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<pre><a name="line-1"></a><span class='hs-comment'>{- arch-tag: Euither utilities
<a name="line-2"></a>Copyright (C) 2004-2006 John Goerzen <jgoerzen@complete.org>
<a name="line-3"></a>
<a name="line-4"></a>This program is free software; you can redistribute it and/or modify
<a name="line-5"></a>it under the terms of the GNU General Public License as published by
<a name="line-6"></a>the Free Software Foundation; either version 2 of the License, or
<a name="line-7"></a>(at your option) any later version.
<a name="line-8"></a>
<a name="line-9"></a>This program is distributed in the hope that it will be useful,
<a name="line-10"></a>but WITHOUT ANY WARRANTY; without even the implied warranty of
<a name="line-11"></a>MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the
<a name="line-12"></a>GNU General Public License for more details.
<a name="line-13"></a>
<a name="line-14"></a>You should have received a copy of the GNU General Public License
<a name="line-15"></a>along with this program; if not, write to the Free Software
<a name="line-16"></a>Foundation, Inc., 59 Temple Place, Suite 330, Boston, MA 02111-1307 USA
<a name="line-17"></a>-}</span>
<a name="line-18"></a>
<a name="line-19"></a><span class='hs-comment'>{- |
<a name="line-20"></a> Module : Data.Either.Utils
<a name="line-21"></a> Copyright : Copyright (C) 2004-2006 John Goerzen
<a name="line-22"></a> License : GNU GPL, version 2 or above
<a name="line-23"></a>
<a name="line-24"></a> Maintainer : John Goerzen <jgoerzen@complete.org>
<a name="line-25"></a> Stability : provisional
<a name="line-26"></a> Portability: portable
<a name="line-27"></a>
<a name="line-28"></a>Utilities for working with the Either data type
<a name="line-29"></a>
<a name="line-30"></a>Copyright (c) 2004 John Goerzen, jgoerzen\@complete.org
<a name="line-31"></a>-}</span>
<a name="line-32"></a><span class='hs-keyword'>module</span> <span class='hs-conid'>Data</span><span class='hs-varop'>.</span><span class='hs-conid'>Either</span><span class='hs-varop'>.</span><span class='hs-conid'>Utils</span>
<a name="line-33"></a> <span class='hs-layout'>(</span>
<a name="line-34"></a> <span class='hs-varid'>maybeToEither</span><span class='hs-layout'>,</span>
<a name="line-35"></a> <span class='hs-varid'>forceEither</span><span class='hs-layout'>,</span>
<a name="line-36"></a> <span class='hs-varid'>forceEitherMsg</span><span class='hs-layout'>,</span>
<a name="line-37"></a> <span class='hs-varid'>eitherToMonadError</span><span class='hs-layout'>,</span>
<a name="line-38"></a> <span class='hs-varid'>fromLeft</span><span class='hs-layout'>,</span> <span class='hs-varid'>fromRight</span><span class='hs-layout'>,</span> <span class='hs-varid'>fromEither</span>
<a name="line-39"></a><span class='hs-layout'>)</span> <span class='hs-keyword'>where</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'>Error</span>
<a name="line-41"></a>
<a name="line-42"></a><a name="maybeToEither"></a><span class='hs-comment'>{- | Converts a Maybe value to an Either value, using the supplied parameter
<a name="line-43"></a>as the Left value if the Maybe is Nothing.
<a name="line-44"></a>
<a name="line-45"></a>This function can be interpreted as:
<a name="line-46"></a>
<a name="line-47"></a>@maybeToEither :: e -> Maybe a -> Either e a@
<a name="line-48"></a>
<a name="line-49"></a>Its definition is given as it is so that it can be used in the Error and related monads.
<a name="line-50"></a>
<a name="line-51"></a>-}</span>
<a name="line-52"></a><span class='hs-definition'>maybeToEither</span> <span class='hs-keyglyph'>::</span> <span class='hs-conid'>MonadError</span> <span class='hs-varid'>e</span> <span class='hs-varid'>m</span> <span class='hs-keyglyph'>=></span>
<a name="line-53"></a> <span class='hs-varid'>e</span> <span class='hs-comment'>-- ^ (Left e) will be returned if the Maybe value is Nothing</span>
<a name="line-54"></a> <span class='hs-keyglyph'>-></span> <span class='hs-conid'>Maybe</span> <span class='hs-varid'>a</span> <span class='hs-comment'>-- ^ (Right a) will be returned if this is (Just a)</span>
<a name="line-55"></a> <span class='hs-keyglyph'>-></span> <span class='hs-varid'>m</span> <span class='hs-varid'>a</span>
<a name="line-56"></a><span class='hs-definition'>maybeToEither</span> <span class='hs-varid'>errorval</span> <span class='hs-conid'>Nothing</span> <span class='hs-keyglyph'>=</span> <span class='hs-varid'>throwError</span> <span class='hs-varid'>errorval</span>
<a name="line-57"></a><span class='hs-definition'>maybeToEither</span> <span class='hs-keyword'>_</span> <span class='hs-layout'>(</span><span class='hs-conid'>Just</span> <span class='hs-varid'>normalval</span><span class='hs-layout'>)</span> <span class='hs-keyglyph'>=</span> <span class='hs-varid'>return</span> <span class='hs-varid'>normalval</span>
<a name="line-58"></a>
<a name="line-59"></a><a name="forceEither"></a><span class='hs-comment'>{- | Pulls a "Right" value out of an Either value. If the Either value is
<a name="line-60"></a>Left, raises an exception with "error". -}</span>
<a name="line-61"></a><span class='hs-definition'>forceEither</span> <span class='hs-keyglyph'>::</span> <span class='hs-conid'>Show</span> <span class='hs-varid'>e</span> <span class='hs-keyglyph'>=></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'>a</span>
<a name="line-62"></a><span class='hs-definition'>forceEither</span> <span class='hs-layout'>(</span><span class='hs-conid'>Left</span> <span class='hs-varid'>x</span><span class='hs-layout'>)</span> <span class='hs-keyglyph'>=</span> <span class='hs-varid'>error</span> <span class='hs-layout'>(</span><span class='hs-varid'>show</span> <span class='hs-varid'>x</span><span class='hs-layout'>)</span>
<a name="line-63"></a><span class='hs-definition'>forceEither</span> <span class='hs-layout'>(</span><span class='hs-conid'>Right</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-64"></a>
<a name="line-65"></a><a name="forceEitherMsg"></a><span class='hs-comment'>{- | Like 'forceEither', but can raise a specific message with the error. -}</span>
<a name="line-66"></a><span class='hs-definition'>forceEitherMsg</span> <span class='hs-keyglyph'>::</span> <span class='hs-conid'>Show</span> <span class='hs-varid'>e</span> <span class='hs-keyglyph'>=></span> <span class='hs-conid'>String</span> <span class='hs-keyglyph'>-></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'>a</span>
<a name="line-67"></a><span class='hs-definition'>forceEitherMsg</span> <span class='hs-varid'>msg</span> <span class='hs-layout'>(</span><span class='hs-conid'>Left</span> <span class='hs-varid'>x</span><span class='hs-layout'>)</span> <span class='hs-keyglyph'>=</span> <span class='hs-varid'>error</span> <span class='hs-varop'>$</span> <span class='hs-varid'>msg</span> <span class='hs-varop'>++</span> <span class='hs-str'>": "</span> <span class='hs-varop'>++</span> <span class='hs-varid'>show</span> <span class='hs-varid'>x</span>
<a name="line-68"></a><span class='hs-definition'>forceEitherMsg</span> <span class='hs-keyword'>_</span> <span class='hs-layout'>(</span><span class='hs-conid'>Right</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-69"></a>
<a name="line-70"></a><a name="eitherToMonadError"></a><span class='hs-comment'>{- | Takes an either and transforms it into something of the more generic
<a name="line-71"></a>MonadError class. -}</span>
<a name="line-72"></a><span class='hs-definition'>eitherToMonadError</span> <span class='hs-keyglyph'>::</span> <span class='hs-conid'>MonadError</span> <span class='hs-varid'>e</span> <span class='hs-varid'>m</span> <span class='hs-keyglyph'>=></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-varid'>a</span>
<a name="line-73"></a><span class='hs-definition'>eitherToMonadError</span> <span class='hs-layout'>(</span><span class='hs-conid'>Left</span> <span class='hs-varid'>x</span><span class='hs-layout'>)</span> <span class='hs-keyglyph'>=</span> <span class='hs-varid'>throwError</span> <span class='hs-varid'>x</span>
<a name="line-74"></a><span class='hs-definition'>eitherToMonadError</span> <span class='hs-layout'>(</span><span class='hs-conid'>Right</span> <span class='hs-varid'>x</span><span class='hs-layout'>)</span> <span class='hs-keyglyph'>=</span> <span class='hs-varid'>return</span> <span class='hs-varid'>x</span>
<a name="line-75"></a>
<a name="line-76"></a>
<a name="line-77"></a><a name="fromLeft"></a><span class='hs-comment'>-- | Take a Left to a value, crashes on a Right</span>
<a name="line-78"></a><span class='hs-definition'>fromLeft</span> <span class='hs-keyglyph'>::</span> <span class='hs-conid'>Either</span> <span class='hs-varid'>a</span> <span class='hs-varid'>b</span> <span class='hs-keyglyph'>-></span> <span class='hs-varid'>a</span>
<a name="line-79"></a><span class='hs-definition'>fromLeft</span> <span class='hs-layout'>(</span><span class='hs-conid'>Left</span> <span class='hs-varid'>a</span><span class='hs-layout'>)</span> <span class='hs-keyglyph'>=</span> <span class='hs-varid'>a</span>
<a name="line-80"></a><span class='hs-definition'>fromLeft</span> <span class='hs-keyword'>_</span> <span class='hs-keyglyph'>=</span> <span class='hs-varid'>error</span> <span class='hs-str'>"Data.Either.Utils.fromLeft: Right"</span>
<a name="line-81"></a>
<a name="line-82"></a><a name="fromRight"></a><span class='hs-comment'>-- | Take a Right to a value, crashes on a Left</span>
<a name="line-83"></a><span class='hs-definition'>fromRight</span> <span class='hs-keyglyph'>::</span> <span class='hs-conid'>Either</span> <span class='hs-varid'>a</span> <span class='hs-varid'>b</span> <span class='hs-keyglyph'>-></span> <span class='hs-varid'>b</span>
<a name="line-84"></a><span class='hs-definition'>fromRight</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-keyglyph'>=</span> <span class='hs-varid'>a</span>
<a name="line-85"></a><span class='hs-definition'>fromRight</span> <span class='hs-keyword'>_</span> <span class='hs-keyglyph'>=</span> <span class='hs-varid'>error</span> <span class='hs-str'>"Data.Either.Utils.fromRight: Left"</span>
<a name="line-86"></a>
<a name="line-87"></a><a name="fromEither"></a><span class='hs-comment'>-- | Take an Either, and return the value inside it</span>
<a name="line-88"></a><span class='hs-definition'>fromEither</span> <span class='hs-keyglyph'>::</span> <span class='hs-conid'>Either</span> <span class='hs-varid'>a</span> <span class='hs-varid'>a</span> <span class='hs-keyglyph'>-></span> <span class='hs-varid'>a</span>
<a name="line-89"></a><span class='hs-definition'>fromEither</span> <span class='hs-layout'>(</span><span class='hs-conid'>Left</span> <span class='hs-varid'>a</span><span class='hs-layout'>)</span> <span class='hs-keyglyph'>=</span> <span class='hs-varid'>a</span>
<a name="line-90"></a><span class='hs-definition'>fromEither</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-keyglyph'>=</span> <span class='hs-varid'>a</span>
<a name="line-91"></a>
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