Re: [Caml-list] Recursive Parametric class type Typing

[Philippe Strauss <philou@philou.ch>]

Jacques answered me about a parametrized class prob. I had some time ago, about an important restriction:

"Object types are allowed to be recursive, but they are restricted to _regular_ types,                                   
where recursive occurences have identical type parameters.                                                              
In particular, this means that in the above line, ('a,'b) node_virt_t and ('b,'c) node_virt_t                           
must have the same parameters, i.e. that 'a = 'b and 'b = 'c, i.e. all your types                                       
end up being identical."

(maybe an FAQ is somewhat missing on the main, inria, ocaml website about that kind of infos.?)

Le 19 mars 2011 à 16:48, Raphael Proust a écrit :

> Hi list,
> 
> Trying to bind a javascript library for js_of_ocaml, I encountered the following
> pattern (here drastically simplified):
> 
> class type ['t] c =
>  object
>    method plus: 't -> unit
>    method minus: unit -> 't
>    method container: unit -> container
>  end
> and container =
>  object
>    method int: unit -> int c
>    method string: unit -> string c
>  end
> 
> The following error is raised at compile time:
> Error: This type string should be an instance of type int
> 
> In the use case:
> - instead of [int] and [string] there are six different [class type]s;
> - [classe type]s have more methods; and
> - there are additional recursive [class type]s
> 
> 
> Why is there such a limitation on the types? Is there a work around that
> wouldn't induce (too  much) code duplication?
> 
> Thanks,
> -- 
> ______________
> Raphaël Proust
> 
> 
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