a trap for the unwary

Today, I thought I had discovered a bug in ghc. Then I tried hbc and Hugs, and they also rejected my program with the same error. nhc98 alone accepts it without complaint. I looked up the Report, and it seems that the program is indeed incorrect. Quick quiz: without running this through a compiler, who can spot the mistake? :-)

module Main where import Char f x = y where y | isSpace x = True y | otherwise = False main = print (f 'x')

Regards, Malcolm