On Wed, Sep 06, 2006 at 11:34:05AM +0200, Pepe Iborra wrote:
Hi Tamas
There are several ways to debug a Haskell program.
The most advanced ones are based in offline analysis of traces, I think Hat [1] is the most up-to-date tool for this. There is a Windows port of Hat at [5].
Another approach is to simply use Debug.Trace. A more powerful alternative for this approach is Hood [2]. Even if it hasn't been updated in some time, Hood works perfectly with the current ghc distribution. Even more, Hugs has it already integrated [3]. You can simply import Observe and use observations directly in your program.
Dear Pepe, Thank you for the information. I finally ended up working with Debug.Trace, and found the bug very quickly. I also tried Hood, but couldn't load it in ghci: import Observe can't find the library, but % locate Observe /usr/lib/ghc-6.4.2/hslibs-imports/util/Observe.hi /usr/lib/ghc-6.4.2/hslibs-imports/util/Observe.p_hi /usr/lib/hugs/libraries/Hugs/Observe.hs /usr/lib/hugs/oldlib/Observe.hs Does importing from hslibs-imports require something special? Quite a bit of philosophical discussion erupted as a result of my original question. I understand the arguments of those who dislike debuggers, but I don't think I could have done without some debugging. It turns out that the problem was not in the algorithm, but in the specification of the equation itself. I was solving a continuous time Hamilton-Jacobi-Bellman equation, and there was a point in the state space where the supremum was infinity, giving stuff like 0/Inf and their ilk. I respecified the problem and now it works. For those who think that it is always possible to break the problem up into small pieces you can test individually (without a debugger): you can't, at least not when solving functional equations. The corner cases (and nonconvergence, when using non-contraction methods, such as PEA) are difficult to catch and reproduce. Thanks for all the suggestions, Tamas