[Caml-list] About polymorphic methods and recursive classes

["Sébastien Briais" <sebastien.briais@epfl.ch>]

Hello,

(I tried to send this mail yesterday, but apparently it did not work.
So I send it again:)

I tried to encode lists with objects.
I defined a class list which is virtual and that contains only a visit 
method
which is polymorphic.
I have defined also a virtual class visitor to visit the lists.
The ocaml toplevel indicated me there was an error in my piece of code
but since I didn't understand the error message, I have simplified my piece
in order to understand which part is not accepted by ocaml.

Here is the simplified code

 > ledit ocaml
         Objective Caml version 3.06

# class virtual ['a] visitor =
   object
     method virtual caseNil : 'a
   end
   and virtual int_list =
   object
     method virtual visit : 'a.('a visitor -> 'a)
   end
   ;;
class virtual ['a] visitor : object method virtual caseNil : 'a end
class virtual int_list : object method virtual visit : 'a visitor -> 'a end

# class nil =
   object
     inherit int_list
     method visit v = v#caseNil
   end
   ;;
This expression has type 'a visitor
It has no method caseNil

This is this last message that I do not understand.

I tried also to remove the 'and' in the recursive class definitions
(since here, the two classes are not really recursive because I have
removed the caseCons method in the visitor) and ocaml accepts this
modified code.

I tried also to instantiate the 'a type parameter with int (so I have
caseNil:int and visit:visitor->int) and this is also accepted.

Why is my code rejected ? What does mean the error message ?

Thank you

Regards

Sébastien


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