Re: desperately seeking RULES help

How does method sharing interact with the ability of the rules engine to "look through" lets? Wouldn't an f rule kick in when fint is seen, by looking through the fint binding? I've been wondering: will pattern matching look through a let even when the let-bound variable is used more than once? I chose "yes" in Pan, though somewhat nervously, since all but one of the uses are free anyway. Cheers, - Conal On Mon, Jun 9, 2008 at 2:38 AM, Simon Peyton-Jones wrote:

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:* glasgow-haskell-users-bounces@haskell.org [mailto: glasgow-haskell-users-bounces@haskell.org] *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 wrote:

Thanks a million, Lennart! -fno-method-sharing was the missing piece. - Conal

On Sat, Jun 7, 2008 at 5:07 AM, Lennart Augustsson 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 :

...

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)))

...

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