[project @ 2001-08-04 06:19:54 by ken]

[ghc-hetmet.git] / ghc / lib / std / PrelST.lhs

diff --git a/ghc/lib/std/PrelST.lhs b/ghc/lib/std/PrelST.lhs
index 580ec93..9efa299 100644 (file)
--- a/ghc/lib/std/PrelST.lhs
+++ b/ghc/lib/std/PrelST.lhs
@@ -1,6 +1,9 @@
+% ------------------------------------------------------------------------------
+% $Id: PrelST.lhs,v 1.20 2001/02/22 16:48:24 qrczak Exp $
 %
-% (c) The GRASP/AQUA Project, Glasgow University, 1992-1996
+% (c) The University of Glasgow, 1992-2000
 %
+
 \section[PrelST]{The @ST@ monad}
 
 \begin{code}
@@ -8,9 +11,11 @@
 
 module PrelST where
 
-import Monad
 import PrelBase
-import PrelGHC
+import PrelShow
+import PrelNum
+
+default ()
 \end{code}
 
 %*********************************************************
@@ -23,46 +28,46 @@ 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}
-newtype ST s a = ST (State# s -> STret s a)
-
-data STret s a = STret (State# s) a
+newtype ST s a = ST (STRep s a)
+type STRep s a = State# s -> (# State# s, a #)
 
 instance Functor (ST s) where
-    map f (ST m) = ST $ \ s ->
-      case (m s) of { STret new_s r ->
-      STret new_s (f r) }
+    fmap f (ST m) = ST $ \ s ->
+      case (m s) of { (# new_s, r #) ->
+      (# new_s, f r #) }
 
 instance Monad (ST s) where
     {-# INLINE return #-}
     {-# INLINE (>>)   #-}
     {-# INLINE (>>=)  #-}
-    return x = ST $ \ s -> STret s x
+    return x = ST $ \ s -> (# s, x #)
     m >> k   =  m >>= \ _ -> k
 
     (ST m) >>= k
       = ST $ \ s ->
-       case (m s) of { STret new_s r ->
+       case (m s) of { (# new_s, r #) ->
        case (k r) of { ST k2 ->
        (k2 new_s) }}
 
+data STret s a = STret (State# s) a
 
-
-fixST :: (a -> ST s a) -> ST s a
-fixST k = ST $ \ s ->
-    let (ST k_r)  = k r
-       ans       = k_r s
-       STret _ r = ans
-    in
-    ans
+-- liftST is useful when we want a lifted result from an ST computation.  See
+-- fixST below.
+liftST :: ST s a -> State# s -> STret s a
+liftST (ST m) = \s -> case m s of (# s', r #) -> STret s' r
 
 {-# NOINLINE unsafeInterleaveST #-}
 unsafeInterleaveST :: ST s a -> ST s a
 unsafeInterleaveST (ST m) = ST ( \ s ->
     let
-       STret _ r = m s
+       r = case m s of (# _, res #) -> res
     in
-    STret s r)
+    (# s, r #)
+  )
 
+instance  Show (ST s a)  where
+    showsPrec _ _  = showString "<<ST action>>"
+    showList      = showList__ (showsPrec 0)
 \end{code}
 
 Definition of runST
@@ -98,32 +103,18 @@ f = let
 All calls to @f@ will share a {\em single} array!  End SLPJ 95/04.
 
 \begin{code}
-{-# NOINLINE runST #-}
-runST :: (All s => ST s a) -> a
-runST st = 
-  case st of
-       ST m -> case m realWorld# of
-                       STret _ r -> r
-\end{code}
-
-%*********************************************************
-%*                                                     *
-\subsection{Ghastly return types}
-%*                                                     *
-%*********************************************************
-
-The @State@ type is the return type of a _ccall_ with no result.  It
-never actually exists, since it's always deconstructed straight away;
-the desugarer ensures this.
-
-\begin{code}
-data State          s     = S#              (State# s)
-data StateAndPtr#    s elt = StateAndPtr#    (State# s) elt 
-
-data StateAndChar#   s     = StateAndChar#   (State# s) Char# 
-data StateAndInt#    s     = StateAndInt#    (State# s) Int# 
-data StateAndWord#   s     = StateAndWord#   (State# s) Word#
-data StateAndFloat#  s     = StateAndFloat#  (State# s) Float# 
-data StateAndDouble# s     = StateAndDouble# (State# s) Double#  
-data StateAndAddr#   s     = StateAndAddr#   (State# s) Addr#
+{-# INLINE runST #-}
+-- 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.
+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 100" says.
+--             SLPJ Apr 99
+{-# INLINE 100 runSTRep #-}
+runSTRep :: (forall s. STRep s a) -> a
+runSTRep st_rep = case st_rep realWorld# of
+                       (# _, r #) -> r
 \end{code}