1508d62653f485ba4b3a321770fcdb119cbd671c
[packages/base.git] / Control / Monad / Fix.hs
1 {-# LANGUAGE Trustworthy #-}
2 {-# LANGUAGE CPP #-}
3
4 -----------------------------------------------------------------------------
5 -- |
6 -- Module : Control.Monad.Fix
7 -- Copyright : (c) Andy Gill 2001,
8 -- (c) Oregon Graduate Institute of Science and Technology, 2002
9 -- License : BSD-style (see the file libraries/base/LICENSE)
10 -- Maintainer : libraries@haskell.org
11 -- Stability : experimental
12 -- Portability : portable
13 --
14 -- Monadic fixpoints.
15 --
16 -- For a detailed discussion, see Levent Erkok's thesis,
17 -- /Value Recursion in Monadic Computations/, Oregon Graduate Institute, 2002.
18 --
19 -----------------------------------------------------------------------------
20
21 module Control.Monad.Fix (
22 MonadFix(mfix),
23 fix
24 ) where
25
26 import Prelude
27 import System.IO
28 import Data.Function (fix)
29 #ifdef __HUGS__
30 import Hugs.Prelude (MonadFix(mfix))
31 #endif
32 #if defined(__GLASGOW_HASKELL__)
33 import GHC.ST
34 #endif
35
36 #ifndef __HUGS__
37 -- | Monads having fixed points with a \'knot-tying\' semantics.
38 -- Instances of 'MonadFix' should satisfy the following laws:
39 --
40 -- [/purity/]
41 -- @'mfix' ('return' . h) = 'return' ('fix' h)@
42 --
43 -- [/left shrinking/ (or /tightening/)]
44 -- @'mfix' (\\x -> a >>= \\y -> f x y) = a >>= \\y -> 'mfix' (\\x -> f x y)@
45 --
46 -- [/sliding/]
47 -- @'mfix' ('Control.Monad.liftM' h . f) = 'Control.Monad.liftM' h ('mfix' (f . h))@,
48 -- for strict @h@.
49 --
50 -- [/nesting/]
51 -- @'mfix' (\\x -> 'mfix' (\\y -> f x y)) = 'mfix' (\\x -> f x x)@
52 --
53 -- This class is used in the translation of the recursive @do@ notation
54 -- supported by GHC and Hugs.
55 class (Monad m) => MonadFix m where
56 -- | The fixed point of a monadic computation.
57 -- @'mfix' f@ executes the action @f@ only once, with the eventual
58 -- output fed back as the input. Hence @f@ should not be strict,
59 -- for then @'mfix' f@ would diverge.
60 mfix :: (a -> m a) -> m a
61 #endif /* !__HUGS__ */
62
63 -- Instances of MonadFix for Prelude monads
64
65 instance MonadFix Maybe where
66 mfix f = let a = f (unJust a) in a
67 where unJust (Just x) = x
68 unJust Nothing = error "mfix Maybe: Nothing"
69
70 instance MonadFix [] where
71 mfix f = case fix (f . head) of
72 [] -> []
73 (x:_) -> x : mfix (tail . f)
74
75 instance MonadFix IO where
76 mfix = fixIO
77
78 instance MonadFix ((->) r) where
79 mfix f = \ r -> let a = f a r in a
80
81 instance MonadFix (Either e) where
82 mfix f = let a = f (unRight a) in a
83 where unRight (Right x) = x
84 unRight (Left _) = error "mfix Either: Left"
85
86 #if defined(__GLASGOW_HASKELL__)
87 instance MonadFix (ST s) where
88 mfix = fixST
89 #endif
90