Polymorphic method and polymorphic type

Polymorphic method and polymorphic type

[SerP]

Simple example. All functions are fake.

class getter = object method add_some p = p+1 end;;
type 'a get_mode = [ `Read | `Watch of (#getter as 'a)];;
let string_of_get_mode (gm: 'a get_mode) = match gm with `Read -> "Read" 
| `Watch g -> let i = g#add_some 1 in ("Watch "^ (string_of_int i));;
class c = object method pgm : 'a. ('a get_mode) -> string = 
string_of_get_mode end;;
--------------
This expression has type 'a. (#getter as 'a) get_mode -> string
but is here used with type 'b. (#getter as 'b) get_mode -> string
Type #getter as 'c = < add_some : int -> int; .. >
is not compatible with type 'c

====================
Please help. Is it my error? how should we do that the right way?

Re: [Caml-list] Polymorphic method and polymorphic type

[SerP]

but if we use simple variant type instead of polymorphic variants it 
works perfect


type 'a get_mode = [ Read | Watch of (#getter as 'a)];;

Re: [Caml-list] Polymorphic method and polymorphic type

[Jacques Garrigue]

From: SerP <serp@stork.ru>

> but if we use simple variant type instead of polymorphic variants it 
> works perfect
> 
> 
> type 'a get_mode = [ Read | Watch of (#getter as 'a)];;

(Better to avoid revised syntax in such cases, it is confusing here)

I was not aware of that, but this is logical since the problem comes
from expansion, and simple variants do not need to be expanded.
So I should correct my phrasing: explicit polymorphic type variables
inside constrained type parameters of type abbreviations are not
supported.

Jacques Garrigue

Re: [Caml-list] Polymorphic method and polymorphic type

[Jacques Garrigue]

From: SerP <serp@stork.ru>
> Simple example. All functions are fake.
> 
> class getter = object method add_some p = p+1 end;;
> type 'a get_mode = [ `Read | `Watch of (#getter as 'a)];;
> let string_of_get_mode (gm: 'a get_mode) = match gm with `Read -> "Read" 
> | `Watch g -> let i = g#add_some 1 in ("Watch "^ (string_of_int i));;
> class c = object method pgm : 'a. ('a get_mode) -> string = 
> string_of_get_mode end;;
> --------------
> This expression has type 'a. (#getter as 'a) get_mode -> string
> but is here used with type 'b. (#getter as 'b) get_mode -> string
> Type #getter as 'c = < add_some : int -> int; .. >
> is not compatible with type 'c
> 
> ====================
> Please help. Is it my error? how should we do that the right way?

Unfortunately, explicitly polymorphic type variables inside
constrained type parameters are not allowed. This is due to the way
type expansion is done inside the type checker...
It should be possible to solve it, but I'm afraid lots of places would
have to be modified to provide comprehensive support.

Currently, the way to do it is to avoid constrained type parameters.

type 'a get_mode = [ `Read | `Watch of 'a];;
let string_of_get_mode (gm: #getter get_mode) =
   match gm with `Read -> "Read" 
  | `Watch g -> let i = g#add_some 1 in ("Watch "^ (string_of_int i));;
class c = object
 method pgm : 'a. (#getter as 'a) get_mode -> string = string_of_get_mode
end;;

This should not be too difficult usually. However, there is no
workaround if the type with the constraint is abstract.

Jacques Garrigue