Damien [Thursday 13 March 2003] :
 > My goal is to define a tree, whose node's type is a class
 > type a_t, containing a method for adding children.
 > but the child type can be any subtype of a_t
 > 
 > * this method can safely be typed [a_t->unit], but this will require a
 > lot of coercions in the rest of the code...

How about adding a coerce method in a_t ? That way, the coercion
appears only in the class definition, not everytime you want to add a
node.

,----
| class type a_t = object
|   method add : a_t -> unit
|   method children : a_t list
|   method coerce : a_t
| end
| 
| class a : a_t =
|   object (self)
|     val mutable children = []
|     method add c = children <- c :: children
|     method children = children
|     method coerce = (self :> a_t)
|   end
`----
 and then :

  parent#add child#coerce

or even :
let adder p c =
  p#add c#coerce

Hope this helps,

-- 
   Olivier

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