[Haskell-cafe] How to debug GHC

[Ben Lippmeier]

>> ... 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.

Example:

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

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

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


Ben.