[Caml-list] polymorphic methods in objects?

[Caml-list] polymorphic methods in objects?

[Lex Stein]

Hi,

Why can't objects have polymorphic methods ? I'm trying a compile and
ocamlc gets upset because I have a method with an unbound type in its
signature (see below for ocamlc error output).

Must methods be type paramaterized through some kind of a template
mechanism? This seems a bit limiting: the methods of an object would have
their type signature fixed across the lifetime of an object.

      method dummy_init : 'a -> unit
      method dump : out_channel -> unit
      method init : unit -> unit
      method join : string -> unit
      method null : unit -> unit
      method resolv : string -> hmap option
    end
The method dummy_init has type 'a -> unit where 'a is unbound
make: *** [pastry.cmo] Error 2

Thanks,

Lex

---
Lex Stein                     http://www.eecs.harvard.edu/~stein/
Harvard Computer Science      TEL: 617-495-5973

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Re: [Caml-list] polymorphic methods in objects?

[Nicolas Cannasse]

> Why can't objects have polymorphic methods ? I'm trying a compile and
> ocamlc gets upset because I have a method with an unbound type in its
> signature (see below for ocamlc error output).
>
> Must methods be type paramaterized through some kind of a template
> mechanism? This seems a bit limiting: the methods of an object would have
> their type signature fixed across the lifetime of an object.
>
>       method dummy_init : 'a -> unit
>       method dump : out_channel -> unit
>       method init : unit -> unit
>       method join : string -> unit
>       method null : unit -> unit
>       method resolv : string -> hmap option
>     end
> The method dummy_init has type 'a -> unit where 'a is unbound

two ways of doing that :

first, having a parameterized object :

class ['a] my_object : object
    method dummy_init : 'a -> unit
end

But perhaps here you really want that any value passed to your object is ok.
So you have to use a polymorphic method, which is not bound to the class
type :

class my_object : object
    method dummy_init : 'a . 'a -> unit
end

This way, you cannot of course store the parameter into a field of my_object
this it is not polymorphic.

Nicolas Cannasse

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Re: [Caml-list] polymorphic methods in objects?

[Karl Zilles]

Nicolas Cannasse wrote:
> two ways of doing that :
> 
> first, having a parameterized object :
> 
> class ['a] my_object : object
>     method dummy_init : 'a -> unit
> end
> 
> But perhaps here you really want that any value passed to your object is ok.
> So you have to use a polymorphic method, which is not bound to the class
> type :
> 
> class my_object : object
>     method dummy_init : 'a . 'a -> unit
> end
> 
> This way, you cannot of course store the parameter into a field of my_object
> this it is not polymorphic.

The stuff Nicolas mentions is covered in sections 3.9 and 3.10 of the 
current manual.  You should probably read these sections, because use of 
polymorphic methods can sometimes require type constraints.

http://caml.inria.fr/ocaml/htmlman/manual005.html#ss:parameterized-classes
http://caml.inria.fr/ocaml/htmlman/manual005.html#ss:polymorphic-methods



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