Question

What does

irrefutable pattern failed for pattern

mean?
What cases will cause this runtime error?

Answer 1

Well, I assume it means what it says – that a pattern doesn’t match but there is no alternative. This example:

But for the program:

g x = let Just y = f x in h y 

GHC reports:

Main: M1.hs:9:11-22:
    Irrefutable pattern failed for pattern Data.Maybe.Just y 

Indicating the source of the failure.

Comes from http://www.haskell.org/haskellwiki/Debugging

The point of the example is that if f x returns Nothing then there is no way GHC can assign a value to y.