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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.

MERGED from commit f55f557
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commit fa366546f73557d83b57403feee1009a0243304b 1 parent 7a6acb1
@pcapriotti pcapriotti authored
Showing with 4 additions and 23 deletions.
  1. +4 −19 Control/Monad.hs
  2. +0 −4 GHC/ST.lhs
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23 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 ()
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4 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
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