[Caml-list] Constraint Inference of a poly-type variable in a class-method

[Caml-list] Constraint Inference of a poly-type variable in a class-method

[Christoph Höger]

Dear all,

As a way to work-around my latest scalability issues, I want to attach a
polymorphic method to a class with this functionality:

let f t = t#s
val f : < s : 'a; .. > -> 'a

I know I can write:

object method f = f end ;;

and

class x = object method f : 'a 'b . (<s : 'a; ..> as 'b) -> 'a = f end ;;

However, the constraint might be somewhat larger, so I want it to be
inferred. Is there a way to have the compiler infer a constraint on a
poly-type variable?

regards,

Christoph


-- 
Christoph Höger

Technische Universität Berlin
Fakultät IV - Elektrotechnik und Informatik
Übersetzerbau und Programmiersprachen

Sekr. TEL12-2, Ernst-Reuter-Platz 7, 10587 Berlin

Tel.: +49 (30) 314-24890
E-Mail: christoph.hoeger@tu-berlin.de
	

Re: [Caml-list] Constraint Inference of a poly-type variable in a class-method

[Jacques Garrigue]

On 2015/12/01 18:31, Christoph Höger wrote:
> 
> Dear all,
> 
> As a way to work-around my latest scalability issues, I want to attach a
> polymorphic method to a class with this functionality:
> 
> let f t = t#s
> val f : < s : 'a; .. > -> 'a
> 
> I know I can write:
> 
> object method f = f end ;;
> 
> and
> 
> class x = object method f : 'a 'b . (<s : 'a; ..> as 'b) -> 'a = f end ;;
> 
> However, the constraint might be somewhat larger, so I want it to be
> inferred. Is there a way to have the compiler infer a constraint on a
> poly-type variable?


No.
Basically, polymorphic method types cannot be compared, i.e. one type is not more general than another, so you always have to be explicit.
This said, if your problem is that you might be calling more methods of t, you can define a class type:
  class type [‘a] t = object method s : ‘a end
Then you can write
  method f : ‘a ‘b. (‘a #t as ‘b) -> ‘a = f

Jacques Garrigue