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<pre><a name="line-1"></a><span class='hs-comment'>-----------------------------------------------------------------------------</span>
<a name="line-2"></a><span class='hs-comment'>-- |</span>
<a name="line-3"></a><span class='hs-comment'>-- Module : Control.Monad.Trans.Reader</span>
<a name="line-4"></a><span class='hs-comment'>-- Copyright : (c) Andy Gill 2001,</span>
<a name="line-5"></a><span class='hs-comment'>-- (c) Oregon Graduate Institute of Science and Technology, 2001</span>
<a name="line-6"></a><span class='hs-comment'>-- License : BSD-style (see the file LICENSE)</span>
<a name="line-7"></a><span class='hs-comment'>--</span>
<a name="line-8"></a><span class='hs-comment'>-- Maintainer : ross@soi.city.ac.uk</span>
<a name="line-9"></a><span class='hs-comment'>-- Stability : experimental</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'>-- Declaration of the 'ReaderT' monad transformer, which adds a static</span>
<a name="line-13"></a><span class='hs-comment'>-- environment to a given monad.</span>
<a name="line-14"></a><span class='hs-comment'>--</span>
<a name="line-15"></a><span class='hs-comment'>-- If the computation is to modify the stored information, use</span>
<a name="line-16"></a><span class='hs-comment'>-- "Control.Monad.Trans.State" instead.</span>
<a name="line-17"></a><span class='hs-comment'>-----------------------------------------------------------------------------</span>
<a name="line-18"></a>
<a name="line-19"></a><span class='hs-keyword'>module</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'>Trans</span><span class='hs-varop'>.</span><span class='hs-conid'>Reader</span> <span class='hs-layout'>(</span>
<a name="line-20"></a> <span class='hs-comment'>-- * The Reader monad</span>
<a name="line-21"></a> <span class='hs-conid'>Reader</span><span class='hs-layout'>,</span>
<a name="line-22"></a> <span class='hs-varid'>reader</span><span class='hs-layout'>,</span>
<a name="line-23"></a> <span class='hs-varid'>runReader</span><span class='hs-layout'>,</span>
<a name="line-24"></a> <span class='hs-varid'>mapReader</span><span class='hs-layout'>,</span>
<a name="line-25"></a> <span class='hs-varid'>withReader</span><span class='hs-layout'>,</span>
<a name="line-26"></a> <span class='hs-comment'>-- * The ReaderT monad transformer</span>
<a name="line-27"></a> <span class='hs-conid'>ReaderT</span><span class='hs-layout'>(</span><span class='hs-keyglyph'>..</span><span class='hs-layout'>)</span><span class='hs-layout'>,</span>
<a name="line-28"></a> <span class='hs-varid'>mapReaderT</span><span class='hs-layout'>,</span>
<a name="line-29"></a> <span class='hs-varid'>withReaderT</span><span class='hs-layout'>,</span>
<a name="line-30"></a> <span class='hs-comment'>-- * Reader operations</span>
<a name="line-31"></a> <span class='hs-varid'>ask</span><span class='hs-layout'>,</span>
<a name="line-32"></a> <span class='hs-varid'>local</span><span class='hs-layout'>,</span>
<a name="line-33"></a> <span class='hs-varid'>asks</span><span class='hs-layout'>,</span>
<a name="line-34"></a> <span class='hs-comment'>-- * Lifting other operations</span>
<a name="line-35"></a> <span class='hs-varid'>liftCallCC</span><span class='hs-layout'>,</span>
<a name="line-36"></a> <span class='hs-varid'>liftCatch</span><span class='hs-layout'>,</span>
<a name="line-37"></a> <span class='hs-layout'>)</span> <span class='hs-keyword'>where</span>
<a name="line-38"></a>
<a name="line-39"></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'>IO</span><span class='hs-varop'>.</span><span class='hs-conid'>Class</span>
<a name="line-40"></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'>Trans</span><span class='hs-varop'>.</span><span class='hs-conid'>Class</span>
<a name="line-41"></a><span class='hs-keyword'>import</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>
<a name="line-42"></a>
<a name="line-43"></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-44"></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-45"></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-46"></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'>Instances</span> <span class='hs-conid'>()</span>
<a name="line-47"></a>
<a name="line-48"></a><a name="Reader"></a><span class='hs-comment'>-- | The parameterizable reader monad.</span>
<a name="line-49"></a><a name="Reader"></a><span class='hs-comment'>--</span>
<a name="line-50"></a><a name="Reader"></a><span class='hs-comment'>-- Computations are functions of a shared environment.</span>
<a name="line-51"></a><a name="Reader"></a><span class='hs-comment'>--</span>
<a name="line-52"></a><a name="Reader"></a><span class='hs-comment'>-- The 'return' function ignores the environment, while @>>=@ passes</span>
<a name="line-53"></a><a name="Reader"></a><span class='hs-comment'>-- the inherited environment to both subcomputations.</span>
<a name="line-54"></a><a name="Reader"></a><span class='hs-keyword'>type</span> <span class='hs-conid'>Reader</span> <span class='hs-varid'>r</span> <span class='hs-keyglyph'>=</span> <span class='hs-conid'>ReaderT</span> <span class='hs-varid'>r</span> <span class='hs-conid'>Identity</span>
<a name="line-55"></a>
<a name="line-56"></a><a name="reader"></a><span class='hs-comment'>-- | Constructor for computations in the reader monad.</span>
<a name="line-57"></a><span class='hs-definition'>reader</span> <span class='hs-keyglyph'>::</span> <span class='hs-layout'>(</span><span class='hs-varid'>r</span> <span class='hs-keyglyph'>-></span> <span class='hs-varid'>a</span><span class='hs-layout'>)</span> <span class='hs-keyglyph'>-></span> <span class='hs-conid'>Reader</span> <span class='hs-varid'>r</span> <span class='hs-varid'>a</span>
<a name="line-58"></a><span class='hs-definition'>reader</span> <span class='hs-varid'>f</span> <span class='hs-keyglyph'>=</span> <span class='hs-conid'>ReaderT</span> <span class='hs-layout'>(</span><span class='hs-conid'>Identity</span> <span class='hs-varop'>.</span> <span class='hs-varid'>f</span><span class='hs-layout'>)</span>
<a name="line-59"></a>
<a name="line-60"></a><a name="runReader"></a><span class='hs-comment'>-- | Runs a @Reader@ and extracts the final value from it.</span>
<a name="line-61"></a><span class='hs-definition'>runReader</span> <span class='hs-keyglyph'>::</span> <span class='hs-conid'>Reader</span> <span class='hs-varid'>r</span> <span class='hs-varid'>a</span> <span class='hs-comment'>-- ^ A @Reader@ to run.</span>
<a name="line-62"></a> <span class='hs-keyglyph'>-></span> <span class='hs-varid'>r</span> <span class='hs-comment'>-- ^ An initial environment.</span>
<a name="line-63"></a> <span class='hs-keyglyph'>-></span> <span class='hs-varid'>a</span>
<a name="line-64"></a><span class='hs-definition'>runReader</span> <span class='hs-varid'>m</span> <span class='hs-keyglyph'>=</span> <span class='hs-varid'>runIdentity</span> <span class='hs-varop'>.</span> <span class='hs-varid'>runReaderT</span> <span class='hs-varid'>m</span>
<a name="line-65"></a>
<a name="line-66"></a><a name="mapReader"></a><span class='hs-comment'>-- | Transform the value returned by a @Reader@.</span>
<a name="line-67"></a><span class='hs-definition'>mapReader</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'>Reader</span> <span class='hs-varid'>r</span> <span class='hs-varid'>a</span> <span class='hs-keyglyph'>-></span> <span class='hs-conid'>Reader</span> <span class='hs-varid'>r</span> <span class='hs-varid'>b</span>
<a name="line-68"></a><span class='hs-definition'>mapReader</span> <span class='hs-varid'>f</span> <span class='hs-keyglyph'>=</span> <span class='hs-varid'>mapReaderT</span> <span class='hs-layout'>(</span><span class='hs-conid'>Identity</span> <span class='hs-varop'>.</span> <span class='hs-varid'>f</span> <span class='hs-varop'>.</span> <span class='hs-varid'>runIdentity</span><span class='hs-layout'>)</span>
<a name="line-69"></a>
<a name="line-70"></a><a name="withReader"></a><span class='hs-comment'>-- | Execute a computation in a modified environment</span>
<a name="line-71"></a><span class='hs-comment'>-- (a specialization of 'withReaderT').</span>
<a name="line-72"></a><span class='hs-definition'>withReader</span>
<a name="line-73"></a> <span class='hs-keyglyph'>::</span> <span class='hs-layout'>(</span><span class='hs-varid'>r'</span> <span class='hs-keyglyph'>-></span> <span class='hs-varid'>r</span><span class='hs-layout'>)</span> <span class='hs-comment'>-- ^ The function to modify the environment.</span>
<a name="line-74"></a> <span class='hs-keyglyph'>-></span> <span class='hs-conid'>Reader</span> <span class='hs-varid'>r</span> <span class='hs-varid'>a</span> <span class='hs-comment'>-- ^ Computation to run in the modified environment.</span>
<a name="line-75"></a> <span class='hs-keyglyph'>-></span> <span class='hs-conid'>Reader</span> <span class='hs-varid'>r'</span> <span class='hs-varid'>a</span>
<a name="line-76"></a><span class='hs-definition'>withReader</span> <span class='hs-keyglyph'>=</span> <span class='hs-varid'>withReaderT</span>
<a name="line-77"></a>
<a name="line-78"></a><a name="ReaderT"></a><span class='hs-comment'>-- | The reader monad transformer,</span>
<a name="line-79"></a><a name="ReaderT"></a><span class='hs-comment'>-- which adds a read-only environment to the given monad.</span>
<a name="line-80"></a><a name="ReaderT"></a><span class='hs-comment'>--</span>
<a name="line-81"></a><a name="ReaderT"></a><span class='hs-comment'>-- The 'return' function ignores the environment, while @>>=@ passes</span>
<a name="line-82"></a><a name="ReaderT"></a><span class='hs-comment'>-- the inherited environment to both subcomputations.</span>
<a name="line-83"></a><a name="ReaderT"></a><span class='hs-keyword'>newtype</span> <span class='hs-conid'>ReaderT</span> <span class='hs-varid'>r</span> <span class='hs-varid'>m</span> <span class='hs-varid'>a</span> <span class='hs-keyglyph'>=</span> <span class='hs-conid'>ReaderT</span> <span class='hs-layout'>{</span>
<a name="line-84"></a> <span class='hs-comment'>-- | The underlying computation, as a function of the environment.</span>
<a name="line-85"></a> <span class='hs-varid'>runReaderT</span> <span class='hs-keyglyph'>::</span> <span class='hs-varid'>r</span> <span class='hs-keyglyph'>-></span> <span class='hs-varid'>m</span> <span class='hs-varid'>a</span>
<a name="line-86"></a> <span class='hs-layout'>}</span>
<a name="line-87"></a>
<a name="line-88"></a><a name="mapReaderT"></a><span class='hs-comment'>-- | Transform the computation inside a @ReaderT@.</span>
<a name="line-89"></a><span class='hs-definition'>mapReaderT</span> <span class='hs-keyglyph'>::</span> <span class='hs-layout'>(</span><span class='hs-varid'>m</span> <span class='hs-varid'>a</span> <span class='hs-keyglyph'>-></span> <span class='hs-varid'>n</span> <span class='hs-varid'>b</span><span class='hs-layout'>)</span> <span class='hs-keyglyph'>-></span> <span class='hs-conid'>ReaderT</span> <span class='hs-varid'>r</span> <span class='hs-varid'>m</span> <span class='hs-varid'>a</span> <span class='hs-keyglyph'>-></span> <span class='hs-conid'>ReaderT</span> <span class='hs-varid'>r</span> <span class='hs-varid'>n</span> <span class='hs-varid'>b</span>
<a name="line-90"></a><span class='hs-definition'>mapReaderT</span> <span class='hs-varid'>f</span> <span class='hs-varid'>m</span> <span class='hs-keyglyph'>=</span> <span class='hs-conid'>ReaderT</span> <span class='hs-varop'>$</span> <span class='hs-varid'>f</span> <span class='hs-varop'>.</span> <span class='hs-varid'>runReaderT</span> <span class='hs-varid'>m</span>
<a name="line-91"></a>
<a name="line-92"></a><a name="withReaderT"></a><span class='hs-comment'>-- | Execute a computation in a modified environment</span>
<a name="line-93"></a><span class='hs-comment'>-- (a more general version of 'local').</span>
<a name="line-94"></a><span class='hs-definition'>withReaderT</span>
<a name="line-95"></a> <span class='hs-keyglyph'>::</span> <span class='hs-layout'>(</span><span class='hs-varid'>r'</span> <span class='hs-keyglyph'>-></span> <span class='hs-varid'>r</span><span class='hs-layout'>)</span> <span class='hs-comment'>-- ^ The function to modify the environment.</span>
<a name="line-96"></a> <span class='hs-keyglyph'>-></span> <span class='hs-conid'>ReaderT</span> <span class='hs-varid'>r</span> <span class='hs-varid'>m</span> <span class='hs-varid'>a</span> <span class='hs-comment'>-- ^ Computation to run in the modified environment.</span>
<a name="line-97"></a> <span class='hs-keyglyph'>-></span> <span class='hs-conid'>ReaderT</span> <span class='hs-varid'>r'</span> <span class='hs-varid'>m</span> <span class='hs-varid'>a</span>
<a name="line-98"></a><span class='hs-definition'>withReaderT</span> <span class='hs-varid'>f</span> <span class='hs-varid'>m</span> <span class='hs-keyglyph'>=</span> <span class='hs-conid'>ReaderT</span> <span class='hs-varop'>$</span> <span class='hs-varid'>runReaderT</span> <span class='hs-varid'>m</span> <span class='hs-varop'>.</span> <span class='hs-varid'>f</span>
<a name="line-99"></a>
<a name="line-100"></a><span class='hs-keyword'>instance</span> <span class='hs-layout'>(</span><span class='hs-conid'>Functor</span> <span class='hs-varid'>m</span><span class='hs-layout'>)</span> <span class='hs-keyglyph'>=></span> <span class='hs-conid'>Functor</span> <span class='hs-layout'>(</span><span class='hs-conid'>ReaderT</span> <span class='hs-varid'>r</span> <span class='hs-varid'>m</span><span class='hs-layout'>)</span> <span class='hs-keyword'>where</span>
<a name="line-101"></a> <span class='hs-varid'>fmap</span> <span class='hs-varid'>f</span> <span class='hs-keyglyph'>=</span> <span class='hs-varid'>mapReaderT</span> <span class='hs-layout'>(</span><span class='hs-varid'>fmap</span> <span class='hs-varid'>f</span><span class='hs-layout'>)</span>
<a name="line-102"></a>
<a name="line-103"></a><span class='hs-keyword'>instance</span> <span class='hs-layout'>(</span><span class='hs-conid'>Applicative</span> <span class='hs-varid'>m</span><span class='hs-layout'>)</span> <span class='hs-keyglyph'>=></span> <span class='hs-conid'>Applicative</span> <span class='hs-layout'>(</span><span class='hs-conid'>ReaderT</span> <span class='hs-varid'>r</span> <span class='hs-varid'>m</span><span class='hs-layout'>)</span> <span class='hs-keyword'>where</span>
<a name="line-104"></a> <span class='hs-varid'>pure</span> <span class='hs-keyglyph'>=</span> <span class='hs-varid'>liftReaderT</span> <span class='hs-varop'>.</span> <span class='hs-varid'>pure</span>
<a name="line-105"></a> <span class='hs-varid'>f</span> <span class='hs-varop'><*></span> <span class='hs-varid'>v</span> <span class='hs-keyglyph'>=</span> <span class='hs-conid'>ReaderT</span> <span class='hs-varop'>$</span> <span class='hs-keyglyph'>\</span> <span class='hs-varid'>r</span> <span class='hs-keyglyph'>-></span> <span class='hs-varid'>runReaderT</span> <span class='hs-varid'>f</span> <span class='hs-varid'>r</span> <span class='hs-varop'><*></span> <span class='hs-varid'>runReaderT</span> <span class='hs-varid'>v</span> <span class='hs-varid'>r</span>
<a name="line-106"></a>
<a name="line-107"></a><span class='hs-keyword'>instance</span> <span class='hs-layout'>(</span><span class='hs-conid'>Alternative</span> <span class='hs-varid'>m</span><span class='hs-layout'>)</span> <span class='hs-keyglyph'>=></span> <span class='hs-conid'>Alternative</span> <span class='hs-layout'>(</span><span class='hs-conid'>ReaderT</span> <span class='hs-varid'>r</span> <span class='hs-varid'>m</span><span class='hs-layout'>)</span> <span class='hs-keyword'>where</span>
<a name="line-108"></a> <span class='hs-varid'>empty</span> <span class='hs-keyglyph'>=</span> <span class='hs-varid'>liftReaderT</span> <span class='hs-varid'>empty</span>
<a name="line-109"></a> <span class='hs-varid'>m</span> <span class='hs-varop'><|></span> <span class='hs-varid'>n</span> <span class='hs-keyglyph'>=</span> <span class='hs-conid'>ReaderT</span> <span class='hs-varop'>$</span> <span class='hs-keyglyph'>\</span> <span class='hs-varid'>r</span> <span class='hs-keyglyph'>-></span> <span class='hs-varid'>runReaderT</span> <span class='hs-varid'>m</span> <span class='hs-varid'>r</span> <span class='hs-varop'><|></span> <span class='hs-varid'>runReaderT</span> <span class='hs-varid'>n</span> <span class='hs-varid'>r</span>
<a name="line-110"></a>
<a name="line-111"></a><span class='hs-keyword'>instance</span> <span class='hs-layout'>(</span><span class='hs-conid'>Monad</span> <span class='hs-varid'>m</span><span class='hs-layout'>)</span> <span class='hs-keyglyph'>=></span> <span class='hs-conid'>Monad</span> <span class='hs-layout'>(</span><span class='hs-conid'>ReaderT</span> <span class='hs-varid'>r</span> <span class='hs-varid'>m</span><span class='hs-layout'>)</span> <span class='hs-keyword'>where</span>
<a name="line-112"></a> <span class='hs-varid'>return</span> <span class='hs-keyglyph'>=</span> <span class='hs-varid'>lift</span> <span class='hs-varop'>.</span> <span class='hs-varid'>return</span>
<a name="line-113"></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-conid'>ReaderT</span> <span class='hs-varop'>$</span> <span class='hs-keyglyph'>\</span> <span class='hs-varid'>r</span> <span class='hs-keyglyph'>-></span> <span class='hs-keyword'>do</span>
<a name="line-114"></a> <span class='hs-varid'>a</span> <span class='hs-keyglyph'><-</span> <span class='hs-varid'>runReaderT</span> <span class='hs-varid'>m</span> <span class='hs-varid'>r</span>
<a name="line-115"></a> <span class='hs-varid'>runReaderT</span> <span class='hs-layout'>(</span><span class='hs-varid'>k</span> <span class='hs-varid'>a</span><span class='hs-layout'>)</span> <span class='hs-varid'>r</span>
<a name="line-116"></a> <span class='hs-varid'>fail</span> <span class='hs-varid'>msg</span> <span class='hs-keyglyph'>=</span> <span class='hs-varid'>lift</span> <span class='hs-layout'>(</span><span class='hs-varid'>fail</span> <span class='hs-varid'>msg</span><span class='hs-layout'>)</span>
<a name="line-117"></a>
<a name="line-118"></a><span class='hs-keyword'>instance</span> <span class='hs-layout'>(</span><span class='hs-conid'>MonadPlus</span> <span class='hs-varid'>m</span><span class='hs-layout'>)</span> <span class='hs-keyglyph'>=></span> <span class='hs-conid'>MonadPlus</span> <span class='hs-layout'>(</span><span class='hs-conid'>ReaderT</span> <span class='hs-varid'>r</span> <span class='hs-varid'>m</span><span class='hs-layout'>)</span> <span class='hs-keyword'>where</span>
<a name="line-119"></a> <span class='hs-varid'>mzero</span> <span class='hs-keyglyph'>=</span> <span class='hs-varid'>lift</span> <span class='hs-varid'>mzero</span>
<a name="line-120"></a> <span class='hs-varid'>m</span> <span class='hs-varop'>`mplus`</span> <span class='hs-varid'>n</span> <span class='hs-keyglyph'>=</span> <span class='hs-conid'>ReaderT</span> <span class='hs-varop'>$</span> <span class='hs-keyglyph'>\</span> <span class='hs-varid'>r</span> <span class='hs-keyglyph'>-></span> <span class='hs-varid'>runReaderT</span> <span class='hs-varid'>m</span> <span class='hs-varid'>r</span> <span class='hs-varop'>`mplus`</span> <span class='hs-varid'>runReaderT</span> <span class='hs-varid'>n</span> <span class='hs-varid'>r</span>
<a name="line-121"></a>
<a name="line-122"></a><span class='hs-keyword'>instance</span> <span class='hs-layout'>(</span><span class='hs-conid'>MonadFix</span> <span class='hs-varid'>m</span><span class='hs-layout'>)</span> <span class='hs-keyglyph'>=></span> <span class='hs-conid'>MonadFix</span> <span class='hs-layout'>(</span><span class='hs-conid'>ReaderT</span> <span class='hs-varid'>r</span> <span class='hs-varid'>m</span><span class='hs-layout'>)</span> <span class='hs-keyword'>where</span>
<a name="line-123"></a> <span class='hs-varid'>mfix</span> <span class='hs-varid'>f</span> <span class='hs-keyglyph'>=</span> <span class='hs-conid'>ReaderT</span> <span class='hs-varop'>$</span> <span class='hs-keyglyph'>\</span> <span class='hs-varid'>r</span> <span class='hs-keyglyph'>-></span> <span class='hs-varid'>mfix</span> <span class='hs-varop'>$</span> <span class='hs-keyglyph'>\</span> <span class='hs-varid'>a</span> <span class='hs-keyglyph'>-></span> <span class='hs-varid'>runReaderT</span> <span class='hs-layout'>(</span><span class='hs-varid'>f</span> <span class='hs-varid'>a</span><span class='hs-layout'>)</span> <span class='hs-varid'>r</span>
<a name="line-124"></a>
<a name="line-125"></a><span class='hs-keyword'>instance</span> <span class='hs-conid'>MonadTrans</span> <span class='hs-layout'>(</span><span class='hs-conid'>ReaderT</span> <span class='hs-varid'>r</span><span class='hs-layout'>)</span> <span class='hs-keyword'>where</span>
<a name="line-126"></a> <span class='hs-varid'>lift</span> <span class='hs-keyglyph'>=</span> <span class='hs-varid'>liftReaderT</span>
<a name="line-127"></a>
<a name="line-128"></a><span class='hs-keyword'>instance</span> <span class='hs-layout'>(</span><span class='hs-conid'>MonadIO</span> <span class='hs-varid'>m</span><span class='hs-layout'>)</span> <span class='hs-keyglyph'>=></span> <span class='hs-conid'>MonadIO</span> <span class='hs-layout'>(</span><span class='hs-conid'>ReaderT</span> <span class='hs-varid'>r</span> <span class='hs-varid'>m</span><span class='hs-layout'>)</span> <span class='hs-keyword'>where</span>
<a name="line-129"></a> <span class='hs-varid'>liftIO</span> <span class='hs-keyglyph'>=</span> <span class='hs-varid'>lift</span> <span class='hs-varop'>.</span> <span class='hs-varid'>liftIO</span>
<a name="line-130"></a>
<a name="line-131"></a><a name="liftReaderT"></a><span class='hs-definition'>liftReaderT</span> <span class='hs-keyglyph'>::</span> <span class='hs-varid'>m</span> <span class='hs-varid'>a</span> <span class='hs-keyglyph'>-></span> <span class='hs-conid'>ReaderT</span> <span class='hs-varid'>r</span> <span class='hs-varid'>m</span> <span class='hs-varid'>a</span>
<a name="line-132"></a><span class='hs-definition'>liftReaderT</span> <span class='hs-varid'>m</span> <span class='hs-keyglyph'>=</span> <span class='hs-conid'>ReaderT</span> <span class='hs-layout'>(</span><span class='hs-varid'>const</span> <span class='hs-varid'>m</span><span class='hs-layout'>)</span>
<a name="line-133"></a>
<a name="line-134"></a><a name="ask"></a><span class='hs-comment'>-- | Fetch the value of the environment.</span>
<a name="line-135"></a><span class='hs-definition'>ask</span> <span class='hs-keyglyph'>::</span> <span class='hs-layout'>(</span><span class='hs-conid'>Monad</span> <span class='hs-varid'>m</span><span class='hs-layout'>)</span> <span class='hs-keyglyph'>=></span> <span class='hs-conid'>ReaderT</span> <span class='hs-varid'>r</span> <span class='hs-varid'>m</span> <span class='hs-varid'>r</span>
<a name="line-136"></a><span class='hs-definition'>ask</span> <span class='hs-keyglyph'>=</span> <span class='hs-conid'>ReaderT</span> <span class='hs-varid'>return</span>
<a name="line-137"></a>
<a name="line-138"></a><a name="local"></a><span class='hs-comment'>-- | Execute a computation in a modified environment</span>
<a name="line-139"></a><span class='hs-comment'>-- (a specialization of 'withReaderT').</span>
<a name="line-140"></a><span class='hs-definition'>local</span> <span class='hs-keyglyph'>::</span> <span class='hs-layout'>(</span><span class='hs-conid'>Monad</span> <span class='hs-varid'>m</span><span class='hs-layout'>)</span>
<a name="line-141"></a> <span class='hs-keyglyph'>=></span> <span class='hs-layout'>(</span><span class='hs-varid'>r</span> <span class='hs-keyglyph'>-></span> <span class='hs-varid'>r</span><span class='hs-layout'>)</span> <span class='hs-comment'>-- ^ The function to modify the environment.</span>
<a name="line-142"></a> <span class='hs-keyglyph'>-></span> <span class='hs-conid'>ReaderT</span> <span class='hs-varid'>r</span> <span class='hs-varid'>m</span> <span class='hs-varid'>a</span> <span class='hs-comment'>-- ^ Computation to run in the modified environment.</span>
<a name="line-143"></a> <span class='hs-keyglyph'>-></span> <span class='hs-conid'>ReaderT</span> <span class='hs-varid'>r</span> <span class='hs-varid'>m</span> <span class='hs-varid'>a</span>
<a name="line-144"></a><span class='hs-definition'>local</span> <span class='hs-keyglyph'>=</span> <span class='hs-varid'>withReaderT</span>
<a name="line-145"></a>
<a name="line-146"></a><a name="asks"></a><span class='hs-comment'>-- | Retrieve a function of the current environment.</span>
<a name="line-147"></a><span class='hs-definition'>asks</span> <span class='hs-keyglyph'>::</span> <span class='hs-layout'>(</span><span class='hs-conid'>Monad</span> <span class='hs-varid'>m</span><span class='hs-layout'>)</span>
<a name="line-148"></a> <span class='hs-keyglyph'>=></span> <span class='hs-layout'>(</span><span class='hs-varid'>r</span> <span class='hs-keyglyph'>-></span> <span class='hs-varid'>a</span><span class='hs-layout'>)</span> <span class='hs-comment'>-- ^ The selector function to apply to the environment.</span>
<a name="line-149"></a> <span class='hs-keyglyph'>-></span> <span class='hs-conid'>ReaderT</span> <span class='hs-varid'>r</span> <span class='hs-varid'>m</span> <span class='hs-varid'>a</span>
<a name="line-150"></a><span class='hs-definition'>asks</span> <span class='hs-varid'>f</span> <span class='hs-keyglyph'>=</span> <span class='hs-varid'>liftM</span> <span class='hs-varid'>f</span> <span class='hs-varid'>ask</span>
<a name="line-151"></a>
<a name="line-152"></a><a name="liftCallCC"></a><span class='hs-comment'>-- | Lift a @callCC@ operation to the new monad.</span>
<a name="line-153"></a><span class='hs-definition'>liftCallCC</span> <span class='hs-keyglyph'>::</span>
<a name="line-154"></a> <span class='hs-layout'>(</span><span class='hs-layout'>(</span><span class='hs-layout'>(</span><span class='hs-varid'>a</span> <span class='hs-keyglyph'>-></span> <span class='hs-varid'>m</span> <span class='hs-varid'>b</span><span class='hs-layout'>)</span> <span class='hs-keyglyph'>-></span> <span class='hs-varid'>m</span> <span class='hs-varid'>a</span><span class='hs-layout'>)</span> <span class='hs-keyglyph'>-></span> <span class='hs-varid'>m</span> <span class='hs-varid'>a</span><span class='hs-layout'>)</span> <span class='hs-comment'>-- ^ @callCC@ on the argument monad.</span>
<a name="line-155"></a> <span class='hs-keyglyph'>-></span> <span class='hs-layout'>(</span><span class='hs-layout'>(</span><span class='hs-varid'>a</span> <span class='hs-keyglyph'>-></span> <span class='hs-conid'>ReaderT</span> <span class='hs-varid'>r</span> <span class='hs-varid'>m</span> <span class='hs-varid'>b</span><span class='hs-layout'>)</span> <span class='hs-keyglyph'>-></span> <span class='hs-conid'>ReaderT</span> <span class='hs-varid'>r</span> <span class='hs-varid'>m</span> <span class='hs-varid'>a</span><span class='hs-layout'>)</span> <span class='hs-keyglyph'>-></span> <span class='hs-conid'>ReaderT</span> <span class='hs-varid'>r</span> <span class='hs-varid'>m</span> <span class='hs-varid'>a</span>
<a name="line-156"></a><span class='hs-definition'>liftCallCC</span> <span class='hs-varid'>callCC</span> <span class='hs-varid'>f</span> <span class='hs-keyglyph'>=</span> <span class='hs-conid'>ReaderT</span> <span class='hs-varop'>$</span> <span class='hs-keyglyph'>\</span> <span class='hs-varid'>r</span> <span class='hs-keyglyph'>-></span>
<a name="line-157"></a> <span class='hs-varid'>callCC</span> <span class='hs-varop'>$</span> <span class='hs-keyglyph'>\</span> <span class='hs-varid'>c</span> <span class='hs-keyglyph'>-></span>
<a name="line-158"></a> <span class='hs-varid'>runReaderT</span> <span class='hs-layout'>(</span><span class='hs-varid'>f</span> <span class='hs-layout'>(</span><span class='hs-conid'>ReaderT</span> <span class='hs-varop'>.</span> <span class='hs-varid'>const</span> <span class='hs-varop'>.</span> <span class='hs-varid'>c</span><span class='hs-layout'>)</span><span class='hs-layout'>)</span> <span class='hs-varid'>r</span>
<a name="line-159"></a>
<a name="line-160"></a><a name="liftCatch"></a><span class='hs-comment'>-- | Lift a @catchError@ operation to the new monad.</span>
<a name="line-161"></a><span class='hs-definition'>liftCatch</span> <span class='hs-keyglyph'>::</span>
<a name="line-162"></a> <span class='hs-layout'>(</span><span class='hs-varid'>m</span> <span class='hs-varid'>a</span> <span class='hs-keyglyph'>-></span> <span class='hs-layout'>(</span><span class='hs-varid'>e</span> <span class='hs-keyglyph'>-></span> <span class='hs-varid'>m</span> <span class='hs-varid'>a</span><span class='hs-layout'>)</span> <span class='hs-keyglyph'>-></span> <span class='hs-varid'>m</span> <span class='hs-varid'>a</span><span class='hs-layout'>)</span> <span class='hs-comment'>-- ^ @catch@ on the argument monad.</span>
<a name="line-163"></a> <span class='hs-keyglyph'>-></span> <span class='hs-conid'>ReaderT</span> <span class='hs-varid'>r</span> <span class='hs-varid'>m</span> <span class='hs-varid'>a</span> <span class='hs-comment'>-- ^ Computation to attempt.</span>
<a name="line-164"></a> <span class='hs-keyglyph'>-></span> <span class='hs-layout'>(</span><span class='hs-varid'>e</span> <span class='hs-keyglyph'>-></span> <span class='hs-conid'>ReaderT</span> <span class='hs-varid'>r</span> <span class='hs-varid'>m</span> <span class='hs-varid'>a</span><span class='hs-layout'>)</span> <span class='hs-comment'>-- ^ Exception handler.</span>
<a name="line-165"></a> <span class='hs-keyglyph'>-></span> <span class='hs-conid'>ReaderT</span> <span class='hs-varid'>r</span> <span class='hs-varid'>m</span> <span class='hs-varid'>a</span>
<a name="line-166"></a><span class='hs-definition'>liftCatch</span> <span class='hs-varid'>f</span> <span class='hs-varid'>m</span> <span class='hs-varid'>h</span> <span class='hs-keyglyph'>=</span>
<a name="line-167"></a> <span class='hs-conid'>ReaderT</span> <span class='hs-varop'>$</span> <span class='hs-keyglyph'>\</span> <span class='hs-varid'>r</span> <span class='hs-keyglyph'>-></span> <span class='hs-varid'>f</span> <span class='hs-layout'>(</span><span class='hs-varid'>runReaderT</span> <span class='hs-varid'>m</span> <span class='hs-varid'>r</span><span class='hs-layout'>)</span> <span class='hs-layout'>(</span><span class='hs-keyglyph'>\</span> <span class='hs-varid'>e</span> <span class='hs-keyglyph'>-></span> <span class='hs-varid'>runReaderT</span> <span class='hs-layout'>(</span><span class='hs-varid'>h</span> <span class='hs-varid'>e</span><span class='hs-layout'>)</span> <span class='hs-varid'>r</span><span class='hs-layout'>)</span>
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