[Caml-list] Unwanted type constraint in mutually recursive parameterized class type definition.

[Caml-list] Unwanted type constraint in mutually recursive parameterized class type definition.

[Philippe Veber]

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Dear camlers,

  I obtain an unexpected answer from the toplevel when I enter the
following definition:

        OCaml version 4.03.0+dev7-2015-02-08

# class type ['a] c =
  object
    method m : d
  end
and d =
  object
    inherit [int] c
  end
;;
class type ['a] c = object constraint 'a = int method m : d end
and d = object method m : d end

What surprises me is the constraint 'a = int in the definition of c. Is
this is a bug or feature?

Cheers,
  Philippe.

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Re: [Caml-list] Unwanted type constraint in mutually recursive parameterized class type definition.

[Jacques Garrigue]

On 2015/07/09 01:24, Philippe Veber wrote:
> 
> Dear camlers,
> 
>   I obtain an unexpected answer from the toplevel when I enter the following definition:
> 
>         OCaml version 4.03.0+dev7-2015-02-08
> 
> # class type ['a] c =
>   object
>     method m : d
>   end
> and d =
>   object
>     inherit [int] c
>   end
> ;;
> class type ['a] c = object constraint 'a = int method m : d end
> and d = object method m : d end
> 
> What surprises me is the constraint 'a = int in the definition of c. Is this is a bug or feature?


This is a feature (or at least, the intended behavior)
For classes, type constraints are inferred automatically, but in a monomorphic way.
A standard workaround is to open the recursion by generalizing definitions:

class type ['a,'d] c0 = object method m : 'd end
class type d = object inherit ['a,d] c0 end
class type ['a] c = ['a,d] c0

Jacques Garrigue

Re: [Caml-list] Unwanted type constraint in mutually recursive parameterized class type definition.

[Philippe Veber]

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This workaround worked just fine, thanks a lot!
ph.


2015-07-09 1:57 GMT+02:00 Jacques Garrigue <garrigue@math.nagoya-u.ac.jp>:

> On 2015/07/09 01:24, Philippe Veber wrote:
> >
> > Dear camlers,
> >
> >   I obtain an unexpected answer from the toplevel when I enter the
> following definition:
> >
> >         OCaml version 4.03.0+dev7-2015-02-08
> >
> > # class type ['a] c =
> >   object
> >     method m : d
> >   end
> > and d =
> >   object
> >     inherit [int] c
> >   end
> > ;;
> > class type ['a] c = object constraint 'a = int method m : d end
> > and d = object method m : d end
> >
> > What surprises me is the constraint 'a = int in the definition of c. Is
> this is a bug or feature?
>
>
> This is a feature (or at least, the intended behavior)
> For classes, type constraints are inferred automatically, but in a
> monomorphic way.
> A standard workaround is to open the recursion by generalizing definitions:
>
> class type ['a,'d] c0 = object method m : 'd end
> class type d = object inherit ['a,d] c0 end
> class type ['a] c = ['a,d] c0
>
> Jacques Garrigue

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