[Haskell-cafe] how do you debug programs?

[Pepe Iborra]

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.
For instance:

import Observe

f' = observe "f" f
f a b = ....

And then in hugs the expression:
> f' 1 2

would output what you want.

Finally, the GHCi debugger project [4] aims to bring dynamic
breakpoints and intermediate values observation to GHCi in a near
future. Right now the tool is only available from the site as a
modified version of GHC, so unfortunately you will have to compile it
yourself if you want to try it.

Cheers
pepe


1. www.haskell.org/hat
2. www.haskell.org/hood
3. http://cvs.haskell.org/Hugs/pages/users_guide/observe.html
4. http://haskell.org/haskellwiki/GHC/GHCiDebugger
5. http://www-users.cs.york.ac.uk/~ndm/projects/windows.php

On 06/09/06, Tamas K Papp <tpapp at princeton.edu> wrote:
> Hi,
>
> I would like to learn a reasonable way (ie how others do it) to debug
> programs in Haskell.  Is it possible to "see" what's going on when a
> function is evaluated?  Eg in
>
> f a b = let c = a+b
>             d = a*b
>         in c+d
>
> evaluating
>
> f 1 2
>
> would output something like
>
> f called with values 1 2
> c is now (+) a b => (+) 1 2 => 3
> d is now (*) a b => (*) 1 2 => 2
> ...
>
> Or maybe I am thinking the wrong way, and functional programming has
> its own debugging style...
>
> Thanks,
>
> Tamas
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