[Haskell] modern language design, stone age tools
Robert.Ennals at cl.cam.ac.uk
Wed Jun 23 04:27:26 EDT 2004
I wrote such a debugger as part of my PhD work. It is called "hsdebug" and you
can read the Haskell Workshop paper on it here:
Unfortunately, HsDebug depends on Optimistic Evaluation, and it seems unlikely
that Optimistic Evaluation will appear in a release version of GHC (too hard
to maintain, and I've gone off to work for Intel Research).
HsDebug is a GDB-style debugger which takes advantage of the fact that
Optimistic Evaluation makes programs evaluate in a largely-strict evaluation
order. It also uses a trick called "transient tail frames" to allow tail calls
to be visible in stack traces.
HsDebug can debug any GHC-compilable Haskell program, including GHC itself.
> Seeing as Haskell is apparently such a popular language these days,
> I don't suppose a working debugger would be too much to ask for, would it?
> Or even just a decent call stack trace when a program terminates with an exception?
> In case you're wondering, yes I have already tried using Hat and Buddha.
> But I'm trying to debug a real application, not a toy one, and neither Hat
> nor Buddha support enough of Haskell. Buddha doesn't even come close,
> as far as I can tell, and Hat lacks support for library modules such as Data.Word.
> I've also tried using ghc's profiling to get stack traces, but that doesn't
> work very well either. Firstly, the stack traces that you get only include
> function names, not line numbers. Secondly, the stack traces are often
> incomplete, for reasons that I don't fully understand. One likely cause is
> tail call optimization. (Is there any option to switch off tail call optimization?
> I didn't see any such option in the ghc man page.)
> Thirdly, profiling seems to be incompatible with the use of ghci; there
> doesn't seem to be any easy way to build a workspace so that you can
> get stack traces and use ghci in that workspace at the same time.
> And ghc is slow enough (even on a 3.2GHz Pentium 4) that recompiling
> the whole workspace is highly unpalettable.
> There's a hell of a lot of languages out there that _do_ support decent
> stack traces when an exception is thrown.
> What's the point of using a high-level functional language if it means
> you're stuck with poor library support and/or stone-age tools?
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