Repository : ssh://darcs.haskell.org//srv/darcs/packages/base On branch : master
http://hackage.haskell.org/trac/ghc/changeset/f55f5574c12ff8dfe57994219eee0702ac8aba2e >--------------------------------------------------------------- commit f55f5574c12ff8dfe57994219eee0702ac8aba2e Author: Paolo Capriotti <[email protected]> Date: Mon Aug 20 16:35:38 2012 +0100 Improve definition of forever (#5205) The previous implementation was: forever a = a >> forever a which can create a space leak in some cases, even with optimizations. The current implementation: forever a = let a' = a >> a' in a' prevents repeated thunk allocations by creating a single thunk for the final result, even without optimizations. >--------------------------------------------------------------- Control/Monad.hs | 23 ++++------------------- GHC/ST.lhs | 4 ---- 2 files changed, 4 insertions(+), 23 deletions(-) diff --git a/Control/Monad.hs b/Control/Monad.hs index 21f0153..eec9465 100644 --- a/Control/Monad.hs +++ b/Control/Monad.hs @@ -190,25 +190,10 @@ f >=> g = \x -> f x >>= g -- | @'forever' act@ repeats the action infinitely. forever :: (Monad m) => m a -> m b -{-# INLINABLE forever #-} -- See Note [Make forever INLINABLE] -forever a = a >> forever a - -{- Note [Make forever INLINABLE] - -If you say x = forever a -you'll get x = a >> a >> a >> a >> ... etc ... -and that can make a massive space leak (see Trac #5205) - -In some monads, where (>>) is expensive, this might be the right -thing, but not in the IO monad. We want to specialise 'forever' for -the IO monad, so that eta expansion happens and there's no space leak. -To achieve this we must make forever INLINABLE, so that it'll get -specialised at call sites. - -Still delicate, though, because it depends on optimisation. But there -really is a space/time tradeoff here, and only optimisation reveals -the "right" answer. --} +{-# INLINE forever #-} +forever a = let a' = a >> a' in a' +-- Use explicit sharing here, as it is prevents a space leak regardless of +-- optimizations. -- | @'void' value@ discards or ignores the result of evaluation, such as the return value of an 'IO' action. void :: Functor f => f a -> f () diff --git a/GHC/ST.lhs b/GHC/ST.lhs index 5983e94..74a299a 100644 --- a/GHC/ST.lhs +++ b/GHC/ST.lhs @@ -27,7 +27,6 @@ module GHC.ST ( import GHC.Base import GHC.Show -import Control.Monad( forever ) default () \end{code} @@ -82,9 +81,6 @@ instance Monad (ST s) where data STret s a = STret (State# s) a -{-# SPECIALISE forever :: ST s a -> ST s b #-} --- See Note [Make forever INLINABLE] in Control.Monad - -- 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 _______________________________________________ Cvs-libraries mailing list [email protected] http://www.haskell.org/mailman/listinfo/cvs-libraries
