a trap for the unwary
Fri, 1 Dec 2000 10:41:32 -0500
Malcolm Wallace writes:
> 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 -- ** The problem line?
> > main = print (f 'x')
Without running this through the compiler, but based on similar
problems I've had recently, I'd assume the problem is the marked
line. Two outer-level patterns are each presented with guards. This
would be correct for a function definition:
> f x = y ()
> y _ | isSpace x = True
> y _ | otherwise = False -- ** Does this work?
This is a tricky issue. I'd like the original program to be all
right. We end up sowing confusion with erroneous programs like this
> f x = y
> y | otherwise = False -- ** Now this pattern overlaps!
> y | isSpace x = True
But of course an analogous problem occurs in the function definition,
and I think can be caught by turning on warnings in ghc.