[project @ 2005-02-03 10:32:11 by ross]

[ghc-base.git] / GHC / ST.lhs

diff --git a/GHC/ST.lhs b/GHC/ST.lhs
index ab1a1b1..cdccb18 100644 (file)
--- a/GHC/ST.lhs
+++ b/GHC/ST.lhs
@@ -1,5 +1,5 @@
 \begin{code}
-{-# OPTIONS -fno-implicit-prelude #-}
+{-# OPTIONS_GHC -fno-implicit-prelude #-}
 -----------------------------------------------------------------------------
 -- |
 -- Module      :  GHC.ST
@@ -14,6 +14,7 @@
 --
 -----------------------------------------------------------------------------
 
+-- #hide
 module GHC.ST where
 
 import GHC.Base
@@ -33,6 +34,18 @@ 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 unstantiated 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'.
 newtype ST s a = ST (STRep s a)
 type STRep s a = State# s -> (# State# s, a #)
 
@@ -45,14 +58,14 @@ 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
 
@@ -70,6 +83,9 @@ 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
@@ -119,6 +135,10 @@ 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 })