[Haskell-cafe] How to debug GHC

Ben Lippmeier Ben.Lippmeier at anu.edu.au
Fri Sep 2 03:10:35 EDT 2005

>> ... It's very hard to debug a large program when you
>> can randomly get messages like "*** Exception: Prelude.head: empty
>> list" and have no idea where they came from. 

As a purely pragmatic suggestion: don't use head, fromJust, last, or any 
other function that is likely to fail in impossible-to-find way, at 
least not directly.

In GHC, you can wrap or replace them with irrefutable patterns which are 
  almost as easy to write, and will give you a sensible error message if 
they fail.


  replace  x        = head xx
  with     (x:_)    = xx

  replace  x        = fromJust mX
  with     (Just x) = mX

  replace  x        = last xx
           y@(_:_)  = xx
           x        = last y


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