[Haskell] modern language design, stone age tools
fjh007 at galois.com
Wed Jun 23 14:58:22 EDT 2004
On 23-Jun-2004, Hal Daume III <hdaume at ISI.EDU> wrote:
> On Wed, 23 Jun 2004, Fergus Henderson wrote:
> > On 23-Jun-2004, MR K P SCHUPKE <k.schupke at imperial.ac.uk> wrote:
> > > This may not be the right answer to the question (which is of
> > > course lets write a debugger) - But I have never used a debugger,
> > > and find them more or less the most unfriendly and useless things
> > So how do you debug problems like "Prelude.head: empty list"
> > in large programs?
> Wasn't addressed to me, but here's what I do:
> write the following function:
> head_ x  = error ("head_: " ++ show x)
> head_ _ l = head l
> and then replace each occurance of "head" with "head_ 1" or "head_ 2"
> etc., so I can know where it failed.
Well, there are quite a lot of such occurrences in the code that I'm working
bash$ find . -name \*.hs -o -name \*.lhs | xargs grep -w head | wc -l
Replacing all of those occurrences by hand is going to be very very
tedious and somewhat time-consuming. Doing it with a script would be
better, but that's not a trivial task.
Even once that is done, there's no guarantee it will actually help to
find the problem. After all, the problem might well be arising from a
call to "head" in one of ghc's standard libraries:
bash$ find ~/ghc6-6.2/hslibs \*.hs -o -name \*.lhs | xargs grep -w head | wc -l
So not only do I have to edit my own code, and the libraries written by
my colleagues, I also need to edit the ghc library code, and figure out
how to build and reinstall the ghc libraries. That could take a long time.
After all that, hopefully I will finally know which function called
"head" with an empty list. But even then there's still no guarantee
that I've actually found the source of the problem; the real problem
might be in that function's caller, or the caller's caller, etc. So I
might have to go through the whole process again.
> (it is rather sad that this is the best approach i could come up
> with...basically tries to get around the [lack of/uselessness of/inability
> to use with ghci] stack traces)
Fergus J. Henderson | "I have always known that the pursuit
Galois Connections, Inc. | of excellence is a lethal habit"
Phone: +1 503 626 6616 | -- the last words of T. S. Garp.
More information about the Haskell