[reiner.pope@gmail.com: [Haskell-cafe] SpecConstr difficulties]

[Reiner Pope]

Thanks for following this up.

Reiner

2009/7/22 Simon Peyton-Jones <simonpj at microsoft.com>:
> 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 at haskell.org [mailto:glasgow-haskell-
> | users-bounces at haskell.org] On Behalf Of Don Stewart
> | Sent: 20 July 2009 17:38
> | To: glasgow-haskell-users at haskell.org
> | Subject: [reiner.pope at gmail.com: [Haskell-cafe] SpecConstr difficulties]
> |
> | Something for SimonPJ
> |
> | ----- Forwarded message from Reiner Pope <reiner.pope at gmail.com> -----
> |
> | Date: Mon, 20 Jul 2009 22:09:09 +0930
> | From: Reiner Pope <reiner.pope at gmail.com>
> | To: Haskell Cafe mailing list <haskell-cafe at haskell.org>
> | 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 at haskell.org
> | http://www.haskell.org/mailman/listinfo/haskell-cafe
> |
> | ----- End forwarded message -----
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