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

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# end;;
class [’a] ref_succ :
’a ->
object
constraint ’a = int
val mutable x : int
method get : int
method set : int -> unit
end
Let us consider a more complex example: define a circle, whose center may be any kind of point. We
put an additional type constraint in method move, since no free variables must remain unaccounted
for by the class type parameters.
# class [’a] circle (c : ’a) =
# object
# val mutable center = c
# method center = center
# method set_center c = center unit)
# end;;
class [’a] circle :
’a ->
object
constraint ’a = < move : int -> unit; .. >
val mutable center : ’a
method center : ’a
method move : int -> unit
method set_center : ’a -> unit
end
An alternate definition of circle, using a constraint clause in the class definition, is shown below.
The type #point used below in the constraint clause is an abbreviation produced by the definition
of class point. This abbreviation unifies with the type of any object belonging to a subclass of class
point. It actually expands to < get_x : int; move : int -> unit; .. >. This leads to the
following alternate definition of circle, which has slightly stronger constraints on its argument, as
we now expect center to have a method get_x.
# class [’a] circle (c : ’a) =
# object
# constraint ’a = #point
# val mutable center = c
# method center = center
# method set_center c = center
object