[Haskell-beginners] Type error in function binding

Hi. I'm trying to write a code which counts the number of  'a's  in a string. it looks like this : mycounter st = if st == [] then 0 else if head st == 'a' then 1 + mycounter tail st else mycounter tail st but it gives this error :  ERROR "deneme.hs":19 - Type error in function binding *** Term           :mycounter *** Type           : [Char] -> b *** Does not match : ([a] -> [a]) -> [Char] -> b *** Because        : unification would give infinite type I have no idea about what I'm doing wrong. Can you help me with this please?