-fwarn-unused-matches

[Malcolm Wallace]

"Serge D. Mechveliani" <mechvel at botik.ru> writes:

>       ghc -c -fwarn-unused-matches
> says
>       Warning: Defined but not used: x
> when compiling the function
> 
>   f :: Eq a => [(a, a)] -> (a, a) -> [(a, a)]
>   f            ps          (x, y) =  [(z, y) | (z, x) <- ps]
> 
> Is it a bug?

No.  The x on the right is in a different scope to the x on the left.
You probably intended to write:

    f            ps          (x, y) =  [(z, y) | (z, x') <- ps, x==x']

Regards,
    Malcolm