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[Caml-list] does class polymorphism need to be so complicated?
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Date: -- (:)
From: Benjamin Geer <ben@s...>
Subject: Re: [Caml-list] does class polymorphism need to be so complicated?
brogoff@speakeasy.net wrote:
> The extension of Brian's code to rows with more than one field is obvious 
> though, isn't it? 
> 
> type fbbq = 
>   <foo: int->int; bar: unit->string; baz: unit->bool;quux: unit->unit>
> 
> class thing_processor = 
>   object 
>     method do_something (o : fbbq) =  
>       (* do something that calls all those methods *)
>     method do_something_else (o : fbbq) =  
>       (* do something else that calls all those methods  *)
>   end

Unless I've missed something, this only works if none of the classes of 
type fbbq have any additional methods.  Here's an example that doesn't work:

type fbbq =
   <foo: int->int; bar: string; baz: bool; quux: unit > ;;

class thing x_init =
    object (self)
      val mutable x = x_init
      method foo y = x + y
      method bar = string_of_int (self#foo 2)
      method baz = (x = 1)
      method quux = print_string (string_of_int x)
  end;;

class extra_thing x_init =
    object (self)
      inherit thing x_init
      method quuux = self#quux; self#quux
  end;;

class thing_processor =
   object
       method process (obj : fbbq) =
	obj#quux;
	print_string (string_of_int (obj#foo 1));
	print_string obj#bar;
	print_string (string_of_bool obj#baz)
   end ;;

let et = new extra_thing 1 ;;
let tp = new thing_processor ;;
tp#process et ;;

The call to 'process' produces the following error:

This expression has type
   extra_thing =
     < bar : string; baz : bool; foo : int -> int; quuux : unit; quux : 
unit >
but is here used with type
   fbbq = < bar : string; baz : bool; foo : int -> int; quux : unit >
Only the first object type has a method quuux

So this approach doesn't fit the requirement, which is to be able to use
a derived class anywhere its base class can be used.

Ben

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