diff options
Diffstat (limited to 'libraries/base/GHC/ST.hs')
| -rw-r--r-- | libraries/base/GHC/ST.hs | 58 |
1 files changed, 3 insertions, 55 deletions
diff --git a/libraries/base/GHC/ST.hs b/libraries/base/GHC/ST.hs index 46c5196c9e..d84dd4d9d1 100644 --- a/libraries/base/GHC/ST.hs +++ b/libraries/base/GHC/ST.hs @@ -18,7 +18,7 @@ module GHC.ST ( ST(..), STret(..), STRep, - fixST, runST, runSTRep, + fixST, runST, -- * Unsafe functions liftST, unsafeInterleaveST @@ -103,62 +103,10 @@ instance Show (ST s a) where showsPrec _ _ = showString "<<ST action>>" showList = showList__ (showsPrec 0) -{- -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. --} - {-# 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. - -- | 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 [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 +runST (ST st_rep) = case runRW# st_rep of (# _, a #) -> a +-- See Note [Definition of runRW#] in GHC.Magic |