How does one debug in haskell? I have a function that I could swear should behave differently than it does, and after tracking down bugs for many hours, I'm wondering if there's any way to step through the evaluation of a haskell function? The other way I would be debugging in an imperative language would be to sprinkle printfs around. Is there any way to do something like that cleanly in haskell? The only thing I can think of would be to modify every function to accept an additional parameter, which seems like it's more likely to introduce bugs than remove them... For what it's worth I'm using ghc, and the only debugging options it seems to have are for debugging the compiler itself. I would be happy to install and use hugs for debugging if it has some nicer debugging mode. 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. :( -- David Roundy http://civet.berkeley.edu/droundy/
On Sat, 5 Oct 2002 18:41:06 -0400
David Roundy
How does one debug in haskell?
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 -- Jón Fairbairn Jon.Fairbairn@cl.cam.ac.uk 31 Chalmers Road jf@cl.cam.ac.uk Cambridge CB1 3SZ +44 1223 570179 (after 14:00 only, please!)
On Sun, Oct 06, 2002 at 12:16:50PM +0100, Jon Fairbairn wrote:
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).
Indeed it was (and still is, to an extent). The difficulty was that I was trying to implement the Hunt-Szymanski LCS algorithm without really understanding how it works, just by implementing the algorithm as original 1977 paper.
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?
Actually, after looking at it again, I realized that the only funky recursion was that I was implementing the equivalent of two nested iterative loops with one recursive fuction. When I broke this into two functions (one of which was a one-liner, and the other almost an identical copy of the code from the original non-working function), everything started working. I think this advice, together with a day of rest, was just what I needed! It took a while to work out a few other bugs (mostly related to the fact that Hunt and Szymanski assumed two strings of equal length for simplicity), but I now have a working and reasonably fast LCS functions. :) My "diff" command (written to use as a speed test) seems just about 10 to 15 times slower than GNU diff for a particular pair of files, which isn't bad considering that I have done nothing to optimise for speed! btw, thank you also to everyone else who gave me advice! -- David Roundy http://civet.berkeley.edu/droundy/
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