(* the signature is general and the structures are the implementation *)
signature abstractFunction = sig
	type element1;
	type element2;
      type element3;
	val f1: element1 -> element2;
	val f2: element1 -> element3;
end;

structure concreteFunction1:abstractFunction = struct

 type element1 = int;
 type element2 = bool;
 type element3 = int;	 
 fun f1 (i) = i=2;
 fun f2 (i) = i+2;
end;

structure concreteFunction2:abstractFunction = struct
  type element1 = int;
  type element2 = bool;
  type element3 = bool;
  fun f1 (i) = i>2;
  fun f2 (i) = i < 2;
end;

(* works because the types of the function are the same *)
val l1 = [concreteFunction1.f1, concreteFunction2.f1]; 

(* doesn't work because the types of the function are different *)
val l2 = [concreteFunction1.f2, concreteFunction2.f2];