how to debug?

Jon Fairbairn
Sun, 06 Oct 2002 12:16:50 +0100

On 2002-10-05 at 18:41EDT David Roundy wrote:
> How does one debug in haskell?

One doesn't. One writes correct code in Haskell ;-b

> I have already isolated my bug within one function, but
> that function has somewhat funky recursion, and uses an
> array (which I'm none too familiar with in haskell), and
> there aren't any smaller parts that I can see to test.  :(

More seriously: It seems to me likely that this function is
too complicated for your current level of understanding
(which probably means it's simply too complicated, full stop).

Often, a better approach than trying to debug a function is
to break the function into smaller parts using higher levels
of abstraction. For example, you say that it involves
"funky" recursion: perhaps it can be rewritten in terms of a
fold or similar?  In other words, can you abstract out the
steps performed at each level of recursion (there may be
several different steps depending on conditions)?  You say
it uses an array: can you use accumArray or accum?

If you can break it up like this, it becomes easier to
determine whether the individual parts are correct without
resorting to debugging. Giving us a skek at the function in
question might educe improved advice, too.


Jón Fairbairn                       
31 Chalmers Road                               
Cambridge CB1 3SZ            +44 1223 570179 (after 14:00 only, please!)