a trap for the unwary

[Malcolm Wallace]

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