help needed for adding isWHNF primop to 5.00.2

Malcolm Wallace
Tue, 31 Jul 2001 14:25:50 +0100

> > I know that Andy Gill was also interested in
> > having similar facilities, perhaps he can help out.
> I wonder if Andy is reading this? Possibly there are other people
> around who are also interested in this. I think the HAT people
> at NHC headquarters are also interested in these things.

Certainly a couple of us at nhc HQ are listening in.  In the old Hat
system, we used an impure test for WHNF.  In the new Hat, it is no
longer needed.  The reason we could abandon it is because we moved
to a scheme for writing the trace to file, using a mechanism pretty
similar to how HOOD does it.

However, it is worth pointing out that this means we store trace
information about (potentially) every reduction that ever happens -
precisely what you are trying to avoid on grounds of cost.  It must
be admitted that Hat currently has a significant slowdown factor
when running programs, but in user tests we have no complaints yet
about speed.  The value of keeping the complete trace information is
that, once you have it, you can ask an enormous number of questions
about it, getting answers rapidly without needing to recompile or rerun
the program.  In the trade-off between information-richness and speed,
I think your average Haskell user trying to find a tricky program
fault is really hungry for all the information they can get, and will
be prepared to wait a little longer provided the info is good enough.

> There once was talk of starting a debugging group for Haskell, but I
> don't know what happened.

The mailing list was set up and still exists,
although I don't think it has ever had any traffic.  I'd be pleased
to see it being used.