Re: The dreaded M-R

Remove the type signature for b and you will see the loss of sharing. It mostly hurts people like John Hughes that don't have the energy to put in type signatures. ;) On the subject of type signatures, I don't want to make them mandatory, but I think they should be strongly encouraged. I don't buy the argument that they make refactoring programs that much harder. It's still very easy to do, the type checker will tell you exactly where. :) -- Lennart Cale Gibbard wrote:

On 28/01/06, Taral wrote:

...

On 1/28/06, Cale Gibbard wrote:

...

Do you have an example of such a program handy?

b = (x, x) where { x :: Num a => a; x = fromInteger 1 }

fromInteger is called twice.

--- mr.hs --- {-# OPTIONS_GHC -fno-monomorphism-restriction #-}

import Debug.Trace

b :: Num a => (a,a) b = (x,x) where x :: Num a => a x = (trace "x" . fromInteger) 1

main = print b ------------ cale@zaphod[~]$ ghci -fno-monomorphism-restriction mr.hs Loading package base-1.0 ... linking ... done. Compiling Main ( mr.hs, interpreted ) Ok, modules loaded: Main. *Main> :t b b :: (Num a) => (a, a) *Main> b (x 1,1) ------------ cale@zaphod[~]$ ghc -fno-monomorphism-restriction -o mr mr.hs && ./mr x (1,1) -----------

If x isn't being shared, then Debug.Trace at least seems incapable of resolving that fact.

Let's try profiling:

--- mr.hs, revised ----- {-# OPTIONS_GHC -fno-monomorphism-restriction #-}

b :: Num a => (a,a) b = (x,x) where x :: Num a => a x = {-# SCC "x" #-} fromInteger 1

main = print b -------------- cale@zaphod[~]$ ghc -fno-monomorphism-restriction -prof -auto-all -o mr mr.hs && ./mr +RTS -p (1,1) cale@zaphod[~]$ cat mr.prof Sat Jan 28 18:21 2006 Time and Allocation Profiling Report (Final)

mr +RTS -p -RTS

total time = 0.00 secs (0 ticks @ 20 ms) total alloc = 17,972 bytes (excludes profiling overheads)

COST CENTRE MODULE %time %alloc

CAF GHC.Handle 0.0 48.2 CAF System.IO 0.0 1.4 CAF Main 0.0 50.3

individual inherited COST CENTRE MODULE no. entries %time %alloc %time %alloc

MAIN MAIN 1 0 0.0 0.0 0.0 100.0 CAF Main 150 6 0.0 50.3 0.0 50.5 b Main 157 1 0.0 0.1 0.0 0.2 x Main 158 1 0.0 0.1 0.0 0.1 main Main 156 1 0.0 0.0 0.0 0.0 CAF System.IO 105 1 0.0 1.4 0.0 1.4 CAF GHC.Handle 103 3 0.0 48.2 0.0 48.2

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One entry to x. So where is this lack of sharing I keep hearing about? Even if I repeat these tests with -fno-cse, the results are the same.

- Cale _______________________________________________ Haskell-prime mailing list Haskell-prime@haskell.org http://haskell.org/mailman/listinfo/haskell-prime