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<title>The Haskell 98 Library Report: Maybe Utilities</title>
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<a name="sect8"></a>
<h2>8<tt>&nbsp;&nbsp;</tt>Maybe Utilities</h2><p>
<table border=2 cellpadding=3>
<tr><td>
<tt><br>
module&nbsp;Maybe(<br>
&nbsp;&nbsp;&nbsp;&nbsp;isJust,&nbsp;isNothing,<br>
&nbsp;&nbsp;&nbsp;&nbsp;fromJust,&nbsp;fromMaybe,&nbsp;listToMaybe,&nbsp;maybeToList,<br>
&nbsp;&nbsp;&nbsp;&nbsp;catMaybes,&nbsp;mapMaybe,<br>
<br>
&nbsp;&nbsp;&nbsp;&nbsp;--&nbsp;...and&nbsp;what&nbsp;the&nbsp;Prelude&nbsp;exports<br>
&nbsp;&nbsp;&nbsp;&nbsp;Maybe(Nothing,&nbsp;Just),<br>
&nbsp;&nbsp;&nbsp;&nbsp;maybe<br>
&nbsp;&nbsp;)&nbsp;where<br>
<br>
isJust,&nbsp;isNothing&nbsp;&nbsp;&nbsp;&nbsp;::&nbsp;Maybe&nbsp;a&nbsp;-&gt;&nbsp;Bool<br>
fromJust&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;::&nbsp;Maybe&nbsp;a&nbsp;-&gt;&nbsp;a<br>
fromMaybe&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;::&nbsp;a&nbsp;-&gt;&nbsp;Maybe&nbsp;a&nbsp;-&gt;&nbsp;a<br>
listToMaybe&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;::&nbsp;[a]&nbsp;-&gt;&nbsp;Maybe&nbsp;a<br>
maybeToList&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;::&nbsp;Maybe&nbsp;a&nbsp;-&gt;&nbsp;[a]<br>
catMaybes&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;::&nbsp;[Maybe&nbsp;a]&nbsp;-&gt;&nbsp;[a]<br>
mapMaybe&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;::&nbsp;(a&nbsp;-&gt;&nbsp;Maybe&nbsp;b)&nbsp;-&gt;&nbsp;[a]&nbsp;-&gt;&nbsp;[b]<br>

</tt></td></tr></table>
<p>
The type constructor <tt>Maybe</tt> is defined in <tt>Prelude</tt> as
<tt><br>

<br>
data&nbsp;Maybe&nbsp;a&nbsp;=&nbsp;Nothing&nbsp;|&nbsp;Just&nbsp;a<br>

<br>

</tt>The purpose of the <tt>Maybe</tt> type is to provide a method of dealing with
illegal or optional values without terminating the program, as would
happen if <tt>error</tt> were used, and without using <tt>IOError</tt> from the <tt>IO
</tt>monad, which would cause the expression to become monadic.  A correct
result is encapsulated by wrapping it in <tt>Just</tt>; an incorrect result
is returned as <tt>Nothing</tt>.<p>
Other operations on <tt>Maybe</tt> are provided as part of the monadic
classes in the Prelude.<a name="Maybe"></a><p>

<a name="sect8.1"></a>
<h3>8.1<tt>&nbsp;&nbsp;</tt>Library <tt>Maybe</tt></h3>

<tt><br>
module&nbsp;Maybe(<br>
&nbsp;&nbsp;&nbsp;&nbsp;isJust,&nbsp;isNothing,<br>
&nbsp;&nbsp;&nbsp;&nbsp;fromJust,&nbsp;fromMaybe,&nbsp;listToMaybe,&nbsp;maybeToList,<br>
&nbsp;&nbsp;&nbsp;&nbsp;catMaybes,&nbsp;mapMaybe,<br>
<br>
&nbsp;&nbsp;&nbsp;&nbsp;--&nbsp;...and&nbsp;what&nbsp;the&nbsp;Prelude&nbsp;exports<br>
&nbsp;&nbsp;&nbsp;&nbsp;Maybe(Nothing,&nbsp;Just),<br>
&nbsp;&nbsp;&nbsp;&nbsp;maybe<br>
&nbsp;&nbsp;)&nbsp;where<br>
<br>
isJust&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;::&nbsp;Maybe&nbsp;a&nbsp;-&gt;&nbsp;Bool<br>
isJust&nbsp;(Just&nbsp;a)&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;=&nbsp;&nbsp;True<br>
isJust&nbsp;Nothing&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;=&nbsp;&nbsp;False<br>
<br>
isNothing	&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;::&nbsp;Maybe&nbsp;a&nbsp;-&gt;&nbsp;Bool<br>
isNothing	&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;=&nbsp;&nbsp;not&nbsp;.&nbsp;isJust<br>
<br>
fromJust&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;::&nbsp;Maybe&nbsp;a&nbsp;-&gt;&nbsp;a<br>
fromJust&nbsp;(Just&nbsp;a)&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;=&nbsp;&nbsp;a<br>
fromJust&nbsp;Nothing&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;=&nbsp;&nbsp;error&nbsp;"Maybe.fromJust:&nbsp;Nothing"<br>
<br>
fromMaybe&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;::&nbsp;a&nbsp;-&gt;&nbsp;Maybe&nbsp;a&nbsp;-&gt;&nbsp;a<br>
fromMaybe&nbsp;d&nbsp;Nothing&nbsp;&nbsp;&nbsp;&nbsp;=&nbsp;&nbsp;d<br>
fromMaybe&nbsp;d&nbsp;(Just&nbsp;a)&nbsp;&nbsp;&nbsp;=&nbsp;&nbsp;a<br>
<br>
maybeToList&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;::&nbsp;Maybe&nbsp;a&nbsp;-&gt;&nbsp;[a]<br>
maybeToList&nbsp;Nothing&nbsp;&nbsp;&nbsp;&nbsp;=&nbsp;&nbsp;[]<br>
maybeToList&nbsp;(Just&nbsp;a)&nbsp;&nbsp;&nbsp;=&nbsp;&nbsp;[a]<br>
<br>
listToMaybe&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;::&nbsp;[a]&nbsp;-&gt;&nbsp;Maybe&nbsp;a<br>
listToMaybe&nbsp;[]&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;=&nbsp;&nbsp;Nothing<br>
listToMaybe&nbsp;(a:_)&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;=&nbsp;&nbsp;Just&nbsp;a<br>
&nbsp;<br>
catMaybes&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;::&nbsp;[Maybe&nbsp;a]&nbsp;-&gt;&nbsp;[a]<br>
catMaybes&nbsp;ms&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;=&nbsp;&nbsp;[&nbsp;m&nbsp;|&nbsp;Just&nbsp;m&nbsp;&lt;-&nbsp;ms&nbsp;]<br>
<br>
mapMaybe&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;::&nbsp;(a&nbsp;-&gt;&nbsp;Maybe&nbsp;b)&nbsp;-&gt;&nbsp;[a]&nbsp;-&gt;&nbsp;[b]<br>
mapMaybe&nbsp;f&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;=&nbsp;&nbsp;catMaybes&nbsp;.&nbsp;map&nbsp;f<br>
<p>
<hr><i>The Haskell 98 Library Report</i><br><a href="index.html">top</a> | <a href="list.html">back</a> | <a href="char.html">next</a> | <a href="libindex.html">contents</a> <br><font size=2>1 February, 1999</font>
</tt>