The -fno-method-sharing flag was supposed to be a bit experimental, which is why it takes the cheap-and-cheerful route of being a static flag. (Only dynamic flags can go in OPTIONS_GHC.)
What it does is this. When you call an overloaded function f :: C a => a -> a, in a function g = ...f...f... you normally get something like this fint :: Int -> Int fint = f Int dCInt g = ...fint...fint... That is, 'fint' extracts the 'f' method from dCInt::C Int, and it's then used repeatedly. With -fno-method-sharing you get g = ...(f Int dCInt) ... (f Int dCInt)... So the record selection is duplicated. It shouldn't make much difference, but of course it *does* when rules are involved, because there are no rules for fint (it's a fresh, local function). Simon From: [EMAIL PROTECTED] [mailto:[EMAIL PROTECTED] On Behalf Of Conal Elliott Sent: 07 June 2008 17:26 To: glasgow-haskell-users@haskell.org Subject: Re: desperately seeking RULES help Is it by intention that -fno-method-sharing works only from the command line, not in an OPTIONS_GHC pragma? On Sat, Jun 7, 2008 at 9:23 AM, Conal Elliott <[EMAIL PROTECTED]<mailto:[EMAIL PROTECTED]>> wrote: Thanks a million, Lennart! -fno-method-sharing was the missing piece. - Conal On Sat, Jun 7, 2008 at 5:07 AM, Lennart Augustsson <[EMAIL PROTECTED]<mailto:[EMAIL PROTECTED]>> wrote: Here's something that actually works. You need to pass -fno-method-sharing on the command line. Instead of using rules on methods it uses rules on global functions, and these global functions don't get inlined until late (after the rule has fired). -- Lennart module F where -- | Domain of a linear map. class AsInt a where toInt' :: a -> Int fromInt' :: Int -> a {-# INLINE[1] toInt #-} toInt :: (AsInt a) => a -> Int toInt = toInt' {-# INLINE[1] fromInt #-} fromInt :: (AsInt a) => Int -> a fromInt = fromInt' {-# RULES "toInt/fromInt" forall m . toInt (fromInt m) = m #-} {-# INLINE onInt #-} onInt :: AsInt a => (Int -> Int) -> (a -> a) onInt f x = fromInt (f (toInt x)) test :: AsInt a => (Int -> Int) -> (Int -> Int) -> (a -> a) test h g = onInt h . onInt g 2008/6/7 Conal Elliott <[EMAIL PROTECTED]<mailto:[EMAIL PROTECTED]>>: > I'm trying to do some fusion in ghc, and I'd greatly appreciate help with > the code below (which is simplified from fusion on linear maps). I've tried > every variation I can think of, and always something prevents the fusion. > > Help, please! Thanks, - Conal > > > {-# OPTIONS_GHC -O2 -Wall -fglasgow-exts -ddump-simpl -ddump-simpl-stats #-} > -- {-# OPTIONS_GHC -ddump-simpl-iterations #-} > > module F where > > -- | Domain of a linear map. > class AsInt a where > toInt :: a -> Int > fromInt :: Int -> a > > {-# RULES > "toInt/fromInt" forall m. toInt (fromInt m) = m > #-} > > {-# INLINE onInt #-} > onInt :: AsInt a => (Int -> Int) -> (a -> a) > onInt f = fromInt . f . toInt > > test :: AsInt a => (Int -> Int) -> (Int -> Int) -> (a -> a) > test h g = onInt h . onInt g > > -- The desired result: > -- > -- test h g > -- == onInt h . onInt g > -- == (fromInt . h . toInt) . (fromInt . g . toInt) > -- == \ a -> (fromInt . h . toInt) ((fromInt . g . toInt) a) > -- == \ a -> (fromInt . h . toInt) (fromInt (g (toInt a))) > -- == \ a -> fromInt (h (toInt (fromInt (g (toInt a))))) > -- == \ a -> fromInt (h (g (toInt a))) > > > > _______________________________________________ > Glasgow-haskell-users mailing list > Glasgow-haskell-users@haskell.org<mailto:Glasgow-haskell-users@haskell.org> > http://www.haskell.org/mailman/listinfo/glasgow-haskell-users > >
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