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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.Fix</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, 2002</span>
<a name="line-6"></a><span class='hs-comment'>-- License : BSD-style (see the file libraries/base/LICENSE)</span>
<a name="line-7"></a><span class='hs-comment'>-- Maintainer : libraries@haskell.org</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'>-- Monadic fixpoints.</span>
<a name="line-12"></a><span class='hs-comment'>--</span>
<a name="line-13"></a><span class='hs-comment'>-- For a detailed discussion, see Levent Erkok's thesis,</span>
<a name="line-14"></a><span class='hs-comment'>-- /Value Recursion in Monadic Computations/, Oregon Graduate Institute, 2002.</span>
<a name="line-15"></a><span class='hs-comment'>--</span>
<a name="line-16"></a><span class='hs-comment'>-----------------------------------------------------------------------------</span>
<a name="line-17"></a>
<a name="line-18"></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'>Fix</span> <span class='hs-layout'>(</span>
<a name="line-19"></a> <span class='hs-conid'>MonadFix</span><span class='hs-layout'>(</span>
<a name="line-20"></a> <span class='hs-varid'>mfix</span> <span class='hs-comment'>-- :: (a -> m a) -> m a</span>
<a name="line-21"></a> <span class='hs-layout'>)</span><span class='hs-layout'>,</span>
<a name="line-22"></a> <span class='hs-varid'>fix</span> <span class='hs-comment'>-- :: (a -> a) -> a</span>
<a name="line-23"></a> <span class='hs-layout'>)</span> <span class='hs-keyword'>where</span>
<a name="line-24"></a>
<a name="line-25"></a><span class='hs-keyword'>import</span> <span class='hs-conid'>Prelude</span>
<a name="line-26"></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-27"></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-28"></a><span class='hs-keyword'>import</span> <span class='hs-conid'>Data</span><span class='hs-varop'>.</span><span class='hs-conid'>Function</span> <span class='hs-layout'>(</span><span class='hs-varid'>fix</span><span class='hs-layout'>)</span>
<a name="line-29"></a><span class='hs-cpp'>#ifdef __HUGS__</span>
<a name="line-30"></a><span class='hs-keyword'>import</span> <span class='hs-conid'>Hugs</span><span class='hs-varop'>.</span><span class='hs-conid'>Prelude</span> <span class='hs-layout'>(</span><span class='hs-conid'>MonadFix</span><span class='hs-layout'>(</span><span class='hs-varid'>mfix</span><span class='hs-layout'>)</span><span class='hs-layout'>)</span>
<a name="line-31"></a><span class='hs-cpp'>#endif</span>
<a name="line-32"></a><span class='hs-cpp'>#if defined(__GLASGOW_HASKELL__)</span>
<a name="line-33"></a><span class='hs-keyword'>import</span> <span class='hs-conid'>GHC</span><span class='hs-varop'>.</span><span class='hs-conid'>ST</span>
<a name="line-34"></a><span class='hs-cpp'>#endif</span>
<a name="line-35"></a>
<a name="line-36"></a><span class='hs-cpp'>#ifndef __HUGS__</span>
<a name="line-37"></a><a name="MonadFix"></a><span class='hs-comment'>-- | Monads having fixed points with a \'knot-tying\' semantics.</span>
<a name="line-38"></a><a name="MonadFix"></a><span class='hs-comment'>-- Instances of 'MonadFix' should satisfy the following laws:</span>
<a name="line-39"></a><a name="MonadFix"></a><span class='hs-comment'>--</span>
<a name="line-40"></a><a name="MonadFix"></a><span class='hs-comment'>-- [/purity/]</span>
<a name="line-41"></a><a name="MonadFix"></a><span class='hs-comment'>-- @'mfix' ('return' . h) = 'return' ('fix' h)@</span>
<a name="line-42"></a><a name="MonadFix"></a><span class='hs-comment'>--</span>
<a name="line-43"></a><a name="MonadFix"></a><span class='hs-comment'>-- [/left shrinking/ (or /tightening/)]</span>
<a name="line-44"></a><a name="MonadFix"></a><span class='hs-comment'>-- @'mfix' (\\x -> a >>= \\y -> f x y) = a >>= \\y -> 'mfix' (\\x -> f x y)@</span>
<a name="line-45"></a><a name="MonadFix"></a><span class='hs-comment'>--</span>
<a name="line-46"></a><a name="MonadFix"></a><span class='hs-comment'>-- [/sliding/]</span>
<a name="line-47"></a><a name="MonadFix"></a><span class='hs-comment'>-- @'mfix' ('Control.Monad.liftM' h . f) = 'Control.Monad.liftM' h ('mfix' (f . h))@,</span>
<a name="line-48"></a><a name="MonadFix"></a><span class='hs-comment'>-- for strict @h@.</span>
<a name="line-49"></a><a name="MonadFix"></a><span class='hs-comment'>--</span>
<a name="line-50"></a><a name="MonadFix"></a><span class='hs-comment'>-- [/nesting/]</span>
<a name="line-51"></a><a name="MonadFix"></a><span class='hs-comment'>-- @'mfix' (\\x -> 'mfix' (\\y -> f x y)) = 'mfix' (\\x -> f x x)@</span>
<a name="line-52"></a><a name="MonadFix"></a><span class='hs-comment'>--</span>
<a name="line-53"></a><a name="MonadFix"></a><span class='hs-comment'>-- This class is used in the translation of the recursive @do@ notation</span>
<a name="line-54"></a><a name="MonadFix"></a><span class='hs-comment'>-- supported by GHC and Hugs.</span>
<a name="line-55"></a><a name="MonadFix"></a><span class='hs-keyword'>class</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'>MonadFix</span> <span class='hs-varid'>m</span> <span class='hs-keyword'>where</span>
<a name="line-56"></a> <span class='hs-comment'>-- | The fixed point of a monadic computation.</span>
<a name="line-57"></a> <span class='hs-comment'>-- @'mfix' f@ executes the action @f@ only once, with the eventual</span>
<a name="line-58"></a> <span class='hs-comment'>-- output fed back as the input. Hence @f@ should not be strict,</span>
<a name="line-59"></a> <span class='hs-comment'>-- for then @'mfix' f@ would diverge.</span>
<a name="line-60"></a> <span class='hs-varid'>mfix</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'>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>
<a name="line-61"></a><span class='hs-cpp'>#endif /* !__HUGS__ */</span>
<a name="line-62"></a>
<a name="line-63"></a><span class='hs-comment'>-- Instances of MonadFix for Prelude monads</span>
<a name="line-64"></a>
<a name="line-65"></a><span class='hs-comment'>-- Maybe:</span>
<a name="line-66"></a><span class='hs-keyword'>instance</span> <span class='hs-conid'>MonadFix</span> <span class='hs-conid'>Maybe</span> <span class='hs-keyword'>where</span>
<a name="line-67"></a> <span class='hs-varid'>mfix</span> <span class='hs-varid'>f</span> <span class='hs-keyglyph'>=</span> <span class='hs-keyword'>let</span> <span class='hs-varid'>a</span> <span class='hs-keyglyph'>=</span> <span class='hs-varid'>f</span> <span class='hs-layout'>(</span><span class='hs-varid'>unJust</span> <span class='hs-varid'>a</span><span class='hs-layout'>)</span> <span class='hs-keyword'>in</span> <span class='hs-varid'>a</span>
<a name="line-68"></a> <span class='hs-keyword'>where</span> <span class='hs-varid'>unJust</span> <span class='hs-layout'>(</span><span class='hs-conid'>Just</span> <span class='hs-varid'>x</span><span class='hs-layout'>)</span> <span class='hs-keyglyph'>=</span> <span class='hs-varid'>x</span>
<a name="line-69"></a> <span class='hs-varid'>unJust</span> <span class='hs-conid'>Nothing</span> <span class='hs-keyglyph'>=</span> <span class='hs-varid'>error</span> <span class='hs-str'>"mfix Maybe: Nothing"</span>
<a name="line-70"></a>
<a name="line-71"></a><span class='hs-comment'>-- List:</span>
<a name="line-72"></a><span class='hs-keyword'>instance</span> <span class='hs-conid'>MonadFix</span> <span class='hs-conid'>[]</span> <span class='hs-keyword'>where</span>
<a name="line-73"></a> <span class='hs-varid'>mfix</span> <span class='hs-varid'>f</span> <span class='hs-keyglyph'>=</span> <span class='hs-keyword'>case</span> <span class='hs-varid'>fix</span> <span class='hs-layout'>(</span><span class='hs-varid'>f</span> <span class='hs-varop'>.</span> <span class='hs-varid'>head</span><span class='hs-layout'>)</span> <span class='hs-keyword'>of</span>
<a name="line-74"></a> <span class='hs-conid'>[]</span> <span class='hs-keyglyph'>-></span> <span class='hs-conid'>[]</span>
<a name="line-75"></a> <span class='hs-layout'>(</span><span class='hs-varid'>x</span><span class='hs-conop'>:</span><span class='hs-keyword'>_</span><span class='hs-layout'>)</span> <span class='hs-keyglyph'>-></span> <span class='hs-varid'>x</span> <span class='hs-conop'>:</span> <span class='hs-varid'>mfix</span> <span class='hs-layout'>(</span><span class='hs-varid'>tail</span> <span class='hs-varop'>.</span> <span class='hs-varid'>f</span><span class='hs-layout'>)</span>
<a name="line-76"></a>
<a name="line-77"></a><span class='hs-comment'>-- IO:</span>
<a name="line-78"></a><span class='hs-keyword'>instance</span> <span class='hs-conid'>MonadFix</span> <span class='hs-conid'>IO</span> <span class='hs-keyword'>where</span>
<a name="line-79"></a> <span class='hs-varid'>mfix</span> <span class='hs-keyglyph'>=</span> <span class='hs-varid'>fixIO</span>
<a name="line-80"></a>
<a name="line-81"></a><span class='hs-comment'>-- Prelude types with Monad instances in Control.Monad.Instances</span>
<a name="line-82"></a>
<a name="line-83"></a><span class='hs-keyword'>instance</span> <span class='hs-conid'>MonadFix</span> <span class='hs-layout'>(</span><span class='hs-layout'>(</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-keyword'>where</span>
<a name="line-84"></a> <span class='hs-varid'>mfix</span> <span class='hs-varid'>f</span> <span class='hs-keyglyph'>=</span> <span class='hs-keyglyph'>\</span> <span class='hs-varid'>r</span> <span class='hs-keyglyph'>-></span> <span class='hs-keyword'>let</span> <span class='hs-varid'>a</span> <span class='hs-keyglyph'>=</span> <span class='hs-varid'>f</span> <span class='hs-varid'>a</span> <span class='hs-varid'>r</span> <span class='hs-keyword'>in</span> <span class='hs-varid'>a</span>
<a name="line-85"></a>
<a name="line-86"></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-87"></a> <span class='hs-varid'>mfix</span> <span class='hs-varid'>f</span> <span class='hs-keyglyph'>=</span> <span class='hs-keyword'>let</span> <span class='hs-varid'>a</span> <span class='hs-keyglyph'>=</span> <span class='hs-varid'>f</span> <span class='hs-layout'>(</span><span class='hs-varid'>unRight</span> <span class='hs-varid'>a</span><span class='hs-layout'>)</span> <span class='hs-keyword'>in</span> <span class='hs-varid'>a</span>
<a name="line-88"></a> <span class='hs-keyword'>where</span> <span class='hs-varid'>unRight</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-89"></a> <span class='hs-varid'>unRight</span> <span class='hs-layout'>(</span><span class='hs-conid'>Left</span> <span class='hs-keyword'>_</span><span class='hs-layout'>)</span> <span class='hs-keyglyph'>=</span> <span class='hs-varid'>error</span> <span class='hs-str'>"mfix Either: Left"</span>
<a name="line-90"></a>
<a name="line-91"></a><span class='hs-cpp'>#if defined(__GLASGOW_HASKELL__)</span>
<a name="line-92"></a><span class='hs-keyword'>instance</span> <span class='hs-conid'>MonadFix</span> <span class='hs-layout'>(</span><span class='hs-conid'>ST</span> <span class='hs-varid'>s</span><span class='hs-layout'>)</span> <span class='hs-keyword'>where</span>
<a name="line-93"></a> <span class='hs-varid'>mfix</span> <span class='hs-keyglyph'>=</span> <span class='hs-varid'>fixST</span>
<a name="line-94"></a><span class='hs-cpp'>#endif</span>
<a name="line-95"></a>
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