Incorrect warning: Pattern match(es) are overlapped

[Regidor]

The following program

main = print $ f ab

a = "foo"
ab = "bar"

f :: String -> Int
f a = 0
f ab = 1

produces

Warning: Pattern match(es) are overlapped
             In the definition of `f': f ab = ...

but the patterns don't overlap, i.e., if a matches, ab does not match, and viceversa.

[duncan]

This is a misunderstanding of the code. I think you're assuming that the 'a' and 'ab' parameters of the function 'f' are standing for the patterns "foo" and "bar", which is not the case.

These 'a' and 'ab' parameters are local variables, they have no relation to the 'a' and 'ab' values you define at the top level.

So if we change the variable names (which is ok as I say, because they're local names) you can see more clearly that they overlap:

f x = 0
f y = 1

both patterns 'x' and 'y' will match any input value, so the first pattern overlaps the second completely. The second case can never happen. So the warning message is correct.

Does that make sense?

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(I'm closing the bug as invalid)