Inferred type is not general enough

Question

I'm wondering if a list is a prefix of a second list, using the following code:

prefix :: [a] -> [b] -> Bool
prefix [] _ = True
prefix _ [] = False
prefix (x:xs) (y:ys) = if (x==y) then prefix xs ys else False

But it returns an error:

Inferred type is not general enough
*** Expression    : prefix
*** Expected type : [a] -> [b] -> Bool
*** Inferred type : [a] -> [a] -> Bool

Can someone help me get this to work?

Answer 1

Your type signature claims the two lists can have different types, but they can't. So the compiler complains that it inferred a type that was less general than the one you asked for.

Answer 2

That's because you're comparing the types you've named a and b using (x==y). Checking the type of ==, that means they are the same type, which has an equality test:

Prelude> :t (==)
(==) :: Eq a => a -> a -> Bool

So the type signature that is inferred is actually Eq a => [a] -> [a] -> Bool.