[Haskell-beginners] Type error in function binding

[bahadýr altan]

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?
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