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background: white; border: 1px #aaa solid; border-collapse: collapse;"> <caption style="height:3em"><big><big>Maybe class (<a href="/index.php?title=Base_package" title="Base package">base</a>)</big></big> </caption> <tbody><tr> <td><tt>import <a rel="nofollow" class="external text" href="http://www.haskell.org/ghc/docs/latest/html/libraries/base/Data-Maybe.html#t%3AMaybe">Data.Maybe</a></tt> </td></tr></tbody></table> <p>The <b>Maybe</b> type is defined as follows: </p> <hask><div class="mw-highlight mw-highlight-lang-haskell mw-content-ltr" dir="ltr"><pre>data Maybe a = Nothing | Just a deriving (Eq, Ord)</pre></div> <p>It allows the programmer to specify something may not be there. </p> <meta property="mw:PageProp/toc" /> <h2><span class="mw-headline" id="Type_equation">Type equation</span></h2> <p><b>Maybe</b> satisfies the <a href="/index.php?title=Type" title="Type">type</a> equation <math class="mwe-math-element" xmlns="http://www.w3.org/1998/Math/MathML"><mstyle displaystyle="true" scriptlevel="0"><mi>F</mi><mi>X</mi><mo>=</mo><mn>1</mn><mo>+</mo><mi>X</mi></mstyle></math>, where the functor <math class="mwe-math-element" xmlns="http://www.w3.org/1998/Math/MathML"><mstyle displaystyle="true" scriptlevel="0"><mi>F</mi></mstyle></math> takes a set to a point plus that set. </p> <h2><span class="mw-headline" id="Comparison_to_imperative_languages">Comparison to imperative languages</span></h2> <p>Imperative languages may support this by rewriting as a <i>union</i> or allow one to use / return <i>NULL</i> (defined in some manner) to specify a value might not be there. </p> <h2><span class="mw-headline" id="Classes">Classes</span></h2> <p>As one can see from the <a href="/index.php?title=Type" title="Type">type</a> definition, <b>Maybe</b> will be an instance of <a href="/index.php?title=Eq&action=edit&redlink=1" class="new" title="Eq (page does not exist)">Eq</a> and <a href="/index.php?title=Ord&action=edit&redlink=1" class="new" title="Ord (page does not exist)">Ord</a> when the base type is. As well, instances of <a href="/index.php?title=Functor" title="Functor">Functor</a> and <a href="/index.php?title=Monad" title="Monad">Monad</a> are defined for <b>Maybe</b>. </p><p>For Functor, the <code>fmap</code> function moves inside the <b>Just</b> <a href="/index.php?title=Constructor" title="Constructor">constructor</a> and is identity on the <b>Nothing</b> <a href="/index.php?title=Constructor" title="Constructor">constructor</a>. </p><p>For Monad, the bind operation passes through <b>Just</b>, while <b>Nothing</b> will force the result to always be <b>Nothing</b>. </p> <h3><span class="mw-headline" id="Maybe_as_a_Monad">Maybe as a Monad</span></h3> <p>Using the <a href="/index.php?title=Monad" title="Monad">Monad</a> class definition can lead to much more compact code. For example: </p> <hask><div class="mw-highlight mw-highlight-lang-haskell mw-content-ltr" dir="ltr"><pre> f :: Int -> Maybe Int f 0 = Nothing f x = Just x g :: Int -> Maybe Int g 100 = Nothing g x = Just x h :: Int -> Maybe Int h x = case f x of Just n -> g n Nothing -> Nothing h' :: Int -> Maybe Int h' x = do n <- f x g n</pre></div> <p>The functions <code>h</code> and <code>h'</code> will give the same results. (<math class="mwe-math-element" xmlns="http://www.w3.org/1998/Math/MathML"><mstyle displaystyle="true" scriptlevel="0"><mi>h</mi><mn>0</mn><mo>=</mo><msup><mi>h</mi><mo>′</mo></msup><mn>0</mn><mo>=</mo><mi>h</mi><mn>1</mn><mn>0</mn><mn>0</mn><mo>=</mo><msup><mi>h</mi><mo>′</mo></msup><mn>1</mn><mn>0</mn><mn>0</mn><mo>=</mo><mi>N</mi><mi>o</mi><mi>t</mi><mi>h</mi><mi>i</mi><mi>n</mi><mi>g</mi><mo>;</mo><mspace width="0.5em"></mspace><mi>h</mi><mi>x</mi><mo>=</mo><msup><mi>h</mi><mo>′</mo></msup><mi>x</mi><mo>=</mo><mi>J</mi><mi>u</mi><mi>s</mi><mi>t</mi><mstyle scriptlevel="0"><mspace width="0.167em"></mspace></mstyle><mi>x</mi></mstyle></math>). In this case the savings in code size is quite modest, stringing together multiple functions like <code>f</code> and <code>g</code> will be more noticeable. </p> <h2><span class="mw-headline" id="Library_functions">Library functions</span></h2> <p>When the module is imported, it supplies a variety of useful functions including: </p> <dl><dt><hask><code class="mw-highlight mw-highlight-lang-haskell mw-content-ltr" dir="ltr">maybe :: b->(a->b) -> Maybe a -> b</code></hask></dt> <dd>Applies the second argument to the third, when it is <i>Just x</i>, otherwise returns the first argument.</dd> <dt><hask><code class="mw-highlight mw-highlight-lang-haskell mw-content-ltr" dir="ltr">isJust</code>, <hask><code class="mw-highlight mw-highlight-lang-haskell mw-content-ltr" dir="ltr">isNothing</code></hask></hask></dt> <dd>Test the argument, returing a Bool based on the constructor.</dd> <dt><hask><code class="mw-highlight mw-highlight-lang-haskell mw-content-ltr" dir="ltr">listToMaybe</code>, <hask><code class="mw-highlight mw-highlight-lang-haskell mw-content-ltr" dir="ltr">maybeToList</code></hask></hask></dt> <dd>Convert to/from a one element or empty list.</dd> <dt><hask><code class="mw-highlight mw-highlight-lang-haskell mw-content-ltr" dir="ltr">mapMaybe</code></hask></dt> <dd>A different way to filter a list.</dd></dl> <p>See the documentation for <a rel="nofollow" class="external text" href="http://www.haskell.org/ghc/docs/latest/html/libraries/base/Data-Maybe.html">Data.Maybe</a> for more explanation and other functions. </p></hask></hask>    </div> <div class="printfooter" data-nosnippet="">Retrieved from "<a dir="ltr" href="https://wiki.haskell.org/index.php?title=Maybe&oldid=66413">https://wiki.haskell.org/index.php?title=Maybe&oldid=66413</a>"</div></div> <div id="catlinks" class="catlinks" data-mw="interface"><div id="mw-normal-catlinks" class="mw-normal-catlinks"><a href="/index.php?title=Special:Categories" title="Special:Categories">Categories</a>: <ul><li><a href="/index.php?title=Category:Pages_with_syntax_highlighting_errors&action=edit&redlink=1" class="new" title="Category:Pages with syntax highlighting errors (page does not exist)">Pages with syntax highlighting errors</a></li><li><a href="/index.php?title=Category:Standard_types" title="Category:Standard types">Standard types</a></li><li><a href="/index.php?title=Category:Monad" title="Category:Monad">Monad</a></li></ul></div></div> </div> </main> </div> <div class="mw-footer-container"> <footer id="footer" class="mw-footer" role="contentinfo" > <ul id="footer-info"> <li id="footer-info-lastmod"> This page was last edited on 11 November 2023, at 00:28.</li> <li id="footer-info-copyright">Recent content is available under <a href="/index.php?title=HaskellWiki:Copyrights" title="HaskellWiki:Copyrights">simple permissive license</a>.</li> </ul> <ul id="footer-places"> <li id="footer-places-privacy"><a href="/index.php?title=HaskellWiki:Privacy_policy">Privacy policy</a></li> <li id="footer-places-about"><a href="/index.php?title=HaskellWiki:About">About HaskellWiki</a></li> <li id="footer-places-disclaimers"><a href="/index.php?title=HaskellWiki:General_disclaimer">Disclaimers</a></li> </ul> <ul id="footer-icons" class="noprint"> <li id="footer-copyrightico"><a href="https://wiki.haskell.org/HaskellWiki:Copyrights"><img src="" alt="simple permissive license" width="88" height="31" loading="lazy"></a></li> </ul> </footer> </div> </div> </div> <div class="vector-settings" id="p-dock-bottom"> <ul> <li> <button class="cdx-button cdx-button--icon-only vector-limited-width-toggle" id=""><span class="vector-icon mw-ui-icon-fullScreen mw-ui-icon-wikimedia-fullScreen"></span> <span>Toggle limited content width</span> </button> </li> </ul> </div> <script>(RLQ=window.RLQ||[]).push(function(){mw.config.set({"wgBackendResponseTime":32,"wgPageParseReport":{"limitreport":{"cputime":"0.020","walltime":"0.038","ppvisitednodes":{"value":116,"limit":1000000},"postexpandincludesize":{"value":393,"limit":2097152},"templateargumentsize":{"value":42,"limit":2097152},"expansiondepth":{"value":3,"limit":100},"expensivefunctioncount":{"value":8,"limit":100},"unstrip-depth":{"value":1,"limit":20},"unstrip-size":{"value":2610,"limit":5000000},"timingprofile":["100.00% 2.001 1 Template:Standard_type","100.00% 2.001 1 -total"]},"cachereport":{"timestamp":"20250403114351","ttl":86400,"transientcontent":false}}});});</script> </body> </html>