Debugging haskell

Ketil Z. Malde
24 Feb 2003 08:42:05 +0100

Joe English <> writes:

> Sengan Baring-Gould wrote:


>> states that "debugging often occupies three-quarters or more
>> of development time".  I don't think that is my experience
>> in Haskell... more like 1/4 at most. I was wondering what
>> others felt.

> Me either; in fact even 1/4 of the time debugging
> sounds quite high.

I think it depends a lot on various factors, and that more important
than language is project size, programmer experience, and effort spent
in the design phase.  For a small, one-programmer project, debugging
is IMHO much simpler than in a multi-programmer, multi-year project,
in particular an ill-designed one.

I think this is a fairly accepted fact in the field of software 

> When I first started using Haskell, most of my time
> went to fighting with the typechecker, but once the
> code checked it almost always Just Worked.  This is a
> pleasant experience.

I can relate to that.  On the other hand, finding the really obscure
bugs is (at least to me) *hard*.  The bugs would probably be hard
regardless of language, except perhaps lazyness-related bugs (if you
can call that kind of undesirable behaviour a bug).

> Nowadays, I spend the most time trying to understand
> the problem, relying on the typechecker to tell me when
> I've misunderstood something.

I think the type syntax itself, just not the checking tools, help.
E.g. reading Paul Graham's "A Plan for Spam", I note that in order to
segregate strings from headers and body, he rather awkwardly prepended
header strings with the originating header -- e.g. "Subject: foo" were
stored as "Subject*foo".  Much nicer to do

        data Token = Header String | Body String deriving ...

If I haven't seen further, it is by standing in the footprints of giants