Remove Control.Parallel*, now in package parallel

[haskell-directory.git] / Control / Monad / ST / Lazy.hs

diff --git a/Control/Monad/ST/Lazy.hs b/Control/Monad/ST/Lazy.hs
index 413659e..5bf1265 100644 (file)
--- a/Control/Monad/ST/Lazy.hs
+++ b/Control/Monad/ST/Lazy.hs
@@ -9,8 +9,8 @@
 -- Portability :  non-portable (requires universal quantification for runST)
 --
 -- This module presents an identical interface to "Control.Monad.ST",
--- but the underlying implementation of the state thread is /lazy/ (in
--- the sense that (@_|_ >> a@ is not necessarily equal to @_|_@).
+-- except that the monad delays evaluation of state operations until
+-- a value depending on them is required.
 --
 -----------------------------------------------------------------------------
 
@@ -20,16 +20,16 @@ module Control.Monad.ST.Lazy (
        runST,
        fixST,
 
-       -- * Unsafe operations
-       unsafeInterleaveST,
-       unsafeIOToST,
+       -- * Converting between strict and lazy 'ST'
+       strictToLazyST, lazyToStrictST,
 
        -- * Converting 'ST' To 'IO'
        RealWorld,
        stToIO,
 
-       -- * Converting between strict and lazy 'ST'
-       strictToLazyST, lazyToStrictST
+       -- * Unsafe operations
+       unsafeInterleaveST,
+       unsafeIOToST
     ) where
 
 import Prelude
@@ -50,6 +50,21 @@ import Hugs.LazyST
 #endif
 
 #ifdef __GLASGOW_HASKELL__
+-- | The lazy state-transformer monad.
+-- A computation of type @'ST' s a@ transforms an internal state indexed
+-- by @s@, and returns a value of type @a@.
+-- The @s@ parameter is either
+--
+-- * an unstantiated type variable (inside invocations of 'runST'), or
+--
+-- * 'RealWorld' (inside invocations of 'stToIO').
+--
+-- It serves to keep the internal states of different invocations of
+-- 'runST' separate from each other and from invocations of 'stToIO'.
+--
+-- The '>>=' and '>>' operations are not strict in the state.  For example,
+--
+-- @'runST' (writeSTRef _|_ v >>= readSTRef _|_ >> return 2) = 2@
 newtype ST s a = ST (State s -> (a, State s))
 data State s = S# (State# s)
 
@@ -76,9 +91,15 @@ instance Monad (ST s) where
            k_a new_s
 
 {-# NOINLINE runST #-}
+-- | Return the value computed by a state transformer computation.
+-- The @forall@ ensures that the internal state used by the 'ST'
+-- computation is inaccessible to the rest of the program.
 runST :: (forall s. ST s a) -> a
 runST st = case st of ST the_st -> let (r,_) = the_st (S# realWorld#) in r
 
+-- | Allow the result of a state transformer computation to be used (lazily)
+-- inside the computation.
+-- Note that if @f@ is strict, @'fixST' f = _|_@.
 fixST :: (a -> ST s a) -> ST s a
 fixST m = ST (\ s -> 
                let 
@@ -123,5 +144,9 @@ unsafeInterleaveST = strictToLazyST . ST.unsafeInterleaveST . lazyToStrictST
 unsafeIOToST :: IO a -> ST s a
 unsafeIOToST = strictToLazyST . ST.unsafeIOToST
 
+-- | A monad transformer embedding lazy state transformers in the 'IO'
+-- monad.  The 'RealWorld' parameter indicates that the internal state
+-- used by the 'ST' computation is a special one supplied by the 'IO'
+-- monad, and thus distinct from those used by invocations of 'runST'.
 stToIO :: ST RealWorld a -> IO a
 stToIO = ST.stToIO . lazyToStrictST