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<title>Data/Functor/Identity.hs</title>
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<pre><a name="line-1"></a><span class='hs-comment'>-- |</span>
<a name="line-2"></a><span class='hs-comment'>-- Module : Data.Functor.Identity</span>
<a name="line-3"></a><span class='hs-comment'>-- Copyright : (c) Andy Gill 2001,</span>
<a name="line-4"></a><span class='hs-comment'>-- (c) Oregon Graduate Institute of Science and Technology 2001</span>
<a name="line-5"></a><span class='hs-comment'>-- License : BSD-style (see the file LICENSE)</span>
<a name="line-6"></a><span class='hs-comment'>--</span>
<a name="line-7"></a><span class='hs-comment'>-- Maintainer : ross@soi.city.ac.uk</span>
<a name="line-8"></a><span class='hs-comment'>-- Stability : experimental</span>
<a name="line-9"></a><span class='hs-comment'>-- Portability : portable</span>
<a name="line-10"></a><span class='hs-comment'>--</span>
<a name="line-11"></a><span class='hs-comment'>-- The identity functor and monad.</span>
<a name="line-12"></a><span class='hs-comment'>--</span>
<a name="line-13"></a><span class='hs-comment'>-- This trivial type constructor serves two purposes:</span>
<a name="line-14"></a><span class='hs-comment'>--</span>
<a name="line-15"></a><span class='hs-comment'>-- * It can be used with functions parameterized by a 'Functor' or 'Monad'.</span>
<a name="line-16"></a><span class='hs-comment'>--</span>
<a name="line-17"></a><span class='hs-comment'>-- * It can be used as a base monad to which a series of monad</span>
<a name="line-18"></a><span class='hs-comment'>-- transformers may be applied to construct a composite monad.</span>
<a name="line-19"></a><span class='hs-comment'>-- Most monad transformer modules include the special case of</span>
<a name="line-20"></a><span class='hs-comment'>-- applying the transformer to 'Identity'. For example, @State s@</span>
<a name="line-21"></a><span class='hs-comment'>-- is an abbreviation for @StateT s 'Identity'@.</span>
<a name="line-22"></a>
<a name="line-23"></a><span class='hs-keyword'>module</span> <span class='hs-conid'>Data</span><span class='hs-varop'>.</span><span class='hs-conid'>Functor</span><span class='hs-varop'>.</span><span class='hs-conid'>Identity</span> <span class='hs-layout'>(</span>
<a name="line-24"></a> <span class='hs-conid'>Identity</span><span class='hs-layout'>(</span><span class='hs-keyglyph'>..</span><span class='hs-layout'>)</span><span class='hs-layout'>,</span>
<a name="line-25"></a> <span class='hs-layout'>)</span> <span class='hs-keyword'>where</span>
<a name="line-26"></a>
<a name="line-27"></a><span class='hs-keyword'>import</span> <span class='hs-conid'>Control</span><span class='hs-varop'>.</span><span class='hs-conid'>Applicative</span>
<a name="line-28"></a><span class='hs-keyword'>import</span> <span class='hs-conid'>Control</span><span class='hs-varop'>.</span><span class='hs-conid'>Monad</span>
<a name="line-29"></a><span class='hs-keyword'>import</span> <span class='hs-conid'>Control</span><span class='hs-varop'>.</span><span class='hs-conid'>Monad</span><span class='hs-varop'>.</span><span class='hs-conid'>Fix</span>
<a name="line-30"></a><span class='hs-keyword'>import</span> <span class='hs-conid'>Data</span><span class='hs-varop'>.</span><span class='hs-conid'>Foldable</span> <span class='hs-layout'>(</span><span class='hs-conid'>Foldable</span><span class='hs-layout'>(</span><span class='hs-varid'>foldMap</span><span class='hs-layout'>)</span><span class='hs-layout'>)</span>
<a name="line-31"></a><span class='hs-keyword'>import</span> <span class='hs-conid'>Data</span><span class='hs-varop'>.</span><span class='hs-conid'>Traversable</span> <span class='hs-layout'>(</span><span class='hs-conid'>Traversable</span><span class='hs-layout'>(</span><span class='hs-varid'>traverse</span><span class='hs-layout'>)</span><span class='hs-layout'>)</span>
<a name="line-32"></a>
<a name="line-33"></a><a name="Identity"></a><span class='hs-comment'>-- | Identity functor and monad.</span>
<a name="line-34"></a><a name="Identity"></a><span class='hs-keyword'>newtype</span> <span class='hs-conid'>Identity</span> <span class='hs-varid'>a</span> <span class='hs-keyglyph'>=</span> <span class='hs-conid'>Identity</span> <span class='hs-layout'>{</span> <span class='hs-varid'>runIdentity</span> <span class='hs-keyglyph'>::</span> <span class='hs-varid'>a</span> <span class='hs-layout'>}</span>
<a name="line-35"></a>
<a name="line-36"></a><span class='hs-comment'>-- ---------------------------------------------------------------------------</span>
<a name="line-37"></a><span class='hs-comment'>-- Identity instances for Functor and Monad</span>
<a name="line-38"></a>
<a name="line-39"></a><span class='hs-keyword'>instance</span> <span class='hs-conid'>Functor</span> <span class='hs-conid'>Identity</span> <span class='hs-keyword'>where</span>
<a name="line-40"></a> <span class='hs-varid'>fmap</span> <span class='hs-varid'>f</span> <span class='hs-varid'>m</span> <span class='hs-keyglyph'>=</span> <span class='hs-conid'>Identity</span> <span class='hs-layout'>(</span><span class='hs-varid'>f</span> <span class='hs-layout'>(</span><span class='hs-varid'>runIdentity</span> <span class='hs-varid'>m</span><span class='hs-layout'>)</span><span class='hs-layout'>)</span>
<a name="line-41"></a>
<a name="line-42"></a><span class='hs-keyword'>instance</span> <span class='hs-conid'>Foldable</span> <span class='hs-conid'>Identity</span> <span class='hs-keyword'>where</span>
<a name="line-43"></a> <span class='hs-varid'>foldMap</span> <span class='hs-varid'>f</span> <span class='hs-layout'>(</span><span class='hs-conid'>Identity</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-44"></a>
<a name="line-45"></a><span class='hs-keyword'>instance</span> <span class='hs-conid'>Traversable</span> <span class='hs-conid'>Identity</span> <span class='hs-keyword'>where</span>
<a name="line-46"></a> <span class='hs-varid'>traverse</span> <span class='hs-varid'>f</span> <span class='hs-layout'>(</span><span class='hs-conid'>Identity</span> <span class='hs-varid'>x</span><span class='hs-layout'>)</span> <span class='hs-keyglyph'>=</span> <span class='hs-conid'>Identity</span> <span class='hs-varop'><$></span> <span class='hs-varid'>f</span> <span class='hs-varid'>x</span>
<a name="line-47"></a>
<a name="line-48"></a><span class='hs-keyword'>instance</span> <span class='hs-conid'>Applicative</span> <span class='hs-conid'>Identity</span> <span class='hs-keyword'>where</span>
<a name="line-49"></a> <span class='hs-varid'>pure</span> <span class='hs-varid'>a</span> <span class='hs-keyglyph'>=</span> <span class='hs-conid'>Identity</span> <span class='hs-varid'>a</span>
<a name="line-50"></a> <span class='hs-conid'>Identity</span> <span class='hs-varid'>f</span> <span class='hs-varop'><*></span> <span class='hs-conid'>Identity</span> <span class='hs-varid'>x</span> <span class='hs-keyglyph'>=</span> <span class='hs-conid'>Identity</span> <span class='hs-layout'>(</span><span class='hs-varid'>f</span> <span class='hs-varid'>x</span><span class='hs-layout'>)</span>
<a name="line-51"></a>
<a name="line-52"></a><span class='hs-keyword'>instance</span> <span class='hs-conid'>Monad</span> <span class='hs-conid'>Identity</span> <span class='hs-keyword'>where</span>
<a name="line-53"></a> <span class='hs-varid'>return</span> <span class='hs-varid'>a</span> <span class='hs-keyglyph'>=</span> <span class='hs-conid'>Identity</span> <span class='hs-varid'>a</span>
<a name="line-54"></a> <span class='hs-varid'>m</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-layout'>(</span><span class='hs-varid'>runIdentity</span> <span class='hs-varid'>m</span><span class='hs-layout'>)</span>
<a name="line-55"></a>
<a name="line-56"></a><span class='hs-keyword'>instance</span> <span class='hs-conid'>MonadFix</span> <span class='hs-conid'>Identity</span> <span class='hs-keyword'>where</span>
<a name="line-57"></a> <span class='hs-varid'>mfix</span> <span class='hs-varid'>f</span> <span class='hs-keyglyph'>=</span> <span class='hs-conid'>Identity</span> <span class='hs-layout'>(</span><span class='hs-varid'>fix</span> <span class='hs-layout'>(</span><span class='hs-varid'>runIdentity</span> <span class='hs-varop'>.</span> <span class='hs-varid'>f</span><span class='hs-layout'>)</span><span class='hs-layout'>)</span>
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