<!DOCTYPE HTML PUBLIC "-//W3C//DTD HTML 4.01 Transitional//EN" "http://www.w3.org/TR/html4/loose.dtd"> <!--Rendered using the Haskell Html Library v0.2--> <HTML ><HEAD ><META HTTP-EQUIV="Content-Type" CONTENT="text/html; charset=UTF-8" ><TITLE >Data.Maybe</TITLE ><LINK HREF="haddock.css" REL="stylesheet" TYPE="text/css" ><SCRIPT SRC="haddock-util.js" TYPE="text/javascript" ></SCRIPT ><SCRIPT TYPE="text/javascript" >window.onload = function () {setSynopsis("mini_Data-Maybe.html")};</SCRIPT ></HEAD ><BODY ><TABLE CLASS="vanilla" CELLSPACING="0" CELLPADDING="0" ><TR ><TD CLASS="topbar" ><TABLE CLASS="vanilla" CELLSPACING="0" CELLPADDING="0" ><TR ><TD ><IMG SRC="haskell_icon.gif" WIDTH="16" HEIGHT="16" ALT=" " ></TD ><TD CLASS="title" >base-4.1.0.0: Basic libraries</TD ><TD CLASS="topbut" ><A HREF="index.html" >Contents</A ></TD ><TD CLASS="topbut" ><A HREF="doc-index.html" >Index</A ></TD ></TR ></TABLE ></TD ></TR ><TR ><TD CLASS="modulebar" ><TABLE CLASS="vanilla" CELLSPACING="0" CELLPADDING="0" ><TR ><TD ><FONT SIZE="6" >Data.Maybe</FONT ></TD ><TD ALIGN="right" ><TABLE CLASS="narrow" CELLSPACING="0" CELLPADDING="0" ><TR ><TD CLASS="infohead" >Portability</TD ><TD CLASS="infoval" >portable</TD ></TR ><TR ><TD CLASS="infohead" >Stability</TD ><TD CLASS="infoval" >stable</TD ></TR ><TR ><TD CLASS="infohead" >Maintainer</TD ><TD CLASS="infoval" >libraries@haskell.org</TD ></TR ></TABLE ></TD ></TR ></TABLE ></TD ></TR ><TR ><TD CLASS="s15" ></TD ></TR ><TR ><TD CLASS="section1" >Description</TD ></TR ><TR ><TD CLASS="doc" >The Maybe type, and associated operations. </TD ></TR ><TR ><TD CLASS="s15" ></TD ></TR ><TR ><TD CLASS="section1" >Synopsis</TD ></TR ><TR ><TD CLASS="s15" ></TD ></TR ><TR ><TD CLASS="body" ><TABLE CLASS="vanilla" CELLSPACING="0" CELLPADDING="0" ><TR ><TD CLASS="decl" ><TABLE CLASS="vanilla" CELLSPACING="0" CELLPADDING="0" ><TR ><TD CLASS="decl" ><SPAN CLASS="keyword" >data</SPAN > <A HREF="#t%3AMaybe" >Maybe</A > a </TD ></TR ><TR ><TD CLASS="body" ><TABLE CLASS="vanilla" CELLSPACING="0" CELLPADDING="0" ><TR ><TD CLASS="decl" >= <A HREF="#v%3ANothing" >Nothing</A ></TD ></TR ><TR ><TD CLASS="decl" >| <A HREF="#v%3AJust" >Just</A > a</TD ></TR ></TABLE ></TD ></TR ></TABLE ></TD ></TR ><TR ><TD CLASS="s8" ></TD ></TR ><TR ><TD CLASS="decl" ><A HREF="#v%3Amaybe" >maybe</A > :: b -> (a -> b) -> <A HREF="Data-Maybe.html#t%3AMaybe" >Maybe</A > a -> b</TD ></TR ><TR ><TD CLASS="s8" ></TD ></TR ><TR ><TD CLASS="decl" ><A HREF="#v%3AisJust" >isJust</A > :: <A HREF="Data-Maybe.html#t%3AMaybe" >Maybe</A > a -> <A HREF="../ghc-prim/GHC-Bool.html#t%3ABool" >Bool</A ></TD ></TR ><TR ><TD CLASS="s8" ></TD ></TR ><TR ><TD CLASS="decl" ><A HREF="#v%3AisNothing" >isNothing</A > :: <A HREF="Data-Maybe.html#t%3AMaybe" >Maybe</A > a -> <A HREF="../ghc-prim/GHC-Bool.html#t%3ABool" >Bool</A ></TD ></TR ><TR ><TD CLASS="s8" ></TD ></TR ><TR ><TD CLASS="decl" ><A HREF="#v%3AfromJust" >fromJust</A > :: <A HREF="Data-Maybe.html#t%3AMaybe" >Maybe</A > a -> a</TD ></TR ><TR ><TD CLASS="s8" ></TD ></TR ><TR ><TD CLASS="decl" ><A HREF="#v%3AfromMaybe" >fromMaybe</A > :: a -> <A HREF="Data-Maybe.html#t%3AMaybe" >Maybe</A > a -> a</TD ></TR ><TR ><TD CLASS="s8" ></TD ></TR ><TR ><TD CLASS="decl" ><A HREF="#v%3AlistToMaybe" >listToMaybe</A > :: [a] -> <A HREF="Data-Maybe.html#t%3AMaybe" >Maybe</A > a</TD ></TR ><TR ><TD CLASS="s8" ></TD ></TR ><TR ><TD CLASS="decl" ><A HREF="#v%3AmaybeToList" >maybeToList</A > :: <A HREF="Data-Maybe.html#t%3AMaybe" >Maybe</A > a -> [a]</TD ></TR ><TR ><TD CLASS="s8" ></TD ></TR ><TR ><TD CLASS="decl" ><A HREF="#v%3AcatMaybes" >catMaybes</A > :: [<A HREF="Data-Maybe.html#t%3AMaybe" >Maybe</A > a] -> [a]</TD ></TR ><TR ><TD CLASS="s8" ></TD ></TR ><TR ><TD CLASS="decl" ><A HREF="#v%3AmapMaybe" >mapMaybe</A > :: (a -> <A HREF="Data-Maybe.html#t%3AMaybe" >Maybe</A > b) -> [a] -> [b]</TD ></TR ></TABLE ></TD ></TR ><TR ><TD CLASS="s15" ></TD ></TR ><TR ><TD CLASS="section1" >Documentation</TD ></TR ><TR ><TD CLASS="s15" ></TD ></TR ><TR ><TD CLASS="decl" ><SPAN CLASS="keyword" >data</SPAN > <A NAME="t:Maybe" ><A NAME="t%3AMaybe" ></A ></A ><B >Maybe</B > a </TD ></TR ><TR ><TD CLASS="body" ><TABLE CLASS="vanilla" CELLSPACING="0" CELLPADDING="0" ><TR ><TD CLASS="ndoc" ><P >The <TT ><A HREF="Data-Maybe.html#t%3AMaybe" >Maybe</A ></TT > type encapsulates an optional value. A value of type <TT ><TT ><A HREF="Data-Maybe.html#t%3AMaybe" >Maybe</A ></TT > a</TT > either contains a value of type <TT >a</TT > (represented as <TT ><TT ><A HREF="Data-Maybe.html#v%3AJust" >Just</A ></TT > a</TT >), or it is empty (represented as <TT ><A HREF="Data-Maybe.html#v%3ANothing" >Nothing</A ></TT >). Using <TT ><A HREF="Data-Maybe.html#t%3AMaybe" >Maybe</A ></TT > is a good way to deal with errors or exceptional cases without resorting to drastic measures such as <TT ><A HREF="Prelude.html#v%3Aerror" >error</A ></TT >. </P ><P >The <TT ><A HREF="Data-Maybe.html#t%3AMaybe" >Maybe</A ></TT > type is also a monad. It is a simple kind of error monad, where all errors are represented by <TT ><A HREF="Data-Maybe.html#v%3ANothing" >Nothing</A ></TT >. A richer error monad can be built using the Data.Either.Either type. </P ></TD ></TR ><TR ><TD CLASS="section4" >Constructors</TD ></TR ><TR ><TD CLASS="body" ><TABLE CLASS="vanilla" CELLSPACING="1" CELLPADDING="0" ><TR ><TD CLASS="arg" ><A NAME="v:Nothing" ><A NAME="v%3ANothing" ></A ></A ><B >Nothing</B ></TD ><TD CLASS="rdoc" ></TD ></TR ><TR ><TD CLASS="arg" ><A NAME="v:Just" ><A NAME="v%3AJust" ></A ></A ><B >Just</B > a</TD ><TD CLASS="rdoc" ></TD ></TR ></TABLE ></TD ></TR ><TR ><TD CLASS="section4" ><IMG SRC="minus.gif" CLASS="coll" ONCLICK="toggle(this,'i:Maybe')" ALT="show/hide" > Instances</TD ></TR ><TR ><TD CLASS="body" ><DIV ID="i:Maybe" STYLE="display:block;" ><TABLE CLASS="vanilla" CELLSPACING="1" CELLPADDING="0" ><TR ><TD CLASS="decl" ><A HREF="Control-Monad.html#t%3AMonad" >Monad</A > <A HREF="Data-Maybe.html#t%3AMaybe" >Maybe</A ></TD ></TR ><TR ><TD CLASS="decl" ><A HREF="Control-Monad.html#t%3AFunctor" >Functor</A > <A HREF="Data-Maybe.html#t%3AMaybe" >Maybe</A ></TD ></TR ><TR ><TD CLASS="decl" ><A HREF="Data-Typeable.html#t%3ATypeable1" >Typeable1</A > <A HREF="Data-Maybe.html#t%3AMaybe" >Maybe</A ></TD ></TR ><TR ><TD CLASS="decl" ><A HREF="Control-Monad-Fix.html#t%3AMonadFix" >MonadFix</A > <A HREF="Data-Maybe.html#t%3AMaybe" >Maybe</A ></TD ></TR ><TR ><TD CLASS="decl" ><A HREF="Control-Monad.html#t%3AMonadPlus" >MonadPlus</A > <A HREF="Data-Maybe.html#t%3AMaybe" >Maybe</A ></TD ></TR ><TR ><TD CLASS="decl" ><A HREF="Control-Applicative.html#t%3AAlternative" >Alternative</A > <A HREF="Data-Maybe.html#t%3AMaybe" >Maybe</A ></TD ></TR ><TR ><TD CLASS="decl" ><A HREF="Control-Applicative.html#t%3AApplicative" >Applicative</A > <A HREF="Data-Maybe.html#t%3AMaybe" >Maybe</A ></TD ></TR ><TR ><TD CLASS="decl" ><A HREF="Data-Foldable.html#t%3AFoldable" >Foldable</A > <A HREF="Data-Maybe.html#t%3AMaybe" >Maybe</A ></TD ></TR ><TR ><TD CLASS="decl" ><A HREF="Data-Traversable.html#t%3ATraversable" >Traversable</A > <A HREF="Data-Maybe.html#t%3AMaybe" >Maybe</A ></TD ></TR ><TR ><TD CLASS="decl" ><A HREF="Data-Eq.html#t%3AEq" >Eq</A > a => <A HREF="Data-Eq.html#t%3AEq" >Eq</A > (<A HREF="Data-Maybe.html#t%3AMaybe" >Maybe</A > a)</TD ></TR ><TR ><TD CLASS="decl" ><A HREF="Data-Data.html#t%3AData" >Data</A > a => <A HREF="Data-Data.html#t%3AData" >Data</A > (<A HREF="Data-Maybe.html#t%3AMaybe" >Maybe</A > a)</TD ></TR ><TR ><TD CLASS="decl" ><A HREF="Data-Ord.html#t%3AOrd" >Ord</A > a => <A HREF="Data-Ord.html#t%3AOrd" >Ord</A > (<A HREF="Data-Maybe.html#t%3AMaybe" >Maybe</A > a)</TD ></TR ><TR ><TD CLASS="decl" ><A HREF="Text-Read.html#t%3ARead" >Read</A > a => <A HREF="Text-Read.html#t%3ARead" >Read</A > (<A HREF="Data-Maybe.html#t%3AMaybe" >Maybe</A > a)</TD ></TR ><TR ><TD CLASS="decl" ><A HREF="Text-Show.html#t%3AShow" >Show</A > a => <A HREF="Text-Show.html#t%3AShow" >Show</A > (<A HREF="Data-Maybe.html#t%3AMaybe" >Maybe</A > a)</TD ></TR ><TR ><TD CLASS="decl" ><A HREF="Data-Monoid.html#t%3AMonoid" >Monoid</A > a => <A HREF="Data-Monoid.html#t%3AMonoid" >Monoid</A > (<A HREF="Data-Maybe.html#t%3AMaybe" >Maybe</A > a)</TD ></TR ></TABLE ></DIV ></TD ></TR ></TABLE ></TD ></TR ><TR ><TD CLASS="s15" ></TD ></TR ><TR ><TD CLASS="decl" ><A NAME="v:maybe" ><A NAME="v%3Amaybe" ></A ></A ><B >maybe</B > :: b -> (a -> b) -> <A HREF="Data-Maybe.html#t%3AMaybe" >Maybe</A > a -> b</TD ></TR ><TR ><TD CLASS="doc" >The <TT ><A HREF="Data-Maybe.html#v%3Amaybe" >maybe</A ></TT > function takes a default value, a function, and a <TT ><A HREF="Data-Maybe.html#t%3AMaybe" >Maybe</A ></TT > value. If the <TT ><A HREF="Data-Maybe.html#t%3AMaybe" >Maybe</A ></TT > value is <TT ><A HREF="Data-Maybe.html#v%3ANothing" >Nothing</A ></TT >, the function returns the default value. Otherwise, it applies the function to the value inside the <TT ><A HREF="Data-Maybe.html#v%3AJust" >Just</A ></TT > and returns the result. </TD ></TR ><TR ><TD CLASS="s15" ></TD ></TR ><TR ><TD CLASS="decl" ><A NAME="v:isJust" ><A NAME="v%3AisJust" ></A ></A ><B >isJust</B > :: <A HREF="Data-Maybe.html#t%3AMaybe" >Maybe</A > a -> <A HREF="../ghc-prim/GHC-Bool.html#t%3ABool" >Bool</A ></TD ></TR ><TR ><TD CLASS="doc" >The <TT ><A HREF="Data-Maybe.html#v%3AisJust" >isJust</A ></TT > function returns <TT ><A HREF="../ghc-prim/GHC-Bool.html#v%3ATrue" >True</A ></TT > iff its argument is of the form <TT >Just _</TT >. </TD ></TR ><TR ><TD CLASS="s15" ></TD ></TR ><TR ><TD CLASS="decl" ><A NAME="v:isNothing" ><A NAME="v%3AisNothing" ></A ></A ><B >isNothing</B > :: <A HREF="Data-Maybe.html#t%3AMaybe" >Maybe</A > a -> <A HREF="../ghc-prim/GHC-Bool.html#t%3ABool" >Bool</A ></TD ></TR ><TR ><TD CLASS="doc" >The <TT ><A HREF="Data-Maybe.html#v%3AisNothing" >isNothing</A ></TT > function returns <TT ><A HREF="../ghc-prim/GHC-Bool.html#v%3ATrue" >True</A ></TT > iff its argument is <TT ><A HREF="Data-Maybe.html#v%3ANothing" >Nothing</A ></TT >. </TD ></TR ><TR ><TD CLASS="s15" ></TD ></TR ><TR ><TD CLASS="decl" ><A NAME="v:fromJust" ><A NAME="v%3AfromJust" ></A ></A ><B >fromJust</B > :: <A HREF="Data-Maybe.html#t%3AMaybe" >Maybe</A > a -> a</TD ></TR ><TR ><TD CLASS="doc" >The <TT ><A HREF="Data-Maybe.html#v%3AfromJust" >fromJust</A ></TT > function extracts the element out of a <TT ><A HREF="Data-Maybe.html#v%3AJust" >Just</A ></TT > and throws an error if its argument is <TT ><A HREF="Data-Maybe.html#v%3ANothing" >Nothing</A ></TT >. </TD ></TR ><TR ><TD CLASS="s15" ></TD ></TR ><TR ><TD CLASS="decl" ><A NAME="v:fromMaybe" ><A NAME="v%3AfromMaybe" ></A ></A ><B >fromMaybe</B > :: a -> <A HREF="Data-Maybe.html#t%3AMaybe" >Maybe</A > a -> a</TD ></TR ><TR ><TD CLASS="doc" >The <TT ><A HREF="Data-Maybe.html#v%3AfromMaybe" >fromMaybe</A ></TT > function takes a default value and and <TT ><A HREF="Data-Maybe.html#t%3AMaybe" >Maybe</A ></TT > value. If the <TT ><A HREF="Data-Maybe.html#t%3AMaybe" >Maybe</A ></TT > is <TT ><A HREF="Data-Maybe.html#v%3ANothing" >Nothing</A ></TT >, it returns the default values; otherwise, it returns the value contained in the <TT ><A HREF="Data-Maybe.html#t%3AMaybe" >Maybe</A ></TT >. </TD ></TR ><TR ><TD CLASS="s15" ></TD ></TR ><TR ><TD CLASS="decl" ><A NAME="v:listToMaybe" ><A NAME="v%3AlistToMaybe" ></A ></A ><B >listToMaybe</B > :: [a] -> <A HREF="Data-Maybe.html#t%3AMaybe" >Maybe</A > a</TD ></TR ><TR ><TD CLASS="doc" >The <TT ><A HREF="Data-Maybe.html#v%3AlistToMaybe" >listToMaybe</A ></TT > function returns <TT ><A HREF="Data-Maybe.html#v%3ANothing" >Nothing</A ></TT > on an empty list or <TT ><TT ><A HREF="Data-Maybe.html#v%3AJust" >Just</A ></TT > a</TT > where <TT >a</TT > is the first element of the list. </TD ></TR ><TR ><TD CLASS="s15" ></TD ></TR ><TR ><TD CLASS="decl" ><A NAME="v:maybeToList" ><A NAME="v%3AmaybeToList" ></A ></A ><B >maybeToList</B > :: <A HREF="Data-Maybe.html#t%3AMaybe" >Maybe</A > a -> [a]</TD ></TR ><TR ><TD CLASS="doc" >The <TT ><A HREF="Data-Maybe.html#v%3AmaybeToList" >maybeToList</A ></TT > function returns an empty list when given <TT ><A HREF="Data-Maybe.html#v%3ANothing" >Nothing</A ></TT > or a singleton list when not given <TT ><A HREF="Data-Maybe.html#v%3ANothing" >Nothing</A ></TT >. </TD ></TR ><TR ><TD CLASS="s15" ></TD ></TR ><TR ><TD CLASS="decl" ><A NAME="v:catMaybes" ><A NAME="v%3AcatMaybes" ></A ></A ><B >catMaybes</B > :: [<A HREF="Data-Maybe.html#t%3AMaybe" >Maybe</A > a] -> [a]</TD ></TR ><TR ><TD CLASS="doc" >The <TT ><A HREF="Data-Maybe.html#v%3AcatMaybes" >catMaybes</A ></TT > function takes a list of <TT ><A HREF="Data-Maybe.html#t%3AMaybe" >Maybe</A ></TT >s and returns a list of all the <TT ><A HREF="Data-Maybe.html#v%3AJust" >Just</A ></TT > values. </TD ></TR ><TR ><TD CLASS="s15" ></TD ></TR ><TR ><TD CLASS="decl" ><A NAME="v:mapMaybe" ><A NAME="v%3AmapMaybe" ></A ></A ><B >mapMaybe</B > :: (a -> <A HREF="Data-Maybe.html#t%3AMaybe" >Maybe</A > b) -> [a] -> [b]</TD ></TR ><TR ><TD CLASS="doc" >The <TT ><A HREF="Data-Maybe.html#v%3AmapMaybe" >mapMaybe</A ></TT > function is a version of <TT ><A HREF="Data-List.html#v%3Amap" >map</A ></TT > which can throw out elements. In particular, the functional argument returns something of type <TT ><TT ><A HREF="Data-Maybe.html#t%3AMaybe" >Maybe</A ></TT > b</TT >. If this is <TT ><A HREF="Data-Maybe.html#v%3ANothing" >Nothing</A ></TT >, no element is added on to the result list. If it just <TT ><TT ><A HREF="Data-Maybe.html#v%3AJust" >Just</A ></TT > b</TT >, then <TT >b</TT > is included in the result list. </TD ></TR ><TR ><TD CLASS="s15" ></TD ></TR ><TR ><TD CLASS="botbar" >Produced by <A HREF="http://www.haskell.org/haddock/" >Haddock</A > version 2.4.2</TD ></TR ></TABLE ></BODY ></HTML >