Typeclass in haskell

Question

I'm extremely new to haskell and was trying to implement a small and a simple function that takes two strings and tells me the number of same characters at the same location.

ed :: (Integral b) => [a] -> [a] -> b
ed _ [] = 0
ed [] _ = 0
ed [] [] = 0
ed (x:xs) (y:ys)
    | x == y = 1 + ed xs ys
    | otherwise = ed xs ys

this doesn't run because my typeclass definition is wrong. I have two strings and need to return an integer and hence the typeclass definition I have written above. Is there something else I need to do?

Answer 1

The type signature should be

ed :: (Eq a, Integral b) => [a] -> [a] -> b

This is because your definition of ed includes the expression x == y. x and y both have type a; to be able to test them for equality, this type must implement the Eq typeclass, which provides the == and /= operators.

The error message you got would have included something like this:

Could not deduce (Eq a) arising from a use of `=='
from the context (Integral b)
  bound by the type signature for ed :: Integral b => [a] -> [a] -> b
  at /home/dave/tmp/so.hs:(2,1)-(5,26)
Possible fix:
  add (Eq a) to the context of
    the type signature for ed :: Integral b => [a] -> [a] -> b

which was trying to tell you this.

(Btw, your code doesn't handle the case when when the strings have different lengths.)