Re: [Haskell-cafe] Haskell stacktrace

Pieter Laeremans wrote:

Hello,

I've written a cgi script in haskell, it crashes sometimes with the error message Prelude . tail : empty list Yup, been there, done that.

First, look for all the uses of "tail" in your program and think hard about all of them. Wrap them in "assert" or "trace" functions to see if there are any clues to be had. Then take a look at the GHC debugger documentation. Debuggers for lazy languages (or indeed any language with closures) is difficult because execution order isn't related to where the faulty data came from. So if I say somewhere x = tail ys and then later on print x the "print" will trigger the error, but the faulty data "ys" may no longer be in scope, so you can't see it.