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%
% (c) The GRASP/AQUA Project, Glasgow University, 1992-1996
%
\section[PrelST]{The @ST@ monad}
\begin{code}
{-# OPTIONS -fno-implicit-prelude #-}
module PrelST where
import Monad
import PrelShow
import PrelBase
import PrelGHC
import PrelNum () -- So that we get the .hi file for system imports
\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}
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 ->
case (m s) of { (# new_s, r #) ->
(# new_s, f r #) }
instance Monad (ST s) where
{-# INLINE return #-}
{-# INLINE (>>) #-}
{-# INLINE (>>=) #-}
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) }}
data STret s a = STret (State# s) a
-- 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
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 ->
let
r = case m s of (# _, res #) -> res
in
(# s, r #)
)
instance Show (ST s a) where
showsPrec _ _ = showString "<<ST action>>"
showList = showList__ (showsPrec 0)
\end{code}
Definition of runST
~~~~~~~~~~~~~~~~~~~
SLPJ 95/04: Why @runST@ must not have an unfolding; consider:
\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'' )
\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'
in
freezeArray# a s''
\end{verbatim}
And now the @newArray#@ binding can be floated to become a CAF, which
is totally and utterly wrong:
\begin{verbatim}
f = let
(a, s') = newArray# 100 [] realWorld#{-NB-} -- YIKES!!!
in
\ x ->
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.
\begin{code}
{-# 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 letting runSTRep be inlined *after* full laziness
-- SLPJ Apr 99
runSTRep :: (forall s. STRep s a) -> a
runSTRep st_rep = case st_rep realWorld# of
(# _, r #) -> r
\end{code}
|
