Debugging haskell

[D. Tweed]

On Mon, 24 Feb 2003, Malcolm Wallace wrote:

> Joe English <jenglish@flightlab.com> writes:
> 
> > Me either; in fact even 1/4 of the time debugging
> > sounds quite high.
> > 
> > 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.
> 
> But surely "fighting the typechecker" is exactly a debugging activity.
> The great benefits are that you remove the bugs before even running
> the code, and you are finding specification errors in addition to
> mere implementation errors.

[standard caveat: I write research code for image processing problems,
which may well be very different to, e.g., code implementing a COM
wrapper.]

Mentally I classify bugs into two kinds: `source code bugs' and
`algorithm bugs'. Source code bugs are when the program you write isn't
doing the same thing as your mental model of your algorithm, algorithm
bugs are when your code is performing the same as your mental
algorithm. You can't really argue that `algorithm bugs' aren't bugs
because

(i) it's easy to have a specification which is precise without
being either obviously acheivable and having no obvious algorithm for
performing it; sometimes the only way to make progress on the program is
to try and iteratively develop an algorithm/implementation to solve it.

(ii) more importantly, when something doesn't work you don't get a clear
indicator of which kind of bug it is, and you often have to engage in
`traditional debugging' to finally see that the code is doing what you
think it is and that your errors are due to an algorithm bug.

If you discard `compliation preventing, very very quick to solve' bugs
(e.g., missing semi-colons in C++, silly typecheck errors in Haskell) I
find that the ratio between source code bugs and algorithm bugs is maybe
1:5. This means that whilst I find Haskell a great deal easier to write
correctly than C++, there's not that much difference between debugging
times for Haskell and C++ because the algorithm level bugs dominate. (In
fact, it's in some ways easier to do algorithm debugging in C++ because
it's very easy to temporarily stash things in global variables so you can
have it available for `debugging processing' in parts of the program where
it isn't available under the normal argument passing. Of course you then
have to clean the mess up...)

And I'd certainly say 3/4 time spent debugging is probably an
underestimate for me (given the def of debugging above).

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