Re: [Caml-list] Pxp objects inside class definition

[Gerd Stolpmann <info@gerd-stolpmann.de>]

Gaurav Chanda wrote:
> Hello
> 
> I am using the Polymorphic XML Parser (PXP) to parse XML documents. I have made 2 files : point.ml and point.mli, which contain simple class definitions.
> 
> The file point.mli contains:
> 
> open Pxp_yacc ;
> open Pxp_document ;
> open Pxp_types ;
> 
> class type point_sig =
>         object
>                 value mutable x : Pxp_document.node 'a;
>         end;
> 
> class point : point_sig ;
> 
> 
> The file point.ml contains:
> 
> open Pxp_yacc ;
> open Pxp_document ;
> open Pxp_types ;
> 
> class type point_sig =
>         object
>                 value mutable x : Pxp_document.node 'a;
>         end ;
> 
> value def = "<def></def>" ;
> value def = parse_wfdocument_entity default_config (from_string def) default_spec ;
> value defr = def#root ;
> 
> class point:point_sig =
>         object
>                 value mutable x = defr;
>         end;
> 
> I get an error message while compilation. My compilation command is : 
> ocamlopt -pp "camlp4r pa_extend.cmo" -I +camlp4 -I Ty/ -I /usr/lib/ocaml/site-lib/netstring -I /usr/lib/ocaml/site-lib/pxp-engine  -I /usr/lib/ocaml/site-lib/pxp-lex-iso88591/ -c point.mli point.ml
> 
> 
> The error message is : 
> 
> File "point.ml", line 15, characters 8-67:
> The class type
>   object
>     val mutable x :
>       ('a Pxp_document.node Pxp_document.extension as 'a) Pxp_document.node
>   end
> is not matched by the class type point_sig
> The class type
>   object
>     val mutable x :
>       ('a Pxp_document.node Pxp_document.extension as 'a) Pxp_document.node
>   end
> is not matched by the class type
>   object
>     val mutable x :
>       ('a Pxp_document.node #Pxp_document.extension as 'a) Pxp_document.node
>   end
[...]
> 
> I am not being able to define PXP objects inside classes. Could you help me solve this problem ?

Hi Garuav,

(the following in classic syntax)

the problem is that  'a Pxp_document.node  is not the full type, because 
there is a constraint on the parameter of the class Pxp_document.node,
and the compiler correctly augments the type such that it reads
('a Pxp_document.node #Pxp_document.extension as 'a) Pxp_document.node.
However, defr has the type
('a Pxp_document.node Pxp_document.extension as 'a) Pxp_document.node,
which is not the same. This is all the long error message wants to tell
you.

The solution is quite simple: Use the complete type instead of
'a Pxp_document.node, i.e.

class type point_sig =
object
   val mutable x :
    ('a Pxp_document.node Pxp_document.extension as 'a) Pxp_document.node
end

Sorry that the types of PXP are that complicated.
('a Pxp_document.node Pxp_document.extension as 'a) Pxp_document.node
reflects the fact that nodes have extensions, and the extensions are
recursively bound to their nodes. This is sometimes a bit unhandy.

Gerd
-- 
------------------------------------------------------------
Gerd Stolpmann * Viktoriastr. 45 * 64293 Darmstadt * Germany
gerd@gerd-stolpmann.de          http://www.gerd-stolpmann.de
------------------------------------------------------------

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