[Haskell-cafe] Haskell stacktrace
paul at cogito.org.uk
Wed Sep 10 14:59:47 EDT 2008
Pieter Laeremans wrote:
> 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
the "print" will trigger the error, but the faulty data "ys" may no
longer be in scope, so you can't see it.
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