The Objective Caml system release 3.06 - The Caml language - Inria

48
int ->
object
val mutable x : int
method distance : #point0 -> int
method get_offset : int
method get_x : int
method move : int -> unit
end
# let p = new distance_point 3 in
# (p#distance (new point 8), p#distance (new colored_point 1 "blue"));;
- : int * int = (5, 2)
Note here the special syntax (#point0 as ’a) we have to use to quantify the extensible part
of #point0. As for the variable binder, it can be omitted in class specifications. If you want
polymorphism inside object field it must be quantified independently.
# class multi_poly =
# object
# method m1 : ’a. (< n1 : ’b. ’b -> ’b; .. > as ’a) -> _ =
# fun o -> o#n1 true, o#n1 "hello"
# method m2 : ’a ’b. (< n2 : ’b -> bool; .. > as ’a) -> ’b -> _ =
# fun o x -> o#n2 x
# end;;
class multi_poly :
object
method m1 : < n1 : ’a. ’a -> ’a; .. > -> bool * string
method m2 : < n2 : ’b -> bool; .. > -> ’b -> bool
end
In method m1, o must be an object with at least a method n1, itself polymorphic. In method m2,
the argument of n2 and x must have the same type, which is quantified at the same level as ’a.
3.11 Using coercions
Subtyping is never implicit. There are, however, two ways to perform subtyping. The most general
construction is fully explicit: both the domain and the codomain of the type coercion must be
given.
We have seen that points and colored points have incompatible types. For instance, they cannot
be mixed in the same list. However, a colored point can be coerced to a point, hiding its color
method:
# let colored_point_to_point cp = (cp : colored_point :> point);;
val colored_point_to_point : colored_point -> point =
# let p = new point 3 and q = new colored_point 4 "blue";;
val p : point =
val q : colored_point =
# let l = [p; (colored_point_to_point q)];;
val l : point list = [; ]