Remove Control.Parallel*, now in package parallel

[haskell-directory.git] / Control / Monad / Fix.hs

diff --git a/Control/Monad/Fix.hs b/Control/Monad/Fix.hs
index b66a6ce..ea481d8 100644 (file)
--- a/Control/Monad/Fix.hs
+++ b/Control/Monad/Fix.hs
@@ -10,8 +10,8 @@
 --
 -- Monadic fixpoints.
 --
--- For more details, see Levent Erkok's thesis,
--- /Value Recursion in Monadic Computations/, 2002.
+-- For a detailed discussion, see Levent Erkok's thesis,
+-- /Value Recursion in Monadic Computations/, Oregon Graduate Institute, 2002.
 --
 -----------------------------------------------------------------------------
 
@@ -24,32 +24,30 @@ module Control.Monad.Fix (
 
 import Prelude
 import System.IO
+import Control.Monad.Instances ()
+import Data.Function (fix)
 
--- | @'fix' f@ is the least fixed point of the function @f@.
-fix :: (a -> a) -> a
-fix f = let x = f x in x
-
--- | Monads having fixed points with a \`knot-tying\' semantics.
--- Instances of 'MonadFix' should satisfy the laws
+-- | Monads having fixed points with a \'knot-tying\' semantics.
+-- Instances of 'MonadFix' should satisfy the following laws:
 --
--- [purity]
+-- [/purity/]
 --     @'mfix' ('return' . h)  =  'return' ('fix' h)@
 --
--- [left shrinking (or tightening)]
---     @'mfix' (\x -> a >>= \y -> f x y)  =  \y -> 'mfix' (\x -> f x y)@
+-- [/left shrinking/ (or /tightening/)]
+--     @'mfix' (\\x -> a >>= \\y -> f x y)  =  a >>= \\y -> 'mfix' (\\x -> f x y)@
 --
--- [sliding]
+-- [/sliding/]
 --     @'mfix' ('Control.Monad.liftM' h . f)  =  'Control.Monad.liftM' h ('mfix' (f . h))@,
 --     for strict @h@.
 --
--- [nesting]
---     @'mfix' (\x -> 'mfix' (\y -> f x y))  =  'mfix' (\x -> f x x)@
+-- [/nesting/]
+--     @'mfix' (\\x -> 'mfix' (\\y -> f x y))  =  'mfix' (\\x -> f x x)@
 --
 -- This class is used in the translation of the recursive @do@ notation
 -- supported by GHC and Hugs.
 class (Monad m) => MonadFix m where
        -- | The fixed point of a monadic computation.
-       -- @'mfix' f@ executes the action 'f' only once, with the eventual
+       -- @'mfix' f@ executes the action @f@ only once, with the eventual
        -- output fed back as the input.  Hence @f@ should not be strict,
        -- for then @'mfix' f@ would diverge.
        mfix :: (a -> m a) -> m a
@@ -70,3 +68,6 @@ instance MonadFix [] where
 -- IO:
 instance MonadFix IO where
     mfix = fixIO 
+
+instance MonadFix ((->) r) where
+    mfix f = \ r -> let a = f a r in a