Re: [Caml-list] Polymorphic methods (longest error message ever!)

[John Max Skaller <skaller@ozemail.com.au>]

Brian Smith wrote:

>
>  # class node : node_type = object (self)
>      method as_variant = `Node (self :> node_type)
>    end;;
>                                 ^^^^
>  This expression cannot be coerced to type
>  node_type = < as_variant : 'a. [> `Node of node_type] as 'a >;
>  it has type < as_variant : 'c; .. > as 'b but is here used with type 'b
>
>  # class node : node_type = object (self)
>      method as_variant : 'a. [> `Node of node_type] as 'a
>                        = `Node (self :>  node_type)
>    end;;
> The class type object method as_variant : [> `Node of node_type] end
> is not matched by the class type node_type
> The class type object method as_variant : [> `Node of node_type] end
> is not matched by the class type
>   object method as_variant : [> `Node of node_type] end
> The method as_variant has type 'a. [> `Node of node_type] as 'a
> but is expected to have type 'b. [> `Node of node_type] as 'b
> The universal variable 'b would escape its scope
>
> I have no idea what the error message means. 

Heh. I can't answer your question, but I have to fall about laughing
(is there a French idiom for that?) at the idea this is a long error 
message.
I often get error messages of 100's of lines, my console buffer isn't set
to 10,000 lines without reason. Just imagine your polymophic variant type

    [> Node of node_type]

was replace statement_t below .. and don't ignore the fact that
the ctor arguments themselves are similar polymorphic variant types
like expr_t ... and this isn't a large variant.

----------------------------------------------
and statement_t =
  [
  | `AST_open of range_srcref * qualified_name_t
  | `AST_comment of string (* for documenting generated code *)

  (* definitions *)
  | `AST_function of range_srcref * id_t * parameter_t list * typecode_t 
* statement_t list
  | `AST_procedure of range_srcref * id_t * parameter_t list * 
statement_t list

  (* types *)
  | `AST_union of range_srcref * id_t * (id_t * typecode_t) list
  | `AST_struct of range_srcref * id_t * (id_t * typecode_t) list
  | `AST_type_alias of range_srcref * id_t * typecode_t

  (* variables *)
  | `AST_val_decl of range_srcref * id_t * typecode_t option * expr_t option
  | `AST_var_decl of range_srcref * id_t * typecode_t option * expr_t option
 
  (* module system *)
  | `AST_untyped_module of range_srcref * id_t * statement_t list 
  | `AST_typed_module of range_srcref * id_t * typecode_t * statement_t 
list 
  | `AST_module_binding of range_srcref * id_t * expr_t
  | `AST_typed_functor of range_srcref * id_t * parameter_t list * 
typecode_t * statement_t list
  | `AST_untyped_functor of range_srcref * id_t * parameter_t list * 
statement_t list
  | `AST_interface of range_srcref * id_t * statement_t list
  | `AST_type of range_srcref * id_t
  | `AST_function_decl of range_srcref * id_t * typecode_t
  | `AST_procedure_decl of range_srcref * id_t * typecode_t

 
  (* control structures: primitives *)
  | `AST_label of range_srcref * id_t (* for testing the code generator! *)
  | `AST_goto of range_srcref * id_t (* for testing the code generator! *)
  | `AST_call of range_srcref * expr_t * expr_t
  | `AST_read of range_srcref * id_t
  | `AST_return of range_srcref * expr_t
  | `AST_nop of range_srcref * string

  | `AST_block of range_srcref * statement_t list
  | `AST_if of
    range_srcref *
    (range_srcref * (expr_t * statement_t)) list *
    (range_srcref * statement_t)
  | `AST_while of range_srcref * expr_t * statement_t

  (* exceptions *)
  | `AST_attempt of
    range_srcref *
    (range_srcref * statement_t) *         (* the attempt *)
    (range_srcref * statement_t list) *    (* the handlers *)
    (range_srcref * statement_t list)      (* the finally block *)
  | `AST_except_handler of range_srcref * id_t * parameter_t list * 
statement_t list
  | `AST_raise of range_srcref * id_t * expr_t

  (* binding structures [prolog] *)
  | `AST_abs_decl of range_srcref * id_t * c_t
  | `AST_const_decl of range_srcref * id_t * typecode_t * c_t
  | `AST_fun_decl of range_srcref * id_t * typecode_t list * typecode_t 
* c_t
  | `AST_proc_decl of range_srcref * id_t * typecode_t list * c_t
  | `AST_header of range_srcref * string
  | `AST_code of range_srcref * string

  | `AST_export of range_srcref * suffixed_name_t * string
  ]


-- 
John Max Skaller, mailto:skaller@ozemail.com.au
snail:10/1 Toxteth Rd, Glebe, NSW 2037, Australia.
voice:61-2-9660-0850




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