\begin{code}
-{-# OPTIONS_GHC -fno-implicit-prelude #-}
+{-# OPTIONS_GHC -XNoImplicitPrelude #-}
+{-# OPTIONS_HADDOCK hide #-}
-----------------------------------------------------------------------------
-- |
-- Module : GHC.ST
\end{code}
%*********************************************************
-%* *
+%* *
\subsection{The @ST@ monad}
-%* *
+%* *
%*********************************************************
The state-transformer monad proper. By default the monad is strict;
-- by @s@, and returns a value of type @a@.
-- The @s@ parameter is either
--
--- * an unstantiated type variable (inside invocations of 'runST'), or
+-- * 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 #)
(ST m) >>= k
= ST (\ s ->
- case (m s) of { (# new_s, r #) ->
- case (k r) of { ST k2 ->
- (k2 new_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 #)
)
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.
-- I'm only letting runSTRep be inlined right at the end, in particular *after* full laziness
-- That's what the "INLINE [0]" says.
--- SLPJ Apr 99
-{-# INLINE [0] runSTRep #-}
+-- 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}