datatype Type = TyVar of string | TyCons of string * Type listRepresentation of a simple type expression language. TyVar a represents a type variable a. TyCons (c,tys) represents a type contructor c.
Example:
TyCons ("int",[])represents the type int.
TyCons ("fun", [TyVar "a", TyCons ("int",[])])represents the type a->int