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<pre><a name="line-1"></a><span class='hs-comment'>{-# OPTIONS_GHC -XNoImplicitPrelude #-}</span>
<a name="line-2"></a><span class='hs-comment'>-----------------------------------------------------------------------------</span>
<a name="line-3"></a><span class='hs-comment'>-- |</span>
<a name="line-4"></a><span class='hs-comment'>-- Module : Data.Maybe</span>
<a name="line-5"></a><span class='hs-comment'>-- Copyright : (c) The University of Glasgow 2001</span>
<a name="line-6"></a><span class='hs-comment'>-- License : BSD-style (see the file libraries/base/LICENSE)</span>
<a name="line-7"></a><span class='hs-comment'>-- </span>
<a name="line-8"></a><span class='hs-comment'>-- Maintainer : libraries@haskell.org</span>
<a name="line-9"></a><span class='hs-comment'>-- Stability : stable</span>
<a name="line-10"></a><span class='hs-comment'>-- Portability : portable</span>
<a name="line-11"></a><span class='hs-comment'>--</span>
<a name="line-12"></a><span class='hs-comment'>-- The Maybe type, and associated operations.</span>
<a name="line-13"></a><span class='hs-comment'>--</span>
<a name="line-14"></a><span class='hs-comment'>-----------------------------------------------------------------------------</span>
<a name="line-15"></a>
<a name="line-16"></a><span class='hs-keyword'>module</span> <span class='hs-conid'>Data</span><span class='hs-varop'>.</span><span class='hs-conid'>Maybe</span>
<a name="line-17"></a> <span class='hs-layout'>(</span>
<a name="line-18"></a> <span class='hs-conid'>Maybe</span><span class='hs-layout'>(</span><span class='hs-conid'>Nothing</span><span class='hs-layout'>,</span><span class='hs-conid'>Just</span><span class='hs-layout'>)</span><span class='hs-comment'>-- instance of: Eq, Ord, Show, Read,</span>
<a name="line-19"></a> <span class='hs-comment'>-- Functor, Monad, MonadPlus</span>
<a name="line-20"></a>
<a name="line-21"></a> <span class='hs-layout'>,</span> <span class='hs-varid'>maybe</span> <span class='hs-comment'>-- :: b -> (a -> b) -> Maybe a -> b</span>
<a name="line-22"></a>
<a name="line-23"></a> <span class='hs-layout'>,</span> <span class='hs-varid'>isJust</span> <span class='hs-comment'>-- :: Maybe a -> Bool</span>
<a name="line-24"></a> <span class='hs-layout'>,</span> <span class='hs-varid'>isNothing</span> <span class='hs-comment'>-- :: Maybe a -> Bool</span>
<a name="line-25"></a> <span class='hs-layout'>,</span> <span class='hs-varid'>fromJust</span> <span class='hs-comment'>-- :: Maybe a -> a</span>
<a name="line-26"></a> <span class='hs-layout'>,</span> <span class='hs-varid'>fromMaybe</span> <span class='hs-comment'>-- :: a -> Maybe a -> a</span>
<a name="line-27"></a> <span class='hs-layout'>,</span> <span class='hs-varid'>listToMaybe</span> <span class='hs-comment'>-- :: [a] -> Maybe a</span>
<a name="line-28"></a> <span class='hs-layout'>,</span> <span class='hs-varid'>maybeToList</span> <span class='hs-comment'>-- :: Maybe a -> [a]</span>
<a name="line-29"></a> <span class='hs-layout'>,</span> <span class='hs-varid'>catMaybes</span> <span class='hs-comment'>-- :: [Maybe a] -> [a]</span>
<a name="line-30"></a> <span class='hs-layout'>,</span> <span class='hs-varid'>mapMaybe</span> <span class='hs-comment'>-- :: (a -> Maybe b) -> [a] -> [b]</span>
<a name="line-31"></a> <span class='hs-layout'>)</span> <span class='hs-keyword'>where</span>
<a name="line-32"></a>
<a name="line-33"></a><span class='hs-cpp'>#ifdef __GLASGOW_HASKELL__</span>
<a name="line-34"></a><span class='hs-keyword'>import</span> <span class='hs-conid'>GHC</span><span class='hs-varop'>.</span><span class='hs-conid'>Base</span>
<a name="line-35"></a><span class='hs-cpp'>#endif</span>
<a name="line-36"></a>
<a name="line-37"></a><span class='hs-cpp'>#ifdef __NHC__</span>
<a name="line-38"></a><span class='hs-keyword'>import</span> <span class='hs-conid'>Prelude</span>
<a name="line-39"></a><span class='hs-keyword'>import</span> <span class='hs-conid'>Prelude</span> <span class='hs-layout'>(</span><span class='hs-conid'>Maybe</span><span class='hs-layout'>(</span><span class='hs-keyglyph'>..</span><span class='hs-layout'>)</span><span class='hs-layout'>,</span> <span class='hs-varid'>maybe</span><span class='hs-layout'>)</span>
<a name="line-40"></a><span class='hs-keyword'>import</span> <span class='hs-conid'>Maybe</span>
<a name="line-41"></a> <span class='hs-layout'>(</span> <span class='hs-varid'>isJust</span>
<a name="line-42"></a> <span class='hs-layout'>,</span> <span class='hs-varid'>isNothing</span>
<a name="line-43"></a> <span class='hs-layout'>,</span> <span class='hs-varid'>fromJust</span>
<a name="line-44"></a> <span class='hs-layout'>,</span> <span class='hs-varid'>fromMaybe</span>
<a name="line-45"></a> <span class='hs-layout'>,</span> <span class='hs-varid'>listToMaybe</span>
<a name="line-46"></a> <span class='hs-layout'>,</span> <span class='hs-varid'>maybeToList</span>
<a name="line-47"></a> <span class='hs-layout'>,</span> <span class='hs-varid'>catMaybes</span>
<a name="line-48"></a> <span class='hs-layout'>,</span> <span class='hs-varid'>mapMaybe</span>
<a name="line-49"></a> <span class='hs-layout'>)</span>
<a name="line-50"></a><span class='hs-cpp'>#else</span>
<a name="line-51"></a>
<a name="line-52"></a><span class='hs-cpp'>#ifndef __HUGS__</span>
<a name="line-53"></a><span class='hs-comment'>-- ---------------------------------------------------------------------------</span>
<a name="line-54"></a><span class='hs-comment'>-- The Maybe type, and instances</span>
<a name="line-55"></a>
<a name="line-56"></a><span class='hs-comment'>-- | The 'Maybe' type encapsulates an optional value. A value of type</span>
<a name="line-57"></a><span class='hs-comment'>-- @'Maybe' a@ either contains a value of type @a@ (represented as @'Just' a@), </span>
<a name="line-58"></a><span class='hs-comment'>-- or it is empty (represented as 'Nothing'). Using 'Maybe' is a good way to </span>
<a name="line-59"></a><span class='hs-comment'>-- deal with errors or exceptional cases without resorting to drastic</span>
<a name="line-60"></a><span class='hs-comment'>-- measures such as 'error'.</span>
<a name="line-61"></a><span class='hs-comment'>--</span>
<a name="line-62"></a><span class='hs-comment'>-- The 'Maybe' type is also a monad. It is a simple kind of error</span>
<a name="line-63"></a><span class='hs-comment'>-- monad, where all errors are represented by 'Nothing'. A richer</span>
<a name="line-64"></a><span class='hs-comment'>-- error monad can be built using the 'Data.Either.Either' type.</span>
<a name="line-65"></a>
<a name="line-66"></a><a name="Maybe"></a><span class='hs-keyword'>data</span> <span class='hs-conid'>Maybe</span> <span class='hs-varid'>a</span> <span class='hs-keyglyph'>=</span> <span class='hs-conid'>Nothing</span> <span class='hs-keyglyph'>|</span> <span class='hs-conid'>Just</span> <span class='hs-varid'>a</span>
<a name="line-67"></a> <span class='hs-keyword'>deriving</span> <span class='hs-layout'>(</span><span class='hs-conid'>Eq</span><span class='hs-layout'>,</span> <span class='hs-conid'>Ord</span><span class='hs-layout'>)</span>
<a name="line-68"></a>
<a name="line-69"></a><span class='hs-keyword'>instance</span> <span class='hs-conid'>Functor</span> <span class='hs-conid'>Maybe</span> <span class='hs-keyword'>where</span>
<a name="line-70"></a> <span class='hs-varid'>fmap</span> <span class='hs-keyword'>_</span> <span class='hs-conid'>Nothing</span> <span class='hs-keyglyph'>=</span> <span class='hs-conid'>Nothing</span>
<a name="line-71"></a> <span class='hs-varid'>fmap</span> <span class='hs-varid'>f</span> <span class='hs-layout'>(</span><span class='hs-conid'>Just</span> <span class='hs-varid'>a</span><span class='hs-layout'>)</span> <span class='hs-keyglyph'>=</span> <span class='hs-conid'>Just</span> <span class='hs-layout'>(</span><span class='hs-varid'>f</span> <span class='hs-varid'>a</span><span class='hs-layout'>)</span>
<a name="line-72"></a>
<a name="line-73"></a><span class='hs-keyword'>instance</span> <span class='hs-conid'>Monad</span> <span class='hs-conid'>Maybe</span> <span class='hs-keyword'>where</span>
<a name="line-74"></a> <span class='hs-layout'>(</span><span class='hs-conid'>Just</span> <span class='hs-varid'>x</span><span class='hs-layout'>)</span> <span class='hs-varop'>>>=</span> <span class='hs-varid'>k</span> <span class='hs-keyglyph'>=</span> <span class='hs-varid'>k</span> <span class='hs-varid'>x</span>
<a name="line-75"></a> <span class='hs-conid'>Nothing</span> <span class='hs-varop'>>>=</span> <span class='hs-keyword'>_</span> <span class='hs-keyglyph'>=</span> <span class='hs-conid'>Nothing</span>
<a name="line-76"></a>
<a name="line-77"></a> <span class='hs-layout'>(</span><span class='hs-conid'>Just</span> <span class='hs-keyword'>_</span><span class='hs-layout'>)</span> <span class='hs-varop'>>></span> <span class='hs-varid'>k</span> <span class='hs-keyglyph'>=</span> <span class='hs-varid'>k</span>
<a name="line-78"></a> <span class='hs-conid'>Nothing</span> <span class='hs-varop'>>></span> <span class='hs-keyword'>_</span> <span class='hs-keyglyph'>=</span> <span class='hs-conid'>Nothing</span>
<a name="line-79"></a>
<a name="line-80"></a> <span class='hs-varid'>return</span> <span class='hs-keyglyph'>=</span> <span class='hs-conid'>Just</span>
<a name="line-81"></a> <span class='hs-varid'>fail</span> <span class='hs-keyword'>_</span> <span class='hs-keyglyph'>=</span> <span class='hs-conid'>Nothing</span>
<a name="line-82"></a>
<a name="line-83"></a><span class='hs-comment'>-- ---------------------------------------------------------------------------</span>
<a name="line-84"></a><span class='hs-comment'>-- Functions over Maybe</span>
<a name="line-85"></a>
<a name="line-86"></a><a name="maybe"></a><span class='hs-comment'>-- | The 'maybe' function takes a default value, a function, and a 'Maybe'</span>
<a name="line-87"></a><span class='hs-comment'>-- value. If the 'Maybe' value is 'Nothing', the function returns the</span>
<a name="line-88"></a><span class='hs-comment'>-- default value. Otherwise, it applies the function to the value inside</span>
<a name="line-89"></a><span class='hs-comment'>-- the 'Just' and returns the result.</span>
<a name="line-90"></a><span class='hs-definition'>maybe</span> <span class='hs-keyglyph'>::</span> <span class='hs-varid'>b</span> <span class='hs-keyglyph'>-></span> <span class='hs-layout'>(</span><span class='hs-varid'>a</span> <span class='hs-keyglyph'>-></span> <span class='hs-varid'>b</span><span class='hs-layout'>)</span> <span class='hs-keyglyph'>-></span> <span class='hs-conid'>Maybe</span> <span class='hs-varid'>a</span> <span class='hs-keyglyph'>-></span> <span class='hs-varid'>b</span>
<a name="line-91"></a><span class='hs-definition'>maybe</span> <span class='hs-varid'>n</span> <span class='hs-keyword'>_</span> <span class='hs-conid'>Nothing</span> <span class='hs-keyglyph'>=</span> <span class='hs-varid'>n</span>
<a name="line-92"></a><span class='hs-definition'>maybe</span> <span class='hs-keyword'>_</span> <span class='hs-varid'>f</span> <span class='hs-layout'>(</span><span class='hs-conid'>Just</span> <span class='hs-varid'>x</span><span class='hs-layout'>)</span> <span class='hs-keyglyph'>=</span> <span class='hs-varid'>f</span> <span class='hs-varid'>x</span>
<a name="line-93"></a><span class='hs-cpp'>#endif /* __HUGS__ */</span>
<a name="line-94"></a>
<a name="line-95"></a><a name="isJust"></a><span class='hs-comment'>-- | The 'isJust' function returns 'True' iff its argument is of the</span>
<a name="line-96"></a><span class='hs-comment'>-- form @Just _@.</span>
<a name="line-97"></a><span class='hs-definition'>isJust</span> <span class='hs-keyglyph'>::</span> <span class='hs-conid'>Maybe</span> <span class='hs-varid'>a</span> <span class='hs-keyglyph'>-></span> <span class='hs-conid'>Bool</span>
<a name="line-98"></a><span class='hs-definition'>isJust</span> <span class='hs-conid'>Nothing</span> <span class='hs-keyglyph'>=</span> <span class='hs-conid'>False</span>
<a name="line-99"></a><span class='hs-definition'>isJust</span> <span class='hs-keyword'>_</span> <span class='hs-keyglyph'>=</span> <span class='hs-conid'>True</span>
<a name="line-100"></a>
<a name="line-101"></a><a name="isNothing"></a><span class='hs-comment'>-- | The 'isNothing' function returns 'True' iff its argument is 'Nothing'.</span>
<a name="line-102"></a><span class='hs-definition'>isNothing</span> <span class='hs-keyglyph'>::</span> <span class='hs-conid'>Maybe</span> <span class='hs-varid'>a</span> <span class='hs-keyglyph'>-></span> <span class='hs-conid'>Bool</span>
<a name="line-103"></a><span class='hs-definition'>isNothing</span> <span class='hs-conid'>Nothing</span> <span class='hs-keyglyph'>=</span> <span class='hs-conid'>True</span>
<a name="line-104"></a><span class='hs-definition'>isNothing</span> <span class='hs-keyword'>_</span> <span class='hs-keyglyph'>=</span> <span class='hs-conid'>False</span>
<a name="line-105"></a>
<a name="line-106"></a><a name="fromJust"></a><span class='hs-comment'>-- | The 'fromJust' function extracts the element out of a 'Just' and</span>
<a name="line-107"></a><span class='hs-comment'>-- throws an error if its argument is 'Nothing'.</span>
<a name="line-108"></a><span class='hs-definition'>fromJust</span> <span class='hs-keyglyph'>::</span> <span class='hs-conid'>Maybe</span> <span class='hs-varid'>a</span> <span class='hs-keyglyph'>-></span> <span class='hs-varid'>a</span>
<a name="line-109"></a><span class='hs-definition'>fromJust</span> <span class='hs-conid'>Nothing</span> <span class='hs-keyglyph'>=</span> <span class='hs-varid'>error</span> <span class='hs-str'>"Maybe.fromJust: Nothing"</span> <span class='hs-comment'>-- yuck</span>
<a name="line-110"></a><span class='hs-definition'>fromJust</span> <span class='hs-layout'>(</span><span class='hs-conid'>Just</span> <span class='hs-varid'>x</span><span class='hs-layout'>)</span> <span class='hs-keyglyph'>=</span> <span class='hs-varid'>x</span>
<a name="line-111"></a>
<a name="line-112"></a><a name="fromMaybe"></a><span class='hs-comment'>-- | The 'fromMaybe' function takes a default value and and 'Maybe'</span>
<a name="line-113"></a><span class='hs-comment'>-- value. If the 'Maybe' is 'Nothing', it returns the default values;</span>
<a name="line-114"></a><span class='hs-comment'>-- otherwise, it returns the value contained in the 'Maybe'.</span>
<a name="line-115"></a><span class='hs-definition'>fromMaybe</span> <span class='hs-keyglyph'>::</span> <span class='hs-varid'>a</span> <span class='hs-keyglyph'>-></span> <span class='hs-conid'>Maybe</span> <span class='hs-varid'>a</span> <span class='hs-keyglyph'>-></span> <span class='hs-varid'>a</span>
<a name="line-116"></a><span class='hs-definition'>fromMaybe</span> <span class='hs-varid'>d</span> <span class='hs-varid'>x</span> <span class='hs-keyglyph'>=</span> <span class='hs-keyword'>case</span> <span class='hs-varid'>x</span> <span class='hs-keyword'>of</span> <span class='hs-layout'>{</span><span class='hs-conid'>Nothing</span> <span class='hs-keyglyph'>-></span> <span class='hs-varid'>d</span><span class='hs-layout'>;</span><span class='hs-conid'>Just</span> <span class='hs-varid'>v</span> <span class='hs-keyglyph'>-></span> <span class='hs-varid'>v</span><span class='hs-layout'>}</span>
<a name="line-117"></a>
<a name="line-118"></a><a name="maybeToList"></a><span class='hs-comment'>-- | The 'maybeToList' function returns an empty list when given</span>
<a name="line-119"></a><span class='hs-comment'>-- 'Nothing' or a singleton list when not given 'Nothing'.</span>
<a name="line-120"></a><span class='hs-definition'>maybeToList</span> <span class='hs-keyglyph'>::</span> <span class='hs-conid'>Maybe</span> <span class='hs-varid'>a</span> <span class='hs-keyglyph'>-></span> <span class='hs-keyglyph'>[</span><span class='hs-varid'>a</span><span class='hs-keyglyph'>]</span>
<a name="line-121"></a><span class='hs-definition'>maybeToList</span> <span class='hs-conid'>Nothing</span> <span class='hs-keyglyph'>=</span> <span class='hs-conid'>[]</span>
<a name="line-122"></a><span class='hs-definition'>maybeToList</span> <span class='hs-layout'>(</span><span class='hs-conid'>Just</span> <span class='hs-varid'>x</span><span class='hs-layout'>)</span> <span class='hs-keyglyph'>=</span> <span class='hs-keyglyph'>[</span><span class='hs-varid'>x</span><span class='hs-keyglyph'>]</span>
<a name="line-123"></a>
<a name="line-124"></a><a name="listToMaybe"></a><span class='hs-comment'>-- | The 'listToMaybe' function returns 'Nothing' on an empty list</span>
<a name="line-125"></a><span class='hs-comment'>-- or @'Just' a@ where @a@ is the first element of the list.</span>
<a name="line-126"></a><span class='hs-definition'>listToMaybe</span> <span class='hs-keyglyph'>::</span> <span class='hs-keyglyph'>[</span><span class='hs-varid'>a</span><span class='hs-keyglyph'>]</span> <span class='hs-keyglyph'>-></span> <span class='hs-conid'>Maybe</span> <span class='hs-varid'>a</span>
<a name="line-127"></a><span class='hs-definition'>listToMaybe</span> <span class='hs-conid'>[]</span> <span class='hs-keyglyph'>=</span> <span class='hs-conid'>Nothing</span>
<a name="line-128"></a><span class='hs-definition'>listToMaybe</span> <span class='hs-layout'>(</span><span class='hs-varid'>a</span><span class='hs-conop'>:</span><span class='hs-keyword'>_</span><span class='hs-layout'>)</span> <span class='hs-keyglyph'>=</span> <span class='hs-conid'>Just</span> <span class='hs-varid'>a</span>
<a name="line-129"></a>
<a name="line-130"></a><a name="catMaybes"></a><span class='hs-comment'>-- | The 'catMaybes' function takes a list of 'Maybe's and returns</span>
<a name="line-131"></a><span class='hs-comment'>-- a list of all the 'Just' values. </span>
<a name="line-132"></a><span class='hs-definition'>catMaybes</span> <span class='hs-keyglyph'>::</span> <span class='hs-keyglyph'>[</span><span class='hs-conid'>Maybe</span> <span class='hs-varid'>a</span><span class='hs-keyglyph'>]</span> <span class='hs-keyglyph'>-></span> <span class='hs-keyglyph'>[</span><span class='hs-varid'>a</span><span class='hs-keyglyph'>]</span>
<a name="line-133"></a><span class='hs-definition'>catMaybes</span> <span class='hs-varid'>ls</span> <span class='hs-keyglyph'>=</span> <span class='hs-keyglyph'>[</span><span class='hs-varid'>x</span> <span class='hs-keyglyph'>|</span> <span class='hs-conid'>Just</span> <span class='hs-varid'>x</span> <span class='hs-keyglyph'><-</span> <span class='hs-varid'>ls</span><span class='hs-keyglyph'>]</span>
<a name="line-134"></a>
<a name="line-135"></a><a name="mapMaybe"></a><span class='hs-comment'>-- | The 'mapMaybe' function is a version of 'map' which can throw</span>
<a name="line-136"></a><span class='hs-comment'>-- out elements. In particular, the functional argument returns</span>
<a name="line-137"></a><span class='hs-comment'>-- something of type @'Maybe' b@. If this is 'Nothing', no element</span>
<a name="line-138"></a><span class='hs-comment'>-- is added on to the result list. If it just @'Just' b@, then @b@ is</span>
<a name="line-139"></a><span class='hs-comment'>-- included in the result list.</span>
<a name="line-140"></a><span class='hs-definition'>mapMaybe</span> <span class='hs-keyglyph'>::</span> <span class='hs-layout'>(</span><span class='hs-varid'>a</span> <span class='hs-keyglyph'>-></span> <span class='hs-conid'>Maybe</span> <span class='hs-varid'>b</span><span class='hs-layout'>)</span> <span class='hs-keyglyph'>-></span> <span class='hs-keyglyph'>[</span><span class='hs-varid'>a</span><span class='hs-keyglyph'>]</span> <span class='hs-keyglyph'>-></span> <span class='hs-keyglyph'>[</span><span class='hs-varid'>b</span><span class='hs-keyglyph'>]</span>
<a name="line-141"></a><span class='hs-definition'>mapMaybe</span> <span class='hs-keyword'>_</span> <span class='hs-conid'>[]</span> <span class='hs-keyglyph'>=</span> <span class='hs-conid'>[]</span>
<a name="line-142"></a><span class='hs-definition'>mapMaybe</span> <span class='hs-varid'>f</span> <span class='hs-layout'>(</span><span class='hs-varid'>x</span><span class='hs-conop'>:</span><span class='hs-varid'>xs</span><span class='hs-layout'>)</span> <span class='hs-keyglyph'>=</span>
<a name="line-143"></a> <span class='hs-keyword'>let</span> <span class='hs-varid'>rs</span> <span class='hs-keyglyph'>=</span> <span class='hs-varid'>mapMaybe</span> <span class='hs-varid'>f</span> <span class='hs-varid'>xs</span> <span class='hs-keyword'>in</span>
<a name="line-144"></a> <span class='hs-keyword'>case</span> <span class='hs-varid'>f</span> <span class='hs-varid'>x</span> <span class='hs-keyword'>of</span>
<a name="line-145"></a> <span class='hs-conid'>Nothing</span> <span class='hs-keyglyph'>-></span> <span class='hs-varid'>rs</span>
<a name="line-146"></a> <span class='hs-conid'>Just</span> <span class='hs-varid'>r</span> <span class='hs-keyglyph'>-></span> <span class='hs-varid'>r</span><span class='hs-conop'>:</span><span class='hs-varid'>rs</span>
<a name="line-147"></a>
<a name="line-148"></a><span class='hs-cpp'>#endif /* else not __NHC__ */</span>
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