# User Error

Gah. I’m trying to write some straightforward combinatorial code in Haskell, and I finally got to the point where I had to start using nontrivial algorithms to make it acceptably fast. When I run those algorithms the first time, I get the following gem:

    *Main> drops 100 2
*** Exception: divide by zero


The “drops” function is now 15 lines of mutually recursive Haskell, and the interactive interpreter conveniently informs me that there was a division by zero. No hint as to where it might be, of course. Come to think of it, this was probably the reason I dropped back to Python after my last Haskell stint.

This came up all the time in OCaml compiler work too. Deep in the middle of an optimization pass, I’d mess up a symbol context. The compiler would conveniently inform me that

    NotFound


OCaml exceptions are monomorphic, so the default exception thrown by map classes couldn’t even include the key, much less something like a stack trace.

The conclusion: any language that claims to be high level should be able to give the exact context in which errors occurred. This means either some form of trace reporting and/or the ability to rule out the possibility of certain errors (dependent types). Yes, both Haskell and OCaml have debuggers for this purpose, but that isn’t good enough; in my case the history function in the Haskell prints out gobs and gobs of tail recursion frames before it ever gets to the real source of the problem. Debugging in high level languages should be easy, not just possible.

Update: the problem turns out to be integer overflow, which means I’m hitting the same language flaw Guido van Rossum described here. Converting my code from Int to Integer requires adding a slew of conversion calls and type annotations because I’m using integers as both container indices and as the results of large combinatorial values. Hopefully Tim Sweeney’s idea will work out and this dichotomy can go away.

Update (16mar2009): Possible hope for the future:

http://ghcmutterings.wordpress.com/2008/12/04/explicit-stack-traces