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>Data.Maybe</TITLE
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><TD CLASS="title"
>base-4.1.0.0: Basic libraries</TD
><TD CLASS="topbut"
><A HREF="index.html"
>Contents</A
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><A HREF="doc-index.html"
>Index</A
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><TD CLASS="modulebar"
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><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 -&gt; (a -&gt; b) -&gt; <A HREF="Data-Maybe.html#t%3AMaybe"
>Maybe</A
> a -&gt; 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 -&gt; <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 -&gt; <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 -&gt; a</TD
></TR
><TR
><TD CLASS="s8"
></TD
></TR
><TR
><TD CLASS="decl"
><A HREF="#v%3AfromMaybe"
>fromMaybe</A
> ::  a -&gt; <A HREF="Data-Maybe.html#t%3AMaybe"
>Maybe</A
> a -&gt; a</TD
></TR
><TR
><TD CLASS="s8"
></TD
></TR
><TR
><TD CLASS="decl"
><A HREF="#v%3AlistToMaybe"
>listToMaybe</A
> ::  [a] -&gt; <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 -&gt; [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] -&gt; [a]</TD
></TR
><TR
><TD CLASS="s8"
></TD
></TR
><TR
><TD CLASS="decl"
><A HREF="#v%3AmapMaybe"
>mapMaybe</A
> ::  (a -&gt; <A HREF="Data-Maybe.html#t%3AMaybe"
>Maybe</A
> b) -&gt; [a] -&gt; [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 =&gt; <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 =&gt; <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 =&gt; <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 =&gt; <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 =&gt; <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 =&gt; <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 -&gt; (a -&gt; b) -&gt; <A HREF="Data-Maybe.html#t%3AMaybe"
>Maybe</A
> a -&gt; 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 -&gt; <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 -&gt; <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 -&gt; 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 -&gt; <A HREF="Data-Maybe.html#t%3AMaybe"
>Maybe</A
> a -&gt; 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] -&gt; <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 -&gt; [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] -&gt; [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 -&gt; <A HREF="Data-Maybe.html#t%3AMaybe"
>Maybe</A
> b) -&gt; [a] -&gt; [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
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