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<pre><a name="line-1"></a><span class='hs-comment'>{-# LANGUAGE CPP #-}</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 : Control.Monad.Trans.Error</span>
<a name="line-5"></a><span class='hs-comment'>-- Copyright : (c) Michael Weber <michael.weber@post.rwth-aachen.de> 2001,</span>
<a name="line-6"></a><span class='hs-comment'>-- (c) Jeff Newbern 2003-2006,</span>
<a name="line-7"></a><span class='hs-comment'>-- (c) Andriy Palamarchuk 2006</span>
<a name="line-8"></a><span class='hs-comment'>-- License : BSD-style (see the file LICENSE)</span>
<a name="line-9"></a><span class='hs-comment'>--</span>
<a name="line-10"></a><span class='hs-comment'>-- Maintainer : ross@soi.city.ac.uk</span>
<a name="line-11"></a><span class='hs-comment'>-- Stability : experimental</span>
<a name="line-12"></a><span class='hs-comment'>-- Portability : portable</span>
<a name="line-13"></a><span class='hs-comment'>--</span>
<a name="line-14"></a><span class='hs-comment'>-- This monad transformer adds the ability to fail or throw exceptions</span>
<a name="line-15"></a><span class='hs-comment'>-- to a monad.</span>
<a name="line-16"></a><span class='hs-comment'>--</span>
<a name="line-17"></a><span class='hs-comment'>-- A sequence of actions succeeds, producing a value, only if all the</span>
<a name="line-18"></a><span class='hs-comment'>-- actions in the sequence are successful. If one fails with an error,</span>
<a name="line-19"></a><span class='hs-comment'>-- the rest of the sequence is skipped and the composite action fails</span>
<a name="line-20"></a><span class='hs-comment'>-- with that error.</span>
<a name="line-21"></a><span class='hs-comment'>--</span>
<a name="line-22"></a><span class='hs-comment'>-- If the value of the error is not required, the variant in</span>
<a name="line-23"></a><span class='hs-comment'>-- "Control.Monad.Trans.Maybe" may be used instead.</span>
<a name="line-24"></a><span class='hs-comment'>-----------------------------------------------------------------------------</span>
<a name="line-25"></a>
<a name="line-26"></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'>Error</span> <span class='hs-layout'>(</span>
<a name="line-27"></a> <span class='hs-comment'>-- * The ErrorT monad transformer</span>
<a name="line-28"></a> <span class='hs-conid'>Error</span><span class='hs-layout'>(</span><span class='hs-keyglyph'>..</span><span class='hs-layout'>)</span><span class='hs-layout'>,</span>
<a name="line-29"></a> <span class='hs-conid'>ErrorList</span><span class='hs-layout'>(</span><span class='hs-keyglyph'>..</span><span class='hs-layout'>)</span><span class='hs-layout'>,</span>
<a name="line-30"></a> <span class='hs-conid'>ErrorT</span><span class='hs-layout'>(</span><span class='hs-keyglyph'>..</span><span class='hs-layout'>)</span><span class='hs-layout'>,</span>
<a name="line-31"></a> <span class='hs-varid'>mapErrorT</span><span class='hs-layout'>,</span>
<a name="line-32"></a> <span class='hs-comment'>-- * Error operations</span>
<a name="line-33"></a> <span class='hs-varid'>throwError</span><span class='hs-layout'>,</span>
<a name="line-34"></a> <span class='hs-varid'>catchError</span><span class='hs-layout'>,</span>
<a name="line-35"></a> <span class='hs-comment'>-- * Lifting other operations</span>
<a name="line-36"></a> <span class='hs-varid'>liftCallCC</span><span class='hs-layout'>,</span>
<a name="line-37"></a> <span class='hs-varid'>liftListen</span><span class='hs-layout'>,</span>
<a name="line-38"></a> <span class='hs-varid'>liftPass</span><span class='hs-layout'>,</span>
<a name="line-39"></a> <span class='hs-comment'>-- * Examples</span>
<a name="line-40"></a> <span class='hs-comment'>-- $examples</span>
<a name="line-41"></a> <span class='hs-layout'>)</span> <span class='hs-keyword'>where</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'>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-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><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-45"></a>
<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'>Applicative</span>
<a name="line-47"></a><span class='hs-keyword'>import</span> <span class='hs-conid'>Control</span><span class='hs-varop'>.</span><span class='hs-conid'>Exception</span> <span class='hs-layout'>(</span><span class='hs-conid'>IOException</span><span class='hs-layout'>)</span>
<a name="line-48"></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-49"></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-50"></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-51"></a><span class='hs-keyword'>import</span> <span class='hs-conid'>System</span><span class='hs-varop'>.</span><span class='hs-conid'>IO</span>
<a name="line-52"></a>
<a name="line-53"></a><span class='hs-keyword'>instance</span> <span class='hs-conid'>MonadPlus</span> <span class='hs-conid'>IO</span> <span class='hs-keyword'>where</span>
<a name="line-54"></a> <span class='hs-varid'>mzero</span> <span class='hs-keyglyph'>=</span> <span class='hs-varid'>ioError</span> <span class='hs-layout'>(</span><span class='hs-varid'>userError</span> <span class='hs-str'>"mzero"</span><span class='hs-layout'>)</span>
<a name="line-55"></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-varid'>m</span> <span class='hs-varop'>`catch`</span> <span class='hs-keyglyph'>\</span><span class='hs-keyword'>_</span> <span class='hs-keyglyph'>-></span> <span class='hs-varid'>n</span>
<a name="line-56"></a>
<a name="line-57"></a><a name="Error"></a><span class='hs-comment'>-- | An exception to be thrown.</span>
<a name="line-58"></a><a name="Error"></a><span class='hs-comment'>--</span>
<a name="line-59"></a><a name="Error"></a><span class='hs-comment'>-- Minimal complete definition: 'noMsg' or 'strMsg'.</span>
<a name="line-60"></a><a name="Error"></a><span class='hs-keyword'>class</span> <span class='hs-conid'>Error</span> <span class='hs-varid'>a</span> <span class='hs-keyword'>where</span>
<a name="line-61"></a> <span class='hs-comment'>-- | Creates an exception without a message.</span>
<a name="line-62"></a> <span class='hs-comment'>-- The default implementation is @'strMsg' \"\"@.</span>
<a name="line-63"></a> <span class='hs-varid'>noMsg</span> <span class='hs-keyglyph'>::</span> <span class='hs-varid'>a</span>
<a name="line-64"></a> <span class='hs-comment'>-- | Creates an exception with a message.</span>
<a name="line-65"></a> <span class='hs-comment'>-- The default implementation of @'strMsg' s@ is 'noMsg'.</span>
<a name="line-66"></a> <span class='hs-varid'>strMsg</span> <span class='hs-keyglyph'>::</span> <span class='hs-conid'>String</span> <span class='hs-keyglyph'>-></span> <span class='hs-varid'>a</span>
<a name="line-67"></a>
<a name="line-68"></a> <span class='hs-varid'>noMsg</span> <span class='hs-keyglyph'>=</span> <span class='hs-varid'>strMsg</span> <span class='hs-str'>""</span>
<a name="line-69"></a> <span class='hs-varid'>strMsg</span> <span class='hs-keyword'>_</span> <span class='hs-keyglyph'>=</span> <span class='hs-varid'>noMsg</span>
<a name="line-70"></a>
<a name="line-71"></a><span class='hs-keyword'>instance</span> <span class='hs-conid'>Error</span> <span class='hs-conid'>IOException</span> <span class='hs-keyword'>where</span>
<a name="line-72"></a> <span class='hs-varid'>strMsg</span> <span class='hs-keyglyph'>=</span> <span class='hs-varid'>userError</span>
<a name="line-73"></a>
<a name="line-74"></a><span class='hs-comment'>-- | A string can be thrown as an error.</span>
<a name="line-75"></a><span class='hs-keyword'>instance</span> <span class='hs-conid'>ErrorList</span> <span class='hs-varid'>a</span> <span class='hs-keyglyph'>=></span> <span class='hs-conid'>Error</span> <span class='hs-keyglyph'>[</span><span class='hs-varid'>a</span><span class='hs-keyglyph'>]</span> <span class='hs-keyword'>where</span>
<a name="line-76"></a> <span class='hs-varid'>strMsg</span> <span class='hs-keyglyph'>=</span> <span class='hs-varid'>listMsg</span>
<a name="line-77"></a>
<a name="line-78"></a><a name="ErrorList"></a><span class='hs-comment'>-- | Workaround so that we can have a Haskell 98 instance @'Error' 'String'@.</span>
<a name="line-79"></a><a name="ErrorList"></a><span class='hs-keyword'>class</span> <span class='hs-conid'>ErrorList</span> <span class='hs-varid'>a</span> <span class='hs-keyword'>where</span>
<a name="line-80"></a> <span class='hs-varid'>listMsg</span> <span class='hs-keyglyph'>::</span> <span class='hs-conid'>String</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-81"></a>
<a name="line-82"></a><span class='hs-keyword'>instance</span> <span class='hs-conid'>ErrorList</span> <span class='hs-conid'>Char</span> <span class='hs-keyword'>where</span>
<a name="line-83"></a> <span class='hs-varid'>listMsg</span> <span class='hs-keyglyph'>=</span> <span class='hs-varid'>id</span>
<a name="line-84"></a>
<a name="line-85"></a><span class='hs-comment'>-- ---------------------------------------------------------------------------</span>
<a name="line-86"></a><span class='hs-comment'>-- Our parameterizable error monad</span>
<a name="line-87"></a>
<a name="line-88"></a><span class='hs-cpp'>#if !(MIN_VERSION_base(4,2,1))</span>
<a name="line-89"></a>
<a name="line-90"></a><span class='hs-comment'>-- These instances are in base-4.3</span>
<a name="line-91"></a>
<a name="line-92"></a><span class='hs-keyword'>instance</span> <span class='hs-conid'>Applicative</span> <span class='hs-layout'>(</span><span class='hs-conid'>Either</span> <span class='hs-varid'>e</span><span class='hs-layout'>)</span> <span class='hs-keyword'>where</span>
<a name="line-93"></a> <span class='hs-varid'>pure</span> <span class='hs-keyglyph'>=</span> <span class='hs-conid'>Right</span>
<a name="line-94"></a> <span class='hs-conid'>Left</span> <span class='hs-varid'>e</span> <span class='hs-varop'><*></span> <span class='hs-keyword'>_</span> <span class='hs-keyglyph'>=</span> <span class='hs-conid'>Left</span> <span class='hs-varid'>e</span>
<a name="line-95"></a> <span class='hs-conid'>Right</span> <span class='hs-varid'>f</span> <span class='hs-varop'><*></span> <span class='hs-varid'>r</span> <span class='hs-keyglyph'>=</span> <span class='hs-varid'>fmap</span> <span class='hs-varid'>f</span> <span class='hs-varid'>r</span>
<a name="line-96"></a>
<a name="line-97"></a><span class='hs-keyword'>instance</span> <span class='hs-conid'>Monad</span> <span class='hs-layout'>(</span><span class='hs-conid'>Either</span> <span class='hs-varid'>e</span><span class='hs-layout'>)</span> <span class='hs-keyword'>where</span>
<a name="line-98"></a> <span class='hs-varid'>return</span> <span class='hs-keyglyph'>=</span> <span class='hs-conid'>Right</span>
<a name="line-99"></a> <span class='hs-conid'>Left</span> <span class='hs-varid'>l</span> <span class='hs-varop'>>>=</span> <span class='hs-keyword'>_</span> <span class='hs-keyglyph'>=</span> <span class='hs-conid'>Left</span> <span class='hs-varid'>l</span>
<a name="line-100"></a> <span class='hs-conid'>Right</span> <span class='hs-varid'>r</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'>r</span>
<a name="line-101"></a>
<a name="line-102"></a><span class='hs-keyword'>instance</span> <span class='hs-conid'>MonadFix</span> <span class='hs-layout'>(</span><span class='hs-conid'>Either</span> <span class='hs-varid'>e</span><span class='hs-layout'>)</span> <span class='hs-keyword'>where</span>
<a name="line-103"></a> <span class='hs-varid'>mfix</span> <span class='hs-varid'>f</span> <span class='hs-keyglyph'>=</span> <span class='hs-keyword'>let</span>
<a name="line-104"></a> <span class='hs-varid'>a</span> <span class='hs-keyglyph'>=</span> <span class='hs-varid'>f</span> <span class='hs-varop'>$</span> <span class='hs-keyword'>case</span> <span class='hs-varid'>a</span> <span class='hs-keyword'>of</span>
<a name="line-105"></a> <span class='hs-conid'>Right</span> <span class='hs-varid'>r</span> <span class='hs-keyglyph'>-></span> <span class='hs-varid'>r</span>
<a name="line-106"></a> <span class='hs-keyword'>_</span> <span class='hs-keyglyph'>-></span> <span class='hs-varid'>error</span> <span class='hs-str'>"empty mfix argument"</span>
<a name="line-107"></a> <span class='hs-keyword'>in</span> <span class='hs-varid'>a</span>
<a name="line-108"></a>
<a name="line-109"></a><span class='hs-cpp'>#endif /* base to 4.2.0.x */</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'>Error</span> <span class='hs-varid'>e</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'>Either</span> <span class='hs-varid'>e</span><span class='hs-layout'>)</span> <span class='hs-keyword'>where</span>
<a name="line-112"></a> <span class='hs-varid'>empty</span> <span class='hs-keyglyph'>=</span> <span class='hs-conid'>Left</span> <span class='hs-varid'>noMsg</span>
<a name="line-113"></a> <span class='hs-conid'>Left</span> <span class='hs-keyword'>_</span> <span class='hs-varop'><|></span> <span class='hs-varid'>n</span> <span class='hs-keyglyph'>=</span> <span class='hs-varid'>n</span>
<a name="line-114"></a> <span class='hs-varid'>m</span> <span class='hs-varop'><|></span> <span class='hs-keyword'>_</span> <span class='hs-keyglyph'>=</span> <span class='hs-varid'>m</span>
<a name="line-115"></a>
<a name="line-116"></a><span class='hs-keyword'>instance</span> <span class='hs-layout'>(</span><span class='hs-conid'>Error</span> <span class='hs-varid'>e</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'>Either</span> <span class='hs-varid'>e</span><span class='hs-layout'>)</span> <span class='hs-keyword'>where</span>
<a name="line-117"></a> <span class='hs-varid'>mzero</span> <span class='hs-keyglyph'>=</span> <span class='hs-conid'>Left</span> <span class='hs-varid'>noMsg</span>
<a name="line-118"></a> <span class='hs-conid'>Left</span> <span class='hs-keyword'>_</span> <span class='hs-varop'>`mplus`</span> <span class='hs-varid'>n</span> <span class='hs-keyglyph'>=</span> <span class='hs-varid'>n</span>
<a name="line-119"></a> <span class='hs-varid'>m</span> <span class='hs-varop'>`mplus`</span> <span class='hs-keyword'>_</span> <span class='hs-keyglyph'>=</span> <span class='hs-varid'>m</span>
<a name="line-120"></a>
<a name="line-121"></a><a name="ErrorT"></a><span class='hs-comment'>-- | The error monad transformer. It can be used to add error handling</span>
<a name="line-122"></a><a name="ErrorT"></a><span class='hs-comment'>-- to other monads.</span>
<a name="line-123"></a><a name="ErrorT"></a><span class='hs-comment'>--</span>
<a name="line-124"></a><a name="ErrorT"></a><span class='hs-comment'>-- The @ErrorT@ Monad structure is parameterized over two things:</span>
<a name="line-125"></a><a name="ErrorT"></a><span class='hs-comment'>--</span>
<a name="line-126"></a><a name="ErrorT"></a><span class='hs-comment'>-- * e - The error type.</span>
<a name="line-127"></a><a name="ErrorT"></a><span class='hs-comment'>--</span>
<a name="line-128"></a><a name="ErrorT"></a><span class='hs-comment'>-- * m - The inner monad.</span>
<a name="line-129"></a><a name="ErrorT"></a><span class='hs-comment'>--</span>
<a name="line-130"></a><a name="ErrorT"></a><span class='hs-comment'>-- The 'return' function yields a successful computation, while @>>=@</span>
<a name="line-131"></a><a name="ErrorT"></a><span class='hs-comment'>-- sequences two subcomputations, failing on the first error.</span>
<a name="line-132"></a><a name="ErrorT"></a><span class='hs-keyword'>newtype</span> <span class='hs-conid'>ErrorT</span> <span class='hs-varid'>e</span> <span class='hs-varid'>m</span> <span class='hs-varid'>a</span> <span class='hs-keyglyph'>=</span> <span class='hs-conid'>ErrorT</span> <span class='hs-layout'>{</span> <span class='hs-varid'>runErrorT</span> <span class='hs-keyglyph'>::</span> <span class='hs-varid'>m</span> <span class='hs-layout'>(</span><span class='hs-conid'>Either</span> <span class='hs-varid'>e</span> <span class='hs-varid'>a</span><span class='hs-layout'>)</span> <span class='hs-layout'>}</span>
<a name="line-133"></a>
<a name="line-134"></a><a name="mapErrorT"></a><span class='hs-comment'>-- | Map the unwrapped computation using the given function.</span>
<a name="line-135"></a><span class='hs-comment'>--</span>
<a name="line-136"></a><span class='hs-comment'>-- * @'runErrorT' ('mapErrorT' f m) = f ('runErrorT' m@)</span>
<a name="line-137"></a><span class='hs-definition'>mapErrorT</span> <span class='hs-keyglyph'>::</span> <span class='hs-layout'>(</span><span class='hs-varid'>m</span> <span class='hs-layout'>(</span><span class='hs-conid'>Either</span> <span class='hs-varid'>e</span> <span class='hs-varid'>a</span><span class='hs-layout'>)</span> <span class='hs-keyglyph'>-></span> <span class='hs-varid'>n</span> <span class='hs-layout'>(</span><span class='hs-conid'>Either</span> <span class='hs-varid'>e'</span> <span class='hs-varid'>b</span><span class='hs-layout'>)</span><span class='hs-layout'>)</span>
<a name="line-138"></a> <span class='hs-keyglyph'>-></span> <span class='hs-conid'>ErrorT</span> <span class='hs-varid'>e</span> <span class='hs-varid'>m</span> <span class='hs-varid'>a</span>
<a name="line-139"></a> <span class='hs-keyglyph'>-></span> <span class='hs-conid'>ErrorT</span> <span class='hs-varid'>e'</span> <span class='hs-varid'>n</span> <span class='hs-varid'>b</span>
<a name="line-140"></a><span class='hs-definition'>mapErrorT</span> <span class='hs-varid'>f</span> <span class='hs-varid'>m</span> <span class='hs-keyglyph'>=</span> <span class='hs-conid'>ErrorT</span> <span class='hs-varop'>$</span> <span class='hs-varid'>f</span> <span class='hs-layout'>(</span><span class='hs-varid'>runErrorT</span> <span class='hs-varid'>m</span><span class='hs-layout'>)</span>
<a name="line-141"></a>
<a name="line-142"></a><span class='hs-keyword'>instance</span> <span class='hs-layout'>(</span><span class='hs-conid'>Functor</span> <span class='hs-varid'>m</span><span class='hs-layout'>)</span> <span class='hs-keyglyph'>=></span> <span class='hs-conid'>Functor</span> <span class='hs-layout'>(</span><span class='hs-conid'>ErrorT</span> <span class='hs-varid'>e</span> <span class='hs-varid'>m</span><span class='hs-layout'>)</span> <span class='hs-keyword'>where</span>
<a name="line-143"></a> <span class='hs-varid'>fmap</span> <span class='hs-varid'>f</span> <span class='hs-keyglyph'>=</span> <span class='hs-conid'>ErrorT</span> <span class='hs-varop'>.</span> <span class='hs-varid'>fmap</span> <span class='hs-layout'>(</span><span class='hs-varid'>fmap</span> <span class='hs-varid'>f</span><span class='hs-layout'>)</span> <span class='hs-varop'>.</span> <span class='hs-varid'>runErrorT</span>
<a name="line-144"></a>
<a name="line-145"></a><span class='hs-keyword'>instance</span> <span class='hs-layout'>(</span><span class='hs-conid'>Functor</span> <span class='hs-varid'>m</span><span class='hs-layout'>,</span> <span class='hs-conid'>Monad</span> <span class='hs-varid'>m</span><span class='hs-layout'>)</span> <span class='hs-keyglyph'>=></span> <span class='hs-conid'>Applicative</span> <span class='hs-layout'>(</span><span class='hs-conid'>ErrorT</span> <span class='hs-varid'>e</span> <span class='hs-varid'>m</span><span class='hs-layout'>)</span> <span class='hs-keyword'>where</span>
<a name="line-146"></a> <span class='hs-varid'>pure</span> <span class='hs-varid'>a</span> <span class='hs-keyglyph'>=</span> <span class='hs-conid'>ErrorT</span> <span class='hs-varop'>$</span> <span class='hs-varid'>return</span> <span class='hs-layout'>(</span><span class='hs-conid'>Right</span> <span class='hs-varid'>a</span><span class='hs-layout'>)</span>
<a name="line-147"></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'>ErrorT</span> <span class='hs-varop'>$</span> <span class='hs-keyword'>do</span>
<a name="line-148"></a> <span class='hs-varid'>mf</span> <span class='hs-keyglyph'><-</span> <span class='hs-varid'>runErrorT</span> <span class='hs-varid'>f</span>
<a name="line-149"></a> <span class='hs-keyword'>case</span> <span class='hs-varid'>mf</span> <span class='hs-keyword'>of</span>
<a name="line-150"></a> <span class='hs-conid'>Left</span> <span class='hs-varid'>e</span> <span class='hs-keyglyph'>-></span> <span class='hs-varid'>return</span> <span class='hs-layout'>(</span><span class='hs-conid'>Left</span> <span class='hs-varid'>e</span><span class='hs-layout'>)</span>
<a name="line-151"></a> <span class='hs-conid'>Right</span> <span class='hs-varid'>k</span> <span class='hs-keyglyph'>-></span> <span class='hs-keyword'>do</span>
<a name="line-152"></a> <span class='hs-varid'>mv</span> <span class='hs-keyglyph'><-</span> <span class='hs-varid'>runErrorT</span> <span class='hs-varid'>v</span>
<a name="line-153"></a> <span class='hs-keyword'>case</span> <span class='hs-varid'>mv</span> <span class='hs-keyword'>of</span>
<a name="line-154"></a> <span class='hs-conid'>Left</span> <span class='hs-varid'>e</span> <span class='hs-keyglyph'>-></span> <span class='hs-varid'>return</span> <span class='hs-layout'>(</span><span class='hs-conid'>Left</span> <span class='hs-varid'>e</span><span class='hs-layout'>)</span>
<a name="line-155"></a> <span class='hs-conid'>Right</span> <span class='hs-varid'>x</span> <span class='hs-keyglyph'>-></span> <span class='hs-varid'>return</span> <span class='hs-layout'>(</span><span class='hs-conid'>Right</span> <span class='hs-layout'>(</span><span class='hs-varid'>k</span> <span class='hs-varid'>x</span><span class='hs-layout'>)</span><span class='hs-layout'>)</span>
<a name="line-156"></a>
<a name="line-157"></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-conid'>Monad</span> <span class='hs-varid'>m</span><span class='hs-layout'>,</span> <span class='hs-conid'>Error</span> <span class='hs-varid'>e</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'>ErrorT</span> <span class='hs-varid'>e</span> <span class='hs-varid'>m</span><span class='hs-layout'>)</span> <span class='hs-keyword'>where</span>
<a name="line-158"></a> <span class='hs-varid'>empty</span> <span class='hs-keyglyph'>=</span> <span class='hs-varid'>mzero</span>
<a name="line-159"></a> <span class='hs-layout'>(</span><span class='hs-varop'><|></span><span class='hs-layout'>)</span> <span class='hs-keyglyph'>=</span> <span class='hs-varid'>mplus</span>
<a name="line-160"></a>
<a name="line-161"></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-conid'>Error</span> <span class='hs-varid'>e</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'>ErrorT</span> <span class='hs-varid'>e</span> <span class='hs-varid'>m</span><span class='hs-layout'>)</span> <span class='hs-keyword'>where</span>
<a name="line-162"></a> <span class='hs-varid'>return</span> <span class='hs-varid'>a</span> <span class='hs-keyglyph'>=</span> <span class='hs-conid'>ErrorT</span> <span class='hs-varop'>$</span> <span class='hs-varid'>return</span> <span class='hs-layout'>(</span><span class='hs-conid'>Right</span> <span class='hs-varid'>a</span><span class='hs-layout'>)</span>
<a name="line-163"></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'>ErrorT</span> <span class='hs-varop'>$</span> <span class='hs-keyword'>do</span>
<a name="line-164"></a> <span class='hs-varid'>a</span> <span class='hs-keyglyph'><-</span> <span class='hs-varid'>runErrorT</span> <span class='hs-varid'>m</span>
<a name="line-165"></a> <span class='hs-keyword'>case</span> <span class='hs-varid'>a</span> <span class='hs-keyword'>of</span>
<a name="line-166"></a> <span class='hs-conid'>Left</span> <span class='hs-varid'>l</span> <span class='hs-keyglyph'>-></span> <span class='hs-varid'>return</span> <span class='hs-layout'>(</span><span class='hs-conid'>Left</span> <span class='hs-varid'>l</span><span class='hs-layout'>)</span>
<a name="line-167"></a> <span class='hs-conid'>Right</span> <span class='hs-varid'>r</span> <span class='hs-keyglyph'>-></span> <span class='hs-varid'>runErrorT</span> <span class='hs-layout'>(</span><span class='hs-varid'>k</span> <span class='hs-varid'>r</span><span class='hs-layout'>)</span>
<a name="line-168"></a> <span class='hs-varid'>fail</span> <span class='hs-varid'>msg</span> <span class='hs-keyglyph'>=</span> <span class='hs-conid'>ErrorT</span> <span class='hs-varop'>$</span> <span class='hs-varid'>return</span> <span class='hs-layout'>(</span><span class='hs-conid'>Left</span> <span class='hs-layout'>(</span><span class='hs-varid'>strMsg</span> <span class='hs-varid'>msg</span><span class='hs-layout'>)</span><span class='hs-layout'>)</span>
<a name="line-169"></a>
<a name="line-170"></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-conid'>Error</span> <span class='hs-varid'>e</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'>ErrorT</span> <span class='hs-varid'>e</span> <span class='hs-varid'>m</span><span class='hs-layout'>)</span> <span class='hs-keyword'>where</span>
<a name="line-171"></a> <span class='hs-varid'>mzero</span> <span class='hs-keyglyph'>=</span> <span class='hs-conid'>ErrorT</span> <span class='hs-varop'>$</span> <span class='hs-varid'>return</span> <span class='hs-layout'>(</span><span class='hs-conid'>Left</span> <span class='hs-varid'>noMsg</span><span class='hs-layout'>)</span>
<a name="line-172"></a> <span class='hs-varid'>m</span> <span class='hs-varop'>`mplus`</span> <span class='hs-varid'>n</span> <span class='hs-keyglyph'>=</span> <span class='hs-conid'>ErrorT</span> <span class='hs-varop'>$</span> <span class='hs-keyword'>do</span>
<a name="line-173"></a> <span class='hs-varid'>a</span> <span class='hs-keyglyph'><-</span> <span class='hs-varid'>runErrorT</span> <span class='hs-varid'>m</span>
<a name="line-174"></a> <span class='hs-keyword'>case</span> <span class='hs-varid'>a</span> <span class='hs-keyword'>of</span>
<a name="line-175"></a> <span class='hs-conid'>Left</span> <span class='hs-keyword'>_</span> <span class='hs-keyglyph'>-></span> <span class='hs-varid'>runErrorT</span> <span class='hs-varid'>n</span>
<a name="line-176"></a> <span class='hs-conid'>Right</span> <span class='hs-varid'>r</span> <span class='hs-keyglyph'>-></span> <span class='hs-varid'>return</span> <span class='hs-layout'>(</span><span class='hs-conid'>Right</span> <span class='hs-varid'>r</span><span class='hs-layout'>)</span>
<a name="line-177"></a>
<a name="line-178"></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-conid'>Error</span> <span class='hs-varid'>e</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'>ErrorT</span> <span class='hs-varid'>e</span> <span class='hs-varid'>m</span><span class='hs-layout'>)</span> <span class='hs-keyword'>where</span>
<a name="line-179"></a> <span class='hs-varid'>mfix</span> <span class='hs-varid'>f</span> <span class='hs-keyglyph'>=</span> <span class='hs-conid'>ErrorT</span> <span class='hs-varop'>$</span> <span class='hs-varid'>mfix</span> <span class='hs-varop'>$</span> <span class='hs-keyglyph'>\</span><span class='hs-varid'>a</span> <span class='hs-keyglyph'>-></span> <span class='hs-varid'>runErrorT</span> <span class='hs-varop'>$</span> <span class='hs-varid'>f</span> <span class='hs-varop'>$</span> <span class='hs-keyword'>case</span> <span class='hs-varid'>a</span> <span class='hs-keyword'>of</span>
<a name="line-180"></a> <span class='hs-conid'>Right</span> <span class='hs-varid'>r</span> <span class='hs-keyglyph'>-></span> <span class='hs-varid'>r</span>
<a name="line-181"></a> <span class='hs-keyword'>_</span> <span class='hs-keyglyph'>-></span> <span class='hs-varid'>error</span> <span class='hs-str'>"empty mfix argument"</span>
<a name="line-182"></a>
<a name="line-183"></a><span class='hs-keyword'>instance</span> <span class='hs-layout'>(</span><span class='hs-conid'>Error</span> <span class='hs-varid'>e</span><span class='hs-layout'>)</span> <span class='hs-keyglyph'>=></span> <span class='hs-conid'>MonadTrans</span> <span class='hs-layout'>(</span><span class='hs-conid'>ErrorT</span> <span class='hs-varid'>e</span><span class='hs-layout'>)</span> <span class='hs-keyword'>where</span>
<a name="line-184"></a> <span class='hs-varid'>lift</span> <span class='hs-varid'>m</span> <span class='hs-keyglyph'>=</span> <span class='hs-conid'>ErrorT</span> <span class='hs-varop'>$</span> <span class='hs-keyword'>do</span>
<a name="line-185"></a> <span class='hs-varid'>a</span> <span class='hs-keyglyph'><-</span> <span class='hs-varid'>m</span>
<a name="line-186"></a> <span class='hs-varid'>return</span> <span class='hs-layout'>(</span><span class='hs-conid'>Right</span> <span class='hs-varid'>a</span><span class='hs-layout'>)</span>
<a name="line-187"></a>
<a name="line-188"></a><span class='hs-keyword'>instance</span> <span class='hs-layout'>(</span><span class='hs-conid'>Error</span> <span class='hs-varid'>e</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'>ErrorT</span> <span class='hs-varid'>e</span> <span class='hs-varid'>m</span><span class='hs-layout'>)</span> <span class='hs-keyword'>where</span>
<a name="line-189"></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-190"></a>
<a name="line-191"></a><a name="throwError"></a><span class='hs-comment'>-- | Signal an error value @e@.</span>
<a name="line-192"></a><span class='hs-comment'>--</span>
<a name="line-193"></a><span class='hs-comment'>-- * @'runErrorT' ('throwErrorT' e) = 'return' ('Left' e)@</span>
<a name="line-194"></a><span class='hs-definition'>throwError</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-conid'>Error</span> <span class='hs-varid'>e</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-conid'>ErrorT</span> <span class='hs-varid'>e</span> <span class='hs-varid'>m</span> <span class='hs-varid'>a</span>
<a name="line-195"></a><span class='hs-definition'>throwError</span> <span class='hs-varid'>l</span> <span class='hs-keyglyph'>=</span> <span class='hs-conid'>ErrorT</span> <span class='hs-varop'>$</span> <span class='hs-varid'>return</span> <span class='hs-layout'>(</span><span class='hs-conid'>Left</span> <span class='hs-varid'>l</span><span class='hs-layout'>)</span>
<a name="line-196"></a>
<a name="line-197"></a><a name="catchError"></a><span class='hs-comment'>-- | Handle an error.</span>
<a name="line-198"></a><span class='hs-definition'>catchError</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-conid'>Error</span> <span class='hs-varid'>e</span><span class='hs-layout'>)</span> <span class='hs-keyglyph'>=></span>
<a name="line-199"></a> <span class='hs-conid'>ErrorT</span> <span class='hs-varid'>e</span> <span class='hs-varid'>m</span> <span class='hs-varid'>a</span> <span class='hs-comment'>-- ^ the inner computation</span>
<a name="line-200"></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'>ErrorT</span> <span class='hs-varid'>e</span> <span class='hs-varid'>m</span> <span class='hs-varid'>a</span><span class='hs-layout'>)</span> <span class='hs-comment'>-- ^ a handler for errors in the inner</span>
<a name="line-201"></a> <span class='hs-comment'>-- computation</span>
<a name="line-202"></a> <span class='hs-keyglyph'>-></span> <span class='hs-conid'>ErrorT</span> <span class='hs-varid'>e</span> <span class='hs-varid'>m</span> <span class='hs-varid'>a</span>
<a name="line-203"></a><a name="catchError"></a><span class='hs-definition'>m</span> <span class='hs-varop'>`catchError`</span> <span class='hs-varid'>h</span> <span class='hs-keyglyph'>=</span> <span class='hs-conid'>ErrorT</span> <span class='hs-varop'>$</span> <span class='hs-keyword'>do</span>
<a name="line-204"></a> <span class='hs-varid'>a</span> <span class='hs-keyglyph'><-</span> <span class='hs-varid'>runErrorT</span> <span class='hs-varid'>m</span>
<a name="line-205"></a> <span class='hs-keyword'>case</span> <span class='hs-varid'>a</span> <span class='hs-keyword'>of</span>
<a name="line-206"></a> <span class='hs-conid'>Left</span> <span class='hs-varid'>l</span> <span class='hs-keyglyph'>-></span> <span class='hs-varid'>runErrorT</span> <span class='hs-layout'>(</span><span class='hs-varid'>h</span> <span class='hs-varid'>l</span><span class='hs-layout'>)</span>
<a name="line-207"></a> <span class='hs-conid'>Right</span> <span class='hs-varid'>r</span> <span class='hs-keyglyph'>-></span> <span class='hs-varid'>return</span> <span class='hs-layout'>(</span><span class='hs-conid'>Right</span> <span class='hs-varid'>r</span><span class='hs-layout'>)</span>
<a name="line-208"></a>
<a name="line-209"></a><a name="liftCallCC"></a><span class='hs-comment'>-- | Lift a @callCC@ operation to the new monad.</span>
<a name="line-210"></a><span class='hs-definition'>liftCallCC</span> <span class='hs-keyglyph'>::</span> <span class='hs-layout'>(</span><span class='hs-layout'>(</span><span class='hs-layout'>(</span><span class='hs-conid'>Either</span> <span class='hs-varid'>e</span> <span class='hs-varid'>a</span> <span class='hs-keyglyph'>-></span> <span class='hs-varid'>m</span> <span class='hs-layout'>(</span><span class='hs-conid'>Either</span> <span class='hs-varid'>e</span> <span class='hs-varid'>b</span><span class='hs-layout'>)</span><span class='hs-layout'>)</span> <span class='hs-keyglyph'>-></span> <span class='hs-varid'>m</span> <span class='hs-layout'>(</span><span class='hs-conid'>Either</span> <span class='hs-varid'>e</span> <span class='hs-varid'>a</span><span class='hs-layout'>)</span><span class='hs-layout'>)</span> <span class='hs-keyglyph'>-></span>
<a name="line-211"></a> <span class='hs-varid'>m</span> <span class='hs-layout'>(</span><span class='hs-conid'>Either</span> <span class='hs-varid'>e</span> <span class='hs-varid'>a</span><span class='hs-layout'>)</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'>a</span> <span class='hs-keyglyph'>-></span> <span class='hs-conid'>ErrorT</span> <span class='hs-varid'>e</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'>ErrorT</span> <span class='hs-varid'>e</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'>ErrorT</span> <span class='hs-varid'>e</span> <span class='hs-varid'>m</span> <span class='hs-varid'>a</span>
<a name="line-212"></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'>ErrorT</span> <span class='hs-varop'>$</span>
<a name="line-213"></a> <span class='hs-varid'>callCC</span> <span class='hs-varop'>$</span> <span class='hs-keyglyph'>\</span><span class='hs-varid'>c</span> <span class='hs-keyglyph'>-></span>
<a name="line-214"></a> <span class='hs-varid'>runErrorT</span> <span class='hs-layout'>(</span><span class='hs-varid'>f</span> <span class='hs-layout'>(</span><span class='hs-keyglyph'>\</span><span class='hs-varid'>a</span> <span class='hs-keyglyph'>-></span> <span class='hs-conid'>ErrorT</span> <span class='hs-varop'>$</span> <span class='hs-varid'>c</span> <span class='hs-layout'>(</span><span class='hs-conid'>Right</span> <span class='hs-varid'>a</span><span class='hs-layout'>)</span><span class='hs-layout'>)</span><span class='hs-layout'>)</span>
<a name="line-215"></a>
<a name="line-216"></a><a name="liftListen"></a><span class='hs-comment'>-- | Lift a @listen@ operation to the new monad.</span>
<a name="line-217"></a><span class='hs-definition'>liftListen</span> <span class='hs-keyglyph'>::</span> <span class='hs-conid'>Monad</span> <span class='hs-varid'>m</span> <span class='hs-keyglyph'>=></span>
<a name="line-218"></a> <span class='hs-layout'>(</span><span class='hs-varid'>m</span> <span class='hs-layout'>(</span><span class='hs-conid'>Either</span> <span class='hs-varid'>e</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-layout'>(</span><span class='hs-conid'>Either</span> <span class='hs-varid'>e</span> <span class='hs-varid'>a</span><span class='hs-layout'>,</span><span class='hs-varid'>w</span><span class='hs-layout'>)</span><span class='hs-layout'>)</span> <span class='hs-keyglyph'>-></span> <span class='hs-conid'>ErrorT</span> <span class='hs-varid'>e</span> <span class='hs-varid'>m</span> <span class='hs-varid'>a</span> <span class='hs-keyglyph'>-></span> <span class='hs-conid'>ErrorT</span> <span class='hs-varid'>e</span> <span class='hs-varid'>m</span> <span class='hs-layout'>(</span><span class='hs-varid'>a</span><span class='hs-layout'>,</span><span class='hs-varid'>w</span><span class='hs-layout'>)</span>
<a name="line-219"></a><span class='hs-definition'>liftListen</span> <span class='hs-varid'>listen</span> <span class='hs-keyglyph'>=</span> <span class='hs-varid'>mapErrorT</span> <span class='hs-varop'>$</span> <span class='hs-keyglyph'>\</span> <span class='hs-varid'>m</span> <span class='hs-keyglyph'>-></span> <span class='hs-keyword'>do</span>
<a name="line-220"></a> <span class='hs-layout'>(</span><span class='hs-varid'>a</span><span class='hs-layout'>,</span> <span class='hs-varid'>w</span><span class='hs-layout'>)</span> <span class='hs-keyglyph'><-</span> <span class='hs-varid'>listen</span> <span class='hs-varid'>m</span>
<a name="line-221"></a> <span class='hs-varid'>return</span> <span class='hs-varop'>$!</span> <span class='hs-varid'>fmap</span> <span class='hs-layout'>(</span><span class='hs-keyglyph'>\</span> <span class='hs-varid'>r</span> <span class='hs-keyglyph'>-></span> <span class='hs-layout'>(</span><span class='hs-varid'>r</span><span class='hs-layout'>,</span> <span class='hs-varid'>w</span><span class='hs-layout'>)</span><span class='hs-layout'>)</span> <span class='hs-varid'>a</span>
<a name="line-222"></a>
<a name="line-223"></a><a name="liftPass"></a><span class='hs-comment'>-- | Lift a @pass@ operation to the new monad.</span>
<a name="line-224"></a><span class='hs-definition'>liftPass</span> <span class='hs-keyglyph'>::</span> <span class='hs-conid'>Monad</span> <span class='hs-varid'>m</span> <span class='hs-keyglyph'>=></span> <span class='hs-layout'>(</span><span class='hs-varid'>m</span> <span class='hs-layout'>(</span><span class='hs-conid'>Either</span> <span class='hs-varid'>e</span> <span class='hs-varid'>a</span><span class='hs-layout'>,</span><span class='hs-varid'>w</span> <span class='hs-keyglyph'>-></span> <span class='hs-varid'>w</span><span class='hs-layout'>)</span> <span class='hs-keyglyph'>-></span> <span class='hs-varid'>m</span> <span class='hs-layout'>(</span><span class='hs-conid'>Either</span> <span class='hs-varid'>e</span> <span class='hs-varid'>a</span><span class='hs-layout'>)</span><span class='hs-layout'>)</span> <span class='hs-keyglyph'>-></span>
<a name="line-225"></a> <span class='hs-conid'>ErrorT</span> <span class='hs-varid'>e</span> <span class='hs-varid'>m</span> <span class='hs-layout'>(</span><span class='hs-varid'>a</span><span class='hs-layout'>,</span><span class='hs-varid'>w</span> <span class='hs-keyglyph'>-></span> <span class='hs-varid'>w</span><span class='hs-layout'>)</span> <span class='hs-keyglyph'>-></span> <span class='hs-conid'>ErrorT</span> <span class='hs-varid'>e</span> <span class='hs-varid'>m</span> <span class='hs-varid'>a</span>
<a name="line-226"></a><span class='hs-definition'>liftPass</span> <span class='hs-varid'>pass</span> <span class='hs-keyglyph'>=</span> <span class='hs-varid'>mapErrorT</span> <span class='hs-varop'>$</span> <span class='hs-keyglyph'>\</span> <span class='hs-varid'>m</span> <span class='hs-keyglyph'>-></span> <span class='hs-varid'>pass</span> <span class='hs-varop'>$</span> <span class='hs-keyword'>do</span>
<a name="line-227"></a> <span class='hs-varid'>a</span> <span class='hs-keyglyph'><-</span> <span class='hs-varid'>m</span>
<a name="line-228"></a> <span class='hs-varid'>return</span> <span class='hs-varop'>$!</span> <span class='hs-keyword'>case</span> <span class='hs-varid'>a</span> <span class='hs-keyword'>of</span>
<a name="line-229"></a> <span class='hs-conid'>Left</span> <span class='hs-varid'>l</span> <span class='hs-keyglyph'>-></span> <span class='hs-layout'>(</span><span class='hs-conid'>Left</span> <span class='hs-varid'>l</span><span class='hs-layout'>,</span> <span class='hs-varid'>id</span><span class='hs-layout'>)</span>
<a name="line-230"></a> <span class='hs-conid'>Right</span> <span class='hs-layout'>(</span><span class='hs-varid'>r</span><span class='hs-layout'>,</span> <span class='hs-varid'>f</span><span class='hs-layout'>)</span> <span class='hs-keyglyph'>-></span> <span class='hs-layout'>(</span><span class='hs-conid'>Right</span> <span class='hs-varid'>r</span><span class='hs-layout'>,</span> <span class='hs-varid'>f</span><span class='hs-layout'>)</span>
<a name="line-231"></a>
<a name="line-232"></a><span class='hs-comment'>{- $examples
<a name="line-233"></a>
<a name="line-234"></a>Wrapping an IO action that can throw an error @e@:
<a name="line-235"></a>
<a name="line-236"></a>> type ErrorWithIO e a = ErrorT e IO a
<a name="line-237"></a>> ==> ErrorT (IO (Either e a))
<a name="line-238"></a>
<a name="line-239"></a>An IO monad wrapped in @StateT@ inside of @ErrorT@:
<a name="line-240"></a>
<a name="line-241"></a>> type ErrorAndStateWithIO e s a = ErrorT e (StateT s IO) a
<a name="line-242"></a>> ==> ErrorT (StateT s IO (Either e a))
<a name="line-243"></a>> ==> ErrorT (StateT (s -> IO (Either e a,s)))
<a name="line-244"></a>
<a name="line-245"></a>-}</span>
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