X-Git-Url: http://git.megacz.com/?a=blobdiff_plain;f=GHC%2FST.lhs;h=909a8da338e287ec8219f6e2874457a141f9e066;hb=a0d7892da0d00fee781a550ef353df8734be5884;hp=c36292cc276f326dc6697d54d75a289aa54f3ae6;hpb=260e7f2ed9a43c6ecf5a556d77817f39ed2893ab;p=haskell-directory.git

diff --git a/GHC/ST.lhs b/GHC/ST.lhs
index c36292c..909a8da 100644
--- a/GHC/ST.lhs
+++ b/GHC/ST.lhs
@@ -1,14 +1,20 @@
-% ------------------------------------------------------------------------------
-% $Id: ST.lhs,v 1.2 2001/12/21 15:07:25 simonmar Exp $
-%
-% (c) The University of Glasgow, 1992-2000
-%
-
-\section[GHC.ST]{The @ST@ monad}
-
 \begin{code}
-{-# OPTIONS -fno-implicit-prelude #-}
-
+{-# OPTIONS_GHC -fno-implicit-prelude #-}
+-----------------------------------------------------------------------------
+-- |
+-- Module      :  GHC.ST
+-- Copyright   :  (c) The University of Glasgow, 1992-2002
+-- License     :  see libraries/base/LICENSE
+-- 
+-- Maintainer  :  cvs-ghc@haskell.org
+-- Stability   :  internal
+-- Portability :  non-portable (GHC Extensions)
+--
+-- The 'ST' Monad.
+--
+-----------------------------------------------------------------------------
+
+-- #hide
 module GHC.ST where
 
 import GHC.Base
@@ -28,6 +34,23 @@ The state-transformer monad proper.  By default the monad is strict;
 too many people got bitten by space leaks when it was lazy.
 
 \begin{code}
+-- | The strict 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 uninstantiated type variable (inside invocations of 'runST'), or
+--
+-- * 'RealWorld' (inside invocations of 'Control.Monad.ST.stToIO').
+--
+-- It serves to keep the internal states of different invocations
+-- of 'runST' separate from each other and from invocations of
+-- 'Control.Monad.ST.stToIO'.
+--
+-- The '>>=' and '>>' operations are strict in the state (though not in
+-- values stored in the state).  For example,
+--
+-- @'runST' (writeSTRef _|_ v >>= f) = _|_@
 newtype ST s a = ST (STRep s a)
 type STRep s a = State# s -> (# State# s, a #)
 
@@ -40,14 +63,14 @@ instance Monad (ST s) where
     {-# INLINE return #-}
     {-# INLINE (>>)   #-}
     {-# INLINE (>>=)  #-}
-    return x = ST $ \ s -> (# s, x #)
-    m >> k   =  m >>= \ _ -> k
+    return x = ST (\ s -> (# s, x #))
+    m >> k   = m >>= \ _ -> k
 
     (ST m) >>= k
-      = ST $ \ s ->
+      = ST (\ s ->
 	case (m s) of { (# new_s, r #) ->
 	case (k r) of { ST k2 ->
-	(k2 new_s) }}
+	(k2 new_s) }})
 
 data STret s a = STret (State# s) a
 
@@ -65,6 +88,9 @@ unsafeInterleaveST (ST m) = ST ( \ s ->
     (# s, 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 k = ST $ \ s ->
     let ans       = liftST (k r) s
@@ -114,13 +140,24 @@ All calls to @f@ will share a {\em single} array!  End SLPJ 95/04.
 -- The INLINE prevents runSTRep getting inlined in *this* module
 -- so that it is still visible when runST is inlined in an importing
 -- module.  Regrettably delicate.  runST is behaving like a wrapper.
+
+-- | 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 = runSTRep (case st of { ST st_rep -> st_rep })
 
 -- I'm only letting runSTRep be inlined right at the end, in particular *after* full laziness
 -- That's what the "INLINE [0]" says.
 -- 		SLPJ Apr 99
-{-# INLINE [0] runSTRep #-}
+-- {-# INLINE [0] runSTRep #-}
+
+-- SDM: further to the above, inline phase 0 is run *before*
+-- full-laziness at the moment, which means that the above comment is
+-- invalid.  Inlining runSTRep doesn't make a huge amount of
+-- difference, anyway.  Hence:
+
+{-# NOINLINE runSTRep #-}
 runSTRep :: (forall s. STRep s a) -> a
 runSTRep st_rep = case st_rep realWorld# of
 	      		(# _, r #) -> r