-% ------------------------------------------------------------------------------
-% $Id: ST.lhs,v 1.1 2001/06/28 14:15:03 simonmar Exp $
-%
-% (c) The University of Glasgow, 1992-2000
-%
-
-\section[GHC.ST]{The @ST@ monad}
-
\begin{code}
-{-# OPTIONS -fno-implicit-prelude #-}
-
+{-# OPTIONS_GHC -XNoImplicitPrelude #-}
+{-# OPTIONS_HADDOCK hide #-}
+-----------------------------------------------------------------------------
+-- |
+-- 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
\end{code}
%*********************************************************
-%* *
+%* *
\subsection{The @ST@ monad}
-%* *
+%* *
%*********************************************************
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 #)
{-# 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 ->
- case (m s) of { (# new_s, r #) ->
- case (k r) of { ST k2 ->
- (k2 new_s) }}
+ = ST (\ s ->
+ case (m s) of { (# new_s, r #) ->
+ case (k r) of { ST k2 ->
+ (k2 new_s) }})
data STret s a = STret (State# s) a
unsafeInterleaveST :: ST s a -> ST s a
unsafeInterleaveST (ST m) = ST ( \ s ->
let
- r = case m s of (# _, res #) -> res
+ r = case m s of (# _, res #) -> res
in
(# 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
- STret _ r = ans
+ STret _ r = ans
in
case ans of STret s' x -> (# s', x #)
instance Show (ST s a) where
showsPrec _ _ = showString "<<ST action>>"
- showList = showList__ (showsPrec 0)
+ showList = showList__ (showsPrec 0)
\end{code}
Definition of runST
\begin{verbatim}
f x =
runST ( \ s -> let
- (a, s') = newArray# 100 [] s
- (_, s'') = fill_in_array_or_something a x s'
- in
- freezeArray# a s'' )
+ (a, s') = newArray# 100 [] s
+ (_, s'') = fill_in_array_or_something a x s'
+ in
+ freezeArray# a s'' )
\end{verbatim}
If we inline @runST@, we'll get:
\begin{verbatim}
f x = let
- (a, s') = newArray# 100 [] realWorld#{-NB-}
- (_, s'') = fill_in_array_or_something a x s'
+ (a, s') = newArray# 100 [] realWorld#{-NB-}
+ (_, s'') = fill_in_array_or_something a x s'
in
freezeArray# a s''
\end{verbatim}
(a, s') = newArray# 100 [] realWorld#{-NB-} -- YIKES!!!
in
\ x ->
- let (_, s'') = fill_in_array_or_something a x s' in
- freezeArray# a s''
+ let (_, s'') = fill_in_array_or_something a x s' in
+ freezeArray# a s''
\end{verbatim}
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 100" says.
--- SLPJ Apr 99
-{-# INLINE 100 runSTRep #-}
+-- That's what the "INLINE [0]" says.
+-- SLPJ Apr 99
+-- {-# 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
+ (# _, r #) -> r
\end{code}