Thanks for following this up. Reiner 2009/7/22 Simon Peyton-Jones :

Thanks.  You've found a bug in 6.10, which is happily fixed in the HEAD and in the upcoming 6.12. Here's the code for g2:

Test.$s$wg2 =  \ (sc_soL :: GHC.Prim.Int#) (sc1_soM :: GHC.Prim.Int#) ->    case GHC.Prim.># sc_soL 10 of _ {      GHC.Bool.False ->        case GHC.Prim.<=# sc1_soM sc_soL of _ {          GHC.Bool.False -> Test.$s$wg2 (GHC.Prim.+# sc_soL 1) sc1_soM;          GHC.Bool.True -> Test.$s$wg2 (GHC.Prim.+# sc_soL 1) sc_soL        };      GHC.Bool.True -> Test.Just @ GHC.Types.Int (GHC.Types.I# sc1_soM)    }

Simon

| -----Original Message----- | From: glasgow-haskell-users-bounces@haskell.org [mailto:glasgow-haskell- | users-bounces@haskell.org] On Behalf Of Don Stewart | Sent: 20 July 2009 17:38 | To: glasgow-haskell-users@haskell.org | Subject: [reiner.pope@gmail.com: [Haskell-cafe] SpecConstr difficulties] | | Something for SimonPJ | | ----- Forwarded message from Reiner Pope ----- | | Date: Mon, 20 Jul 2009 22:09:09 +0930 | From: Reiner Pope | To: Haskell Cafe mailing list | Subject: [Haskell-cafe] SpecConstr difficulties | | Hi everyone, | | I've been having some trouble getting SpecConstr to work as I want it | to. The following example (see end of email) came up in some code I | was playing around with. The loops g1 and g2 both compute the same | thing: the maximum element of the "list" (which has been fused away) | of numbers from 1 to 10. | | Since 'maximum' is a foldl1 not a foldl, I use a strict Maybe type as | an accumulator. The Maybe gets filled after the first element is seen, | so the Maybe is a Just for almost the entire running of the loop. | | It would be good to have this recognised by SpecConstr, to create an | optimised loop for the Just case. This does indeed happen for g1, but | not for g2. | | My difficulty is that I am only able to produce code similar to g2, | i.e. where the pattern matching is in a separate function, 'expose', | because the 'expose' function is implemented in a type-class, like in | stream-fusion. Is there some way to keep the SpecConstr while leaving | the 'expose' as a separate function? | | Here is the code: | | > {-# LANGUAGE BangPatterns #-} | > | > module Test(ans1,ans2) where | > | > import Prelude hiding(Maybe(..)) | > | > data Maybe a = Just !a | Nothing | > | > Nothing `mapp` Just b = Just b | > Just a `mapp` Just b = Just (max a b) | > | > ans1 = g1 Nothing (0::Int) | > | > g1 m !n = case m of | >            Nothing -> if n > 10 then m else g1 (m `mapp` Just n) (n+1) | >            Just x -> if n > 10 then m else g1 (m `mapp` Just n) (n+1) | > | > ans2 = g2 Nothing (0::Int) | > | > g2 m !n = expose m (if n > 10 then m else g2 (m `mapp` Just n) (n+1)) | > expose Nothing  b = b | > expose (Just a) b = a `seq` b | | On a similar note, when I was having difficulties with this problem, I | started to wonder if it would be possible to come up with a more | direct way to tell GHC, "do SpecConstr on this variable". From reading | the source code of the stream-fusion package, it seems that the | current way of doing this is with 'expose' functions like I wrote | below. Could we instead have a {-# SPECCONSTR #-} pragma, to be used | on function arguments, like: | | foo {-# SPECCONSTR #-} x y {-# SPECCONSTR #-} z = ... | | This pragma should say to the GHC something like "ignore conditions | H2, H5 and H6 of the SpecConstr paper, for this function and this | argument". | | Cheers, | Reiner | _______________________________________________ | Haskell-Cafe mailing list | Haskell-Cafe@haskell.org | http://www.haskell.org/mailman/listinfo/haskell-cafe | | ----- End forwarded message ----- | _______________________________________________ | Glasgow-haskell-users mailing list | Glasgow-haskell-users@haskell.org | http://www.haskell.org/mailman/listinfo/glasgow-haskell-users

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