how to debug?

[Zdenek Dvorak]

Hello,

>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.  :(

aside from (useful) general advices already given, just few tips that I
found useful when I needed to debug:

-- to print debug dumps, you may use 'trace' function from IOExts (but 
beware,
   it may affect behavior of your program if used incorrectly)
-- other way (safer, but slower to learn and use) is to use Hood library
-- if your program fails with some really informative error message of type
   '*** Exception: Maybe.fromJust: Nothing', compiling it with profiling
   (-prof -auto-all) and running with +RTS -xc will give you a stack dump
   (pretty useful, well hidden feature :-)

Zdenek Dvorak

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