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

[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