error.txt

Illegal instance declaration for `Monoid [Char]'
      (All instance types must be of the form (T a1 ... an)
       where a1 ... an are *distinct type variables*,
       and each type variable appears at most once in the instance head.
       Use -XFlexibleInstances if you want to disable this.)
    In the instance declaration for `Monoid [Char]'
Failed, modules loaded: none.

HigerKinds.hs

class Monoid a where
      append :: a -> a -> a
      identity :: a 

foldLeft :: [a] -> b -> (b -> a -> b) -> b
foldLeft [] x f = x
foldLeft (h:l) x f = foldLeft l (f x h) f


instance Monoid Int where
         append x y = x + y
         identity = 0
 
instance Monoid [Char] where
         append x y = x ++ y
         identity = ""
         
 
mysum :: (Monoid a) => [a] -> a
mysum l = foldLeft l identity  append 

main :: IO ()
main = do 
              let x = [1::Int, 2, 3]
                  y = ["1", "2", "3"]
              print (mysum x)
              print (mysum y)