Haskell - Pattern match(es) are overlapped

test :: String -> String -> Int

test' x y n = n
test' "" (y:ys) n = error "error"
test' (x:xs) "" n = error "error"
test' (x:xs) (y:ys) n =
        if      x == y
        then    test'  xs ys n
        else    test'  xs ys (n+1)
test a b = test' a b 0

When I compile this, I get this output:

Warning: Pattern match(es) are overlapped

And the answer is always "0", which is not what I intended. What is the开发者_运维百科 problem with the code and how to fix it?

test' x y n = n will match for every call, the other patterns won't be considered. I think this case should be test' "" "" n = n. You get the same result if you move your original line at the end (when all other cases fail), but then you should write test' _ _ n = n which shows that you deliberately ignore some of the arguments.

[Edit]

A shorter solution would be:

test a b | length a == length b = sum $ map fromEnum $ zipWith (/=) a b
         | otherwise = error "error" 

The zipWith expression generates a list of Bool which is True for every difference. The function fromEnum maps False to 0 and True to 1.

The patterns are tried in order. The first of your patterns for test' always matches, so that case is always used. The first case should probably be

test' "" "" n = n

instead.