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The problem is that class ['a] store after adding o1 only accepts objects that 
have both an id method AND a hi method.  So to make it work you need to do 
something like this:


let o1 = object method id = 1  method hi = "hi" end;;
let o2 = object method id = 2  method ho = "ho" end;;
class ['a] store = object 
  val mutable ids = []
  method add (o: 'a) =  ids <- o#id :: ids
end;;
let s = new store ;;
s#add (o1 :> <id:int>);;
s#add (o2 :> <id:int>);;

Note how I coerce o1 and o2 both to a subtype before adding.  Now the problem 
goes away because to the s#add method o1 and o2 have the same type.



BUT, what I think you really want is a polymorphic method, not a polymorphic 
class.  The manual has a decent explanation under polymohrphic methods in the 
Objects section.  So I think you want this:


let o1 = object method id = 1  method hi = "hi" end;;
let o2 = object method id = 2  method ho = "ho" end;;
class store = object 
  val mutable ids = []
  method add : 'a. <id:int; ..> as 'a -> unit = 
    fun o -> ids <- o#id :: ids
end;;

let s = new store ;;
s#add o1;;
s#add o2;;



Peng



On Thursday 06 March 2008 07:52:38 am Michael Wohlwend wrote:
> I try to do the following:
>
> I have some objects (all with an id method):
> let o1 = object method id = 1  method hi = "hi" end
> let o2 = object method id = 2  method ho = "ho" end
>
> and now:
>
> class store = object
> 	val mutable  ids = []
> 	method add o =  ids <- o#id :: ids
> end
>
> this doesn't work, since the hidden type variable in the argument to add (I
> think). But why can't the compiler just set o to be of type < obj:int; ..>
> ?. So I try:
> class ['a] store = object
> 	val mutable  ids = []
> 	method add (o: 'a) =  ids <- o#id :: ids
> end;;
>
> ...but...
>
> let s = new store ;;
> give the type:
> val s : < id : '_a; _.. > store = <obj>
>
> so this doesn't work:
>
> s#add o1;
> s#add o2
>
> his expression has type < hi : string; id : int > but is here used with
> type < ha : string; id : int >
> The second object type has no method ho
>
> If I define a function, say:
> let f o = o#id;;
> I get :
> val f : < id : 'a; .. > -> 'a = <fun>
> so this works:
> f o1; f o2
>
>
> why does this not work with methods?
>
> cheers
>  Michael
>
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