strictness of interpreted haskell implementations

[Duncan Coutts]

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