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To: caml-list@inria.fr
Subject: [Caml-list] implicit subtyping in method argument and parameterized class
From: Pixel <pixel@mandrakesoft.com>
Date: 05 Sep 2002 01:18:19 +0200
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--=-=-=

With ocaml 3.06 we are now able to replace

    method with_foo : foo -> unit = ...
with
    method with_foo : 'a. (#foo as 'a) -> unit = ...

and get implicit subtyping when calling with_foo.


But I can't make it work when "foo" is a parametric variable:

    method with_foo : 'foo -> unit = ...
works whereas
    method with_foo : 'a. (#'foo as 'a) -> unit = ...
gives "Syntax error", and
    method with_foo : 'a. ('foo as 'a) -> unit = ...
gives "This type has no row variable"

Is there a solution?

thanks.


Full example: 

--=-=-=
Content-Disposition: attachment;
  filename=implicit-method-argument-subtyping.ml

module Before_ocaml_3_06 = struct
  class foo = object method foo = () end
  class foo' = object inherit foo method foo' = () end
  
  class on_any_foo =
    object
        method with_foo : foo -> unit = fun foo -> foo#foo
    end
  
  class ['foo] on_that_foo =
    object
        method with_foo : 'foo -> unit = fun foo -> foo#foo
    end
  
  class on_foo  = [foo ] on_that_foo
  class on_foo' = [foo'] on_that_foo
  
  let _ = 
    let foo  = new foo in
    let foo' = new foo' in
  
    let on_any_foo = new on_any_foo in
    let on_foo  = new on_foo in
    let on_foo' = new on_foo' in
  
    on_any_foo #with_foo (foo' :> foo) ; (* explicit subtyping *)
    on_foo     #with_foo (foo' :> foo) ; (* explicit subtyping *)
    on_foo'    #with_foo foo' ;
    ()
end

module Ocaml_3_06 = struct
  class foo = object method foo = () end
  class foo' = object inherit foo method foo' = () end
  
  class on_any_foo =
    object
        method with_foo : 'a. (#foo as 'a) -> unit = fun foo -> foo#foo
    end
  
  class ['foo] on_that_foo =
    object
        method with_foo : 'foo -> unit = fun foo -> foo#foo
        (* could be something like: *)
        (* method with_foo : 'a. (#'foo as 'a) -> unit = fun foo -> foo#foo *)
        (*   and calling: [foo] on_that_foo *)
        (* method with_foo : 'a. ('foo as 'a) -> unit = fun foo -> foo#foo *)
        (*   and calling: [#foo] on_that_foo *)
    end
  
  class on_foo  = [foo ] on_that_foo
  class on_foo' = [foo'] on_that_foo
  
  let _ = 
    let foo  = new foo in
    let foo' = new foo' in
  
    let on_any_foo = new on_any_foo in
    let on_foo  = new on_foo in
    let on_foo' = new on_foo' in
  
    on_any_foo #with_foo foo' ; (* implicit subtyping *)
    on_foo     #with_foo (foo' :> foo) ; (* explicit subtyping *)
    on_foo'    #with_foo foo' ;
    ()
end

--=-=-=


--
programming languages addict      http://merd.net/pixel/language-study/

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