Curious behaviour of irrefutable pattern.

Hi, consider this program:

module Main (f, main) where

f ~(a:as) = 1 + f as

main = print $ f (error "Foobar!")

Obviously, the program should result in an error - the irrefutable pattern of f always succeeds, so f calls itself recursively ad infinitum and the result is <<Loop>> or nontermination. Or so I thought. Running this program after compiling with ghc -O (with 6.4.2, 6.6 and 6.7) results in: a.out: Foobar! So the 'error "Foobar!"' got evaluated. The compiler somehow replaced one 'bottom' with another which is arguably allowed. I wasn't able to come up with an example where it turned a bottom into a value or vice versa. Anyway, this looks suspicious to me, so what's happening here? regards, Bertram