Re: strictness of interpreted haskell implementations
Sorry to revive a year-old thread, but... On Fri, 25 Apr 2008 at 20:17:53 +0100 Duncan Coutts wrote:
On Fri, 2008-04-25 at 09:08 -0700, Don Stewart wrote:
Geraint.Jones:
Are there well-known differences in the implementations of Haskell in ghci and hugs? I've got some moderately intricate code (simulations of pipelined processors) that behave differently - apparently because ghci Haskell is stricter than hugs Haskell, and I cannot find any obviously relevant claims about strictness in the documentation.
I think they should give the same answer. It sounds like a bug in one implementation or the other.
Hugs does no optimisations, while GHC does a truckload, including strictness analysis. Some of these optimisations prevent space leaks.
Though none should change the static semantics.
Post the code. Even if you don't have time to track down the difference, someone might.
At the time I was reluctant to impose all the code on anyone and I found it hard to cut the example down to a manageable size. I've just got it down to a one-liner: it's the implementation of what I think ought to be strict fields in records: data S = S { a :: Int, b :: ! Int } I think ghci is correct: *Main> a (S { a = 0, b = 1 }) 0 *Main> a (S { a = 0, b = undefined }) *** Exception: Prelude.undefined and that hugs had been concealing a bug in my program by not demanding one of the fields of S when it ought to: Main> a (S { a = 0, b = 1 }) 0 Main> a (S { a = 0, b = undefined }) 0 Ho hum. Is this a "known difference"? (What makes you think I'm teaching the same course again this year?)
On Tue, 2009-05-05 at 00:43 +0100, Geraint Jones wrote:
Sorry to revive a year-old thread, but...
On Fri, 25 Apr 2008 at 20:17:53 +0100 Duncan Coutts wrote:
On Fri, 2008-04-25 at 09:08 -0700, Don Stewart wrote:
Geraint.Jones:
Are there well-known differences in the implementations of Haskell in ghci and hugs? I've got some moderately intricate code (simulations of pipelined processors) that behave differently - apparently because ghci Haskell is stricter than hugs Haskell, and I cannot find any obviously relevant claims about strictness in the documentation.
I think they should give the same answer. It sounds like a bug in one implementation or the other.
Hugs does no optimisations, while GHC does a truckload, including strictness analysis. Some of these optimisations prevent space leaks.
Though none should change the static semantics.
Post the code. Even if you don't have time to track down the difference, someone might.
At the time I was reluctant to impose all the code on anyone and I found it hard to cut the example down to a manageable size. I've just got it down to a one-liner: it's the implementation of what I think ought to be strict fields in records:
data S = S { a :: Int, b :: ! Int }
I think ghci is correct:
*Main> a (S { a = 0, b = 1 }) 0 *Main> a (S { a = 0, b = undefined }) *** Exception: Prelude.undefined
and that hugs had been concealing a bug in my program by not demanding one of the fields of S when it ought to:
Main> a (S { a = 0, b = 1 }) 0 Main> a (S { a = 0, b = undefined }) 0
Ho hum. Is this a "known difference"?
It's certainly a bug. I suspect it is not well known. It's not documented at http://cvs.haskell.org/Hugs/pages/users_guide/haskell98.html#BUGS-HASKELL98 Also, if we instead define: data S' = S' Int !Int a' (S' x _) = x b' (S' _ x) = x Then: Main> a' (S' 0 undefined) Program error: Prelude.undefined Which is clearly inconsistent. There's something wrong in hugs with the strictness annotations on data defined using the record syntax.
(What makes you think I'm teaching the same course again this year?)
:-) As an ex teaching assistant my recommendation is "Use ghci!". Duncan
Hi
data S = S { a :: Int, b :: ! Int }
Main> a (S { a = 0, b = 1 }) 0 Main> a (S { a = 0, b = undefined }) 0
Ho hum. Is this a "known difference"?
I've submitted a bug: http://hackage.haskell.org/trac/hugs/ticket/92
As an ex teaching assistant my recommendation is "Use ghci!".
I helped to teach using WinHugs, which was quite nice. Auto reload cuts out one very frequent source of problems. Thanks Neil
Hello Neil, Monday, May 18, 2009, 8:14:56 PM, you wrote:
As an ex teaching assistant my recommendation is "Use ghci!".
I helped to teach using WinHugs, which was quite nice. Auto reload cuts out one very frequent source of problems.
i think we should fill a ticket against it. auto-save in editor + auto-reload make really incredible environment -- Best regards, Bulat mailto:Bulat.Ziganshin@gmail.com
participants (4)
-
Bulat Ziganshin -
Duncan Coutts -
Geraint Jones -
Neil Mitchell