+% ------------------------------------------------------------------------------
+% $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}
module PrelST where
-import Monad
import PrelBase
-import PrelGHC
+import PrelShow
+import PrelNum
+
+default ()
\end{code}
%*********************************************************
too many people got bitten by space leaks when it was lazy.
\begin{code}
-newtype ST s a = ST (State# s -> (# 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
fmap f (ST m) = ST $ \ s ->
liftST :: ST s a -> State# s -> STret s a
liftST (ST m) = \s -> case m s of (# s', r #) -> STret s' r
-fixST :: (a -> ST s a) -> ST s a
-fixST k = ST $ \ s ->
- let ans = liftST (k r) s
- STret _ r = ans
- in
- case ans of STret s' x -> (# s', x #)
-
{-# NOINLINE unsafeInterleaveST #-}
unsafeInterleaveST :: ST s a -> ST s a
unsafeInterleaveST (ST m) = ST ( \ s ->
All calls to @f@ will share a {\em single} array! End SLPJ 95/04.
\begin{code}
-{-# NOINLINE runST #-}
+{-# 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 =
- case st of
- ST m -> case m realWorld# of
- (# _, 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)
+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}