This error is usually caused by a type mismatch. In your code, the type signature of the defined match function is (Int-> b)-> [Token]-> Either String b1, while the type of matchInt function in the instance is (Int-> b)-> [Token]-> Either String b. Here b1 and b are treated as different type variables, resulting in a type mismatch error. To solve this problem, you need to make sure that the type of the match function and the matchInt function are exactly the same, that is, they both use the same type variable b. You may need to check the implementation and type declaration of the function to ensure that they are coordinated and consistent in type.