The principal type needs an equality type class constraint:
isPrefix :: Eq a => [a] -> [a] -> Bool
isPrefix [] _ = True
isPrefix (_:_) [] = False
isPrefix (x:xs) (y:ys) = x == y && isPrefix xs ys
The empty list is a prefix of every list, witnessed by the second list itself as the remaining suffix. A nonempty prefix cannot fit in the empty list. Otherwise the first elements must agree, and the remaining first list must be a prefix of the remaining second list. These cases prove correctness by structural induction on the candidate prefix. The Eq a constraint is required by x == y; no ordering constraint is needed.

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