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[Caml-list] subtyping and inheritance (bug?)
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Date: 2002-06-25 (21:35)
From: Brian Rogoff <bpr@a...>
Subject: Re: [Caml-list] subtyping and inheritance (bug?)
Eray Ozkural writes:
> On Tuesday 25 June 2002 05:54, Jacques Garrigue wrote:
> > Sure. The children_set method contains a contravariant occurence of 'self.
> > It cannot be compatible with its subclasses.  You don't need to have
> > mutable data to have variance incompatibilities.
> > No bug, business as usual.
> Which programming style would express the intended semantics, then? I guess we 
> would need a slightly different formulation.

Yes, you need a different formulation. If you want to have a tree class but
want to create a family of color_trees which can be coerced to each other
and put on the same list, something like this 

class ['a] tree =
    val v = (None : string option)
    val ch = ([] : 'a list)
    method value_get = v
    method value_set x = {< v = x >}
    method children_get = ch
    method children_set x = {< ch = x >}

class color_tree =
    inherit [color_tree] tree
    method color = "green"

class pine_tree =
    val mutable age = 0.0
    inherit color_tree
    method color = "evergreen"
    method age = age

class fake_tree =
    inherit color_tree
    method color = "olivegreen"
    method material = "fantastic plastic"

now these can all be coreced to color_tree, and they have a colortree list 
type on method children_get. 

let colortree = new color_tree;;
let pinetree = new pine_tree;;
let faketree = new fake_tree;;

let trees = [colortree; 
	     (pinetree :> color_tree);
	     (faketree :> color_tree)];;

faketree#children_set trees;;

If you create a new class, 

class palm_tree =
    inherit [palm_tree] tree
    method color = "palmgreen"
    method fronds = true

then you won't be able to do the coercion. 

let palmtree = new palm_tree;;
(palmtree :> color_tree);; (* ERROR! *)

-- Brian

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