[Haskell-cafe] Prelude.undefined

Alexander Dunlap alexander.dunlap at gmail.com
Wed Mar 3 01:24:23 EST 2010

On Tue, Mar 2, 2010 at 9:06 PM, Tom Hawkins <tomahawkins at gmail.com> wrote:
> How do I track down an reference to an undefined value?  My program
> must not be using a library correctly because the program makes no
> direct use of 'undefined'.  Running with +RTS -xc yields:
> <GHC.Err.CAF>Test: Prelude.undefined
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While the debugger, etc., are very useful tools, I find that often the
easiest way to track down this sort of bug is to test your code
function-by-function. Make sure each function does what you want it to
when run in isolation; this will quickly lead to tracking down the
caller of "undefined". (If your code is not structured in a way that
allows this sort of testing, you might consider restructuring it to
make it more modular.)


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