[commit: base] master: Make 'forever' inlinable (fixes Trac #5205) (ae10342)

[Simon Peyton-Jones]

I did, yes. Works ok for me.  'forever' does has an INLINABLE pragma..
so I'm confused

S

| -----Original Message-----
| From: Manuel M T Chakravarty [mailto:mchakravarty at me.com]
| Sent: 10 June 2011 02:08
| To: Simon Peyton-Jones
| Cc: cvs-libraries at haskell.org
| Subject: Re: [commit: base] master: Make 'forever' inlinable (fixes Trac #5205)
| (ae10342)
| 
| Simon,
| 
| Did you validate this?  I get
| 
| > libraries/base/GHC/ST.lhs:78:1:
| >     You cannot SPECIALISE `forever'
| >       because its definition has no INLINE/INLINABLE pragma
| >       (or you compiled its defining module without -O)
| > make[1]: *** [libraries/base/dist-install/doc/html/base/base.haddock] Error 1
| 
| with a standard validate run on Mac OS X using 7.0.3 as the bootstrap compiler.
| 
| Manuel
| 
| 
| 
| > Repository : ssh://darcs.haskell.org//srv/darcs/packages/base
| >
| > On branch  : master
| >
| >
| http://hackage.haskell.org/trac/ghc/changeset/ae10342b49b95393b09ffee8df8c84740969996
| 8
| >
| >> ---------------------------------------------------------------
| >
| > commit ae10342b49b95393b09ffee8df8c847409699968
| > Author: Simon Peyton Jones <simonpj at microsoft.com>
| > Date:   Thu Jun 9 20:44:21 2011 +0100
| >
| >    Make 'forever' inlinable (fixes Trac #5205)
| >
| >    See Note [Make forever INLINABLE] in Control.Monad
| >
| >> ---------------------------------------------------------------
| >
| > Control/Monad.hs |   18 ++++++++++++++++++
| > GHC/ST.lhs       |    4 ++++
| > 2 files changed, 22 insertions(+), 0 deletions(-)
| >
| > diff --git a/Control/Monad.hs b/Control/Monad.hs
| > index 2bbfc57..75b9d0b 100644
| > --- a/Control/Monad.hs
| > +++ b/Control/Monad.hs
| > @@ -189,8 +189,26 @@ 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.
| > +-}
| > +
| > -- | @'void' value@ discards or ignores the result of evaluation, such as the
| return value of an 'IO' action.
| > void :: Functor f => f a -> f ()
| > void = fmap (const ())
| > diff --git a/GHC/ST.lhs b/GHC/ST.lhs
| > index dc62000..a66a5f8 100644
| > --- a/GHC/ST.lhs
| > +++ b/GHC/ST.lhs
| > @@ -20,6 +20,7 @@ module GHC.ST where
| >
| > import GHC.Base
| > import GHC.Show
| > +import Control.Monad( forever )
| >
| > default ()
| > \end{code}
| > @@ -74,6 +75,9 @@ 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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