The behaviour of the compiler for your code is strange: I don't  see why 
the message error say the following:
    #session_t as 'a = < id : int; location : 'b. #location_t; .. >
where location is required to be a #location_t for all 'b (which I 
assume is the implicit self type variable of the open_type #location_t).
This seems to restrictive. Maybe a bug ?

Anymay, putting an explicit parameter in the session_t type solves the 
issue:

class type ['a] session_t =
 object
   method id:int
   method location: #location_t as 'a
 end

let check_session (session:'a #session_t) = begin
 print_int session#id;
 print_int session#location#a;
 print_string session#location#b
end

Tiphaine Turpin




SerP a écrit :
>
>
> class type location_t =
>  object
>    method a: int;
>    method b: string;
>  end;
>
> class type session_t =
>  object
>    method id:int;
>    method location: #location_t;
>  end;
>
> value check_session (session:#session_t) = begin
>  print_int session#id;
>  print_int session#location#a;
>  print_string session#location#b;
> end;
>
> class location =
>  object
>    method a = 1;
>    method b = "b";
>    method c = ["a";"b"];
>  end;
>
> class session =
>  object     value location = new location;
>    method id = 1;
>    method location = location;
>  end;
>
> value _ =
>  let s = new session in
>  check_session s;
> ====================================
> Error: This expression has type session = < id : int; location : 
> location >
>       but is here used with type
>         #session_t as 'a = < id : int; location : 'b. #location_t; .. >
>       Type location = < a : int; b : string; c : string list >
>       is not compatible with type 'b
>       The second object type has no method c
> --------------------------------------------
> But,
>    value check_session (session:< id : int; location : < a : int; b : 
> string; .. >; .. >) = begin .....
>
> Compiled properly.
>
> Please help. Is exists any way to define open class type like inline 
> type definition.
> Write every function with inline type definition in mli files is 
> terrible.
>
>
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