2 id="default_and_when" class="raku-h2"><a name="index-entry-default_when-default_and_when" data-indexedheader="Control flow;default when"></a><a href="#Control_flow" title="go to top of document">default and when<a class="raku-anchor" title="direct link" href="#default_and_when">§</a></a></h2>  A block containing a <code>default</code> statement will be left immediately when the sub-block after the <code>default</code> statement is left. It is as though the rest of the statements in the block were skipped. <div class="raku-code raku-lang"> <button class="copy-code" title="Copy code"></button> <label>Raku highlighting</label> <div> <pre class="nohighlights">given 42 { "This says".say; $_ == 42 and ( default { "This says, too".say; 43; } ); "This never says".say; } # The above block evaluates to 43</pre> </div> </div> A <code>when</code> statement will also do this (but a <code>when</code> statement modifier will not.)In addition, <code>when</code> statements <code>smartmatch</code> the topic (<code>$_</code>) against a supplied expression such that it is possible to check against values, regular expressions, and types when specifying a match. <div class="raku-code raku-lang"> <button class="copy-code" title="Copy code"></button> <label>Raku highlighting</label> <div> <pre class="nohighlights">for 42, 43, "foo", 44, "bar" { when Int { .say } when /:i ^Bar/ { .say } default { say "Not an Int or a Bar" } } # OUTPUT: «42␤43␤Not an Int or a Bar␤44␤Bar␤»</pre> </div> </div> In this form, the <code>given</code>/<code>when</code> construct acts much like a set of <code>if</code>/<code>elsif</code>/<code>else</code> statements. Be careful with the order of the <code>when</code> statements. The following code says <code>"Int"</code> not <code>42</code>. <div class="raku-code raku-lang"> <button class="copy-code" title="Copy code"></button> <label>Raku highlighting</label> <div> <pre class="nohighlights">given 42 { when Int { say "Int" } when 42 { say 42 } default { say "huh?" } } # OUTPUT: «Int␤»</pre> </div> </div> When a <code>when</code> statement or <code>default</code> statement causes the outer block to return, nesting <code>when</code> or <code>default</code> blocks do not count as the outer block, so you can nest these statements and still be in the same "switch" just so long as you do not open a new block: <div class="raku-code raku-lang"> <button class="copy-code" title="Copy code"></button> <label>Raku highlighting</label> <div> <pre class="nohighlights">given 42 { when Int { when 42 { say 42 } say "Int" } default { say "huh?" } } # OUTPUT: «42␤»</pre> </div> </div> <code>when</code> statements can smartmatch against <a href="/language/syntax#Signature_literals">Signatures</a>. <h2 id="proceed_and_succeed" class="raku-h2"><a name="index-entry-proceed-proceed" data-indexedheader="Control flow;proceed"></a><a href="#Control_flow" title="go to top of document">proceed<a class="raku-anchor" title="direct link" href="#proceed_and_succeed">§</a></a></h2>  Both <code>proceed</code> and <code>succeed</code> are meant to be used only from inside <code>when</code> or <code>default</code> blocks.The <code>proceed</code> statement will immediately leave the <code>when</code> or <code>default</code> block, skipping the rest of the statements, and resuming after the block. This prevents the <code>when</code> or <code>default</code> from exiting the outer block. <div class="raku-code raku-lang"> <button class="copy-code" title="Copy code"></button> <label>Raku highlighting</label> <div> <pre class="nohighlights">given * { default { proceed; "This never says".say } } "This says".say; </pre> </div> </div> This is most often used to enter multiple <code>when</code> blocks. <code>proceed</code> will resume matching after a successful match, like so: <div class="raku-code raku-lang"> <button class="copy-code" title="Copy code"></button> <label>Raku highlighting</label> <div> <pre class="nohighlights">given 42 { when Int { say "Int"; proceed } when 42 { say 42 } when 40..* { say "greater than 40" } default { say "huh?" } } # OUTPUT: «Int␤» # OUTPUT: «42␤»</pre> </div> </div> Note that the <code>when 40..*</code> match didn't occur. For this to match such cases as well, one would need a <code>proceed</code> in the <code>when 42</code> block.This is not like a <code>C</code> <code>switch</code> statement, because the <code>proceed</code> does not merely enter the directly following block, it attempts to match the <code>given</code> value once more, consider this code: <div class="raku-code raku-lang"> <button class="copy-code" title="Copy code"></button> <label>Raku highlighting</label> <div> <pre class="nohighlights">given 42 { when Int { "Int".say; proceed } when 43 { 43.say } when 42 { 42.say } default { "got change for an existential answer?".say } } # OUTPUT: «Int␤» # OUTPUT: «42␤»</pre> </div> </div> ...which matches the <a href="/type/Int"><code>Int</code></a>, skips <code>43</code> since the value doesn't match, matches <code>42</code> since this is the next positive match, but doesn't enter the <code>default</code> block since the <code>when 42</code> block doesn't contain a <code>proceed</code>.By contrast, the <code>succeed</code> keyword short-circuits execution and exits the entire <code>given</code> block at that point. It may also take an argument to specify a final value for the block. <div class="raku-code raku-lang"> <button class="copy-code" title="Copy code"></button> <label>Raku highlighting</label> <div> <pre class="nohighlights">say do given 42 { when Int { succeed "Found"; say "never this!"; } when 42 { say 42 } default { say "dunno?" } } # OUTPUT: «Found␤»</pre> </div> </div> If you are not inside a when or default block, it is an error to try to use <code>proceed</code> or <code>succeed</code>.Also remember, the <code>when</code> statement modifier form does not cause any blocks to be left, and any <code>succeed</code> or <code>proceed</code> in such a statement applies to the surrounding clause, if there is one: <div class="raku-code raku-lang"> <button class="copy-code" title="Copy code"></button> <label>Raku highlighting</label> <div> <pre class="nohighlights">given 42 { { say "This says" } when Int; "This says too".say; when * > 41 { { "And this says".say; proceed } when * > 41; "This never says".say; } "This also says".say; } # OUTPUT: «This says␤This says too␤And this says␤This also says␤»</pre> </div> </div> <h2 id="given_as_a_statement" class="raku-h2"><a name="index-entry-given_statement-given_as_a_statement" data-indexedheader="Control flow;given statement"></a><a href="#Control_flow" title="go to top of document">given as a statement<a class="raku-anchor" title="direct link" href="#given_as_a_statement">§</a></a></h2>  <code>given</code> can follow a statement to set the topic in the statement it follows. <div class="raku-code raku-lang"> <button class="copy-code" title="Copy code"></button> <label>Raku highlighting</label> <div> <pre class="nohighlights">.say given "foo"; # OUTPUT: «foo␤» printf "%s %02i.%02i.%i", <Mo Tu We Th Fr Sa Su>[.day-of-week - 1], .day, .month, .year given DateTime.now; # OUTPUT: «Sa 03.06.2016»</pre> </div> </div> <h1 id="loop" class="raku-h1"><a name="index-entry-loop-loop" data-indexedheader="Control flow;loop"></a><a href="#Control_flow" title="go to top of document">loop<a class="raku-anchor" title="direct link" href="#loop">§</a></a></h1>  The <code>loop</code> statement takes three statements in parentheses separated by <code>;</code> that take the roles of initializer, conditional and incrementer, respectively. The initializer is executed once before the conditional is first tested. In case the initializer involves a variable declaration, the variable is declared as a lexical variable in the loop's outer or containing scope so that it can be used in code following the loop statement. The conditional is executed before each iteration and coerced to <a href="/type/Bool"><code>Bool</code></a>; if <code>False</code> the loop is stopped. The incrementer is executed after each iteration, and before the conditional is tested again. <div class="raku-code raku-lang"> <button class="copy-code" title="Copy code"></button> <label>Raku highlighting</label> <div> <pre class="nohighlights">loop (my $i = 0; $i < 10; $i++) { # A typical loop say $i; } my @str = "However Long".comb; # Our very own .char routine: loop (my $l = 0;;) { # Declare $l in outer scope last if !@str[$l++] # and count chars until we hit } # an undefined element (Any) say "The string is {--$l} chars long.";</pre> </div> </div> The infinite loop does not require parentheses. <div class="raku-code raku-lang"> <button class="copy-code" title="Copy code"></button> <label>Raku highlighting</label> <div> <pre class="nohighlights">loop { say 'forever' } </pre> </div> </div> The <code>loop</code> statement may be used to produce values from the result of each run of the attached block if it appears in lists: <div class="raku-code raku-lang"> <button class="copy-code" title="Copy code"></button> <label>Raku highlighting</label> <div> <pre class="nohighlights">(loop ( my $i = 0; $i++ < 3;) { $i * 2 }).say; # OUTPUT: «(2 4 6)␤» my @a = (loop ( my $j = 0; $j++ < 3;) { $j * 2 }); @a.say; # OUTPUT: «[2 4 6]␤» my @b = do loop ( my $k = 0; $k++ < 3;) { $k * 2 }; @b.say; # same thing</pre> </div> </div> Unlike a <code>for</code> loop, one should not rely on whether returned values are produced lazily. It would probably be best to use <code>eager</code> to guarantee that a loop whose return value may be used actually runs: <div class="raku-code raku-lang"> <button class="copy-code" title="Copy code"></button> <label>Raku highlighting</label> <div> <pre class="nohighlights">sub heads-in-a-row { (eager loop (; 2.rand < 1;) { "heads".say }) }</pre> </div> </div> <h1 id="while,_until" class="raku-h1"><a name="index-entry-while_until-while,_until" data-indexedheader="Control flow;while until"></a><a href="#Control_flow" title="go to top of document">while, until<a class="raku-anchor" title="direct link" href="#while,_until">§</a></a></h1>  The <code>while</code> statement executes the block as long as its condition is true. So <div class="raku-code raku-lang"> <button class="copy-code" title="Copy code"></button> <label>Raku highlighting</label> <div> <pre class="nohighlights">my $x = 1; while $x < 4 { print $x++; } print "\n"; # OUTPUT: «123␤»</pre> </div> </div> Similarly, the <code>until</code> statement executes the block as long as the expression is false. <div class="raku-code raku-lang"> <button class="copy-code" title="Copy code"></button> <label>Raku highlighting</label> <div> <pre class="nohighlights">my $x = 1; until $x > 3 { print $x++; } print "\n"; # OUTPUT: «123␤»</pre> </div> </div> The condition for <code>while</code> or <code>until</code> can be parenthesized, but there must be a space between the keyword and the opening parenthesis of the condition.Both <code>while</code> and <code>until</code> can be used as statement modifiers. E. g. <div class="raku-code raku-lang"> <button class="copy-code" title="Copy code"></button> <label>Raku highlighting</label> <div> <pre class="nohighlights">my $x = 42; $x-- while $x > 12</pre> </div> </div> Also see <code>repeat/while</code> and <code>repeat/until</code> below.All these forms may produce a return value the same way <code>loop</code> does. <h1 id="repeat/while,_repeat/until" class="raku-h1"><a name="index-entry-repeat-repeat/while,_repeat/until" data-indexedheader="Control flow;repeat"></a><a href="#Control_flow" title="go to top of document">repeat/while, repeat/until<a class="raku-anchor" title="direct link" href="#repeat/while,_repeat/until">§</a></a></h1>  Executes the block at least once and, if the condition allows, repeats that execution. This differs from <code>while</code>/<code>until</code> in that the condition is evaluated at the end of the loop, even if it appears at the front. <div class="raku-code raku-lang"> <button class="copy-code" title="Copy code"></button> <label>Raku highlighting</label> <div> <pre class="nohighlights">my $x = -42; repeat { $x++; } while $x < 5; $x.say; # OUTPUT: «5␤» repeat { $x++; } while $x < 5; $x.say; # OUTPUT: «6␤» repeat while $x < 10 { $x++; } $x.say; # OUTPUT: «10␤» repeat while $x < 10 { $x++; } $x.say; # OUTPUT: «11␤» repeat { $x++; } until $x >= 15; $x.say; # OUTPUT: «15␤» repeat { $x++; } until $x >= 15; $x.say; # OUTPUT: «16␤» repeat until $x >= 20 { $x++; } $x.say; # OUTPUT: «20␤» repeat until $x >= 20 { $x++; } $x.say; # OUTPUT: «21␤»</pre> </div> </div> All these forms may produce a return value the same way <code>loop</code> does. <h1 id="return" class="raku-h1"><a name="index-entry-return-return" data-indexedheader="Control flow;return"></a><a href="#Control_flow" title="go to top of document">return<a class="raku-anchor" title="direct link" href="#return">§</a></a></h1>  The sub <code>return</code> will stop execution of a subroutine or method, run all relevant <a href="/language/phasers#Block_phasers">phasers</a> and provide the given return value to the caller. The default return value is <a href="/type/Nil"><code>Nil</code></a>. If a return <a href="/language/signatures#Constraining_return_types">type constraint</a> is provided it will be checked unless the return value is <a href="/type/Nil"><code>Nil</code></a>. If the type check fails the exception <a href="/type/X/TypeCheck/Return"><code>X::TypeCheck::Return</code></a> is thrown. If it passes a control exception is raised and can be caught with <a href="/language/phasers#CONTROL">CONTROL</a>.Any <code>return</code> in a block is tied to the first <a href="/type/Routine"><code>Routine</code></a> in the outer lexical scope of that block, no matter how deeply nested. Please note that a <code>return</code> in the root of a package will fail at runtime. A <code>return</code> in a block that is evaluated lazily (e.g. inside <code>map</code>) may find the outer lexical routine gone by the time the block is executed. In almost any case <code>last</code> is the better alternative. Please check <a href="/language/functions#Return_values">the functions documentation</a> for more information on how return values are handled and produced. <h1 id="return-rw" class="raku-h1"><a name="index-entry-return-rw-return-rw" data-indexedheader="Control flow;return-rw"></a><a href="#Control_flow" title="go to top of document">return-rw<a class="raku-anchor" title="direct link" href="#return-rw">§</a></a></h1>  The sub <code>return</code> will return values, not containers. Those are immutable and will lead to runtime errors when attempted to be mutated. <div class="raku-code raku-lang"> <button class="copy-code" title="Copy code"></button> <label>Raku highlighting</label> <div> <pre class="nohighlights">sub s(){ my $a = 41; return $a }; say ++s(); CATCH { default { say .^name, ': ', .Str } }; # OUTPUT: «X::Multi::NoMatch.new(dispatcher …</pre> </div> </div> To return a mutable container, use <code>return-rw</code>. <div class="raku-code raku-lang"> <button class="copy-code" title="Copy code"></button> <label>Raku highlighting</label> <div> <pre class="nohighlights">sub s(){ my $a = 41; return-rw $a }; say ++s(); # OUTPUT: «42␤»</pre> </div> </div> The same rules as for <code>return</code> regarding phasers and control exceptions apply. <h1 id="fail" class="raku-h1"><a name="index-entry-fail-fail" data-indexedheader="Control flow;fail"></a><a href="#Control_flow" title="go to top of document">fail<a class="raku-anchor" title="direct link" href="#fail">§</a></a></h1>  Leaves the current routine and returns the provided <a href="/type/Exception"><code>Exception</code></a> or <a href="/type/Str"><code>Str</code></a> wrapped inside a <a href="/type/Failure"><code>Failure</code></a>, after all relevant <a href="/language/phasers#Block_phasers">phasers</a> are executed. If the caller activated fatal exceptions via the pragma <code>use fatal;</code>, the exception is thrown instead of being returned as a <a href="/type/Failure"><code>Failure</code></a>. <div class="raku-code raku-lang"> <button class="copy-code" title="Copy code"></button> <label>Raku highlighting</label> <div> <pre class="nohighlights">sub f { fail "WELP!" }; say f; CATCH { default { say .^name, ': ', .Str } } # OUTPUT: «X::AdHoc: WELP!␤»</pre> </div> </div> <h1 id="once" class="raku-h1"><a name="index-entry-once-once" data-indexedheader="Control flow;once"></a><a href="#Control_flow" title="go to top of document">once<a class="raku-anchor" title="direct link" href="#once">§</a></a></h1>  A block or statement prefixed with <code>once</code> will be executed exactly once, even if placed inside a loop or a recursive routine. <div class="raku-code raku-lang"> <button class="copy-code" title="Copy code"></button> <label>Raku highlighting</label> <div> <pre class="nohighlights">my $guard; loop { once $guard = 3; last if $guard-- <= 0; once { put 'once' }; print 'many' } # OUTPUT: «once␤manymanymany»</pre> </div> </div> This works per "clone" of the containing code object, so: <div class="raku-code raku-lang"> <button class="copy-code" title="Copy code"></button> <label>Raku highlighting</label> <div> <pre class="nohighlights">({ once 42.say } xx 3).map: {$_(), $_()}; # says 42 thrice</pre> </div> </div> Note that this is not a thread-safe construct when the same clone of the same block is run by multiple threads. Also remember that methods only have one clone per class, not per object. <h1 id="LABELs" class="raku-h1"><a href="#Control_flow" title="go to top of document">LABELs<a class="raku-anchor" title="direct link" href="#LABELs">§</a></a></h1> <code>while</code>, <code>until</code>, <code>loop</code> and <code>for</code> loops can all take a label, which can be used to identify them for <code>next</code>, <code>last</code>, and <code>redo</code>. Nested loops are supported, for instance: <div class="raku-code raku-lang"> <button class="copy-code" title="Copy code"></button> <label>Raku highlighting</label> <div> <pre class="nohighlights">OUTAHERE: while True { for 1,2,3 -> $n { last OUTAHERE if $n == 2; } }</pre> </div> </div> Labels can be used also within nested loops to name each loop, for instance: <div class="raku-code raku-lang"> <button class="copy-code" title="Copy code"></button> <label>Raku highlighting</label> <div> <pre class="nohighlights">OUTAHERE: loop ( my $i = 1; True; $i++ ) { OUTFOR: for 1,2,3 -> $n { # exits the for loop before its natural end last OUTFOR if $n == 2; } # exits the infinite loop last OUTAHERE if $i >= 2; } </pre> </div> </div> <h1 id="next" class="raku-h1"><a name="index-entry-next-next" data-indexedheader="Control flow;next"></a><a href="#Control_flow" title="go to top of document">next<a class="raku-anchor" title="direct link" href="#next">§</a></a></h1>  The <code>next</code> command starts the next iteration of the loop. So the code <div class="raku-code raku-lang"> <button class="copy-code" title="Copy code"></button> <label>Raku highlighting</label> <div> <pre class="nohighlights">my @x = 1, 2, 3, 4, 5; for @x -> $x { next if $x == 3; print $x; } </pre> </div> </div> prints "1245".You can also use <code>next</code> in a <code>map</code>: the above example then looks like: <div class="raku-code raku-lang"> <button class="copy-code" title="Copy code"></button> <label>Raku highlighting</label> <div> <pre class="nohighlights">my @x = 1, 2, 3, 4, 5; print @x.map: -> $x { next if $x == 3; $x } </pre> </div> </div> prints "1 2 4 5" because a space is added between entries of a <a href="/type/Seq"><code>Seq</code></a> when it is stringified. Note that that <code>print</code> was not put inside the block of the <code>map</code>, as it generally considered bad practice to run a <code>map</code> for its side-effects (in this case, the <code>print</code>.If the <a href="/language/phasers#NEXT"><code>NEXT</code> phaser</a> is present, it runs before the next iteration: <div class="raku-code raku-lang"> <button class="copy-code" title="Copy code"></button> <label>Raku highlighting</label> <div> <pre class="nohighlights">my Int $i = 0; while ($i < 10) { if ($i % 2 == 0) { next; } say "$i is odd."; NEXT { $i++; } } # OUTPUT: «1 is odd.␤3 is odd.␤5 is odd.␤7 is odd.␤9 is odd.␤» </pre> </div> </div> In version 6.e.PREVIEW (available as of the 2021.07 Rakudo compiler release), it is also possible to return a value with the <code>next</code> statement. This is particularly useful when using it in a <code>map</code>: <div class="raku-code raku-lang"> <button class="copy-code" title="Copy code"></button> <label>Raku highlighting</label> <div> <pre class="nohighlights">my @x = 1, 2, 3, 4, 5; print @x.map: -> $x { next 42 if $x == 3; $x } </pre> </div> </div> prints "1 2 42 4 5".In a <a href="/language/concurrency#whenever">whenever</a> block, <code>next</code> immediately exits the block for the current value: <div class="raku-code raku-lang"> <button class="copy-code" title="Copy code"></button> <label>Raku highlighting</label> <div> <pre class="nohighlights">react { whenever Supply.interval(1) { next if .is-prime; say $_; done if $_ == 4; } }</pre> </div> </div> prints "0", "1" and "4" - integers from 0 to 4 with primes skipped.*Since version 6.d, the <code>next</code> command in a loop that collects its last statement values returns <code>Empty</code> for the iterations they run on.* <h1 id="last" class="raku-h1"><a name="index-entry-last-last" data-indexedheader="Control flow;last"></a><a href="#Control_flow" title="go to top of document">last<a class="raku-anchor" title="direct link" href="#last">§</a></a></h1>  The <code>last</code> command immediately exits the loop in question. <div class="raku-code raku-lang"> <button class="copy-code" title="Copy code"></button> <label>Raku highlighting</label> <div> <pre class="nohighlights">my @x = 1, 2, 3, 4, 5; for @x -> $x { last if $x == 3; print $x; } </pre> </div> </div> prints "12".You can also use <code>last</code> in a <code>map</code>: the above example then looks like: <div class="raku-code raku-lang"> <button class="copy-code" title="Copy code"></button> <label>Raku highlighting</label> <div> <pre class="nohighlights">my @x = 1, 2, 3, 4, 5; print @x.map: -> $x { last if $x == 3; $x } </pre> </div> </div> prints "1 2" because a space is added between entries of a <a href="/type/Seq"><code>Seq</code></a> when it is stringified. Note that that <code>print</code> was not put inside the block of the <code>map</code>, as it generally considered bad practice to run a <code>map</code> for its side-effects (in this case, the <code>print</code>.If the <a href="/language/phasers#LAST"><code>LAST</code> phaser</a> is present, it runs before exiting the loop: <div class="raku-code raku-lang"> <button class="copy-code" title="Copy code"></button> <label>Raku highlighting</label> <div> <pre class="nohighlights">my Int $i = 1; while ($i < 10) { if ($i % 5 == 0) { last; } LAST { say "The last number was $i."; } NEXT { $i++; } } # OUTPUT: «The last number was 5.␤» </pre> </div> </div> Since version 6.d, the <code>last</code> command in a loop that collects its last statement values returns <code>Empty</code> for the iterations they run on.In version 6.e.PREVIEW (available as of the 2021.07 Rakudo compiler release), it is also possible to return a value with the <code>last</code> statement. This is particularly useful when using it in a <code>map</code>: <div class="raku-code raku-lang"> <button class="copy-code" title="Copy code"></button> <label>Raku highlighting</label> <div> <pre class="nohighlights">my @x = 1, 2, 3, 4, 5; print @x.map: -> $x { last 42 if $x == 3; $x } </pre> </div> </div> print "1 2 42". <h1 id="redo" class="raku-h1"><a name="index-entry-redo-redo" data-indexedheader="Control flow;redo"></a><a href="#Control_flow" title="go to top of document">redo<a class="raku-anchor" title="direct link" href="#redo">§</a></a></h1>  The <code>redo</code> command restarts the loop block without evaluating the conditional again. <div class="raku-code raku-lang"> <button class="copy-code" title="Copy code"></button> <label>Raku highlighting</label> <div> <pre class="nohighlights">for 1..5 -> $current-level { state $total-attempts = 0; $total-attempts++; print("Entering #$current-level. "); if $total-attempts %% 3 { redo; } } # OUTPUT: «Entering #1... Entering #2... Entering #3... Entering #3... Entering #4... Entering #5... Entering #5... » </pre> </div> </div> </div></div></section> <section class="page-footnotes"> <div class="container"> <div id="_Footnotes" class="footnotes"> <div class="footnote" id="fn1">1<a class="footnote-linkback" href="#fnret1"> [↑] </a>There are other ways to modify their default behavior; they are discussed in other sections.</div> </div> </div> </section> </div> </div> </div> <footer class="footer main-footer"> <div class="container px-4"> <nav class="level"> <div class="level-left"> <div class="level-item"> <a href="/about">About</a> </div> <div class="level-item"> <a id="toggle-theme">Toggle theme</a> </div> <div class="level-item" title="8e397fdce 2024-12-26"> <a>Commit</a> </div> </div> <div class="level-right"> <div class="level-item"> <a href="/license">License</a> </div> </div> </nav> </div> </footer> </body> </html>

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