Date: Fri, 15 Jan 1999 16:02:42 +0100
From: Jerome Vouillon <Jerome.Vouillon@inria.fr>
To: Markus Mottl <mottl@miss.wu-wien.ac.at>, OCAML <caml-list@inria.fr>
Subject: Re: subtyping and inheritance
In-Reply-To: <199901111852.TAA00822@miss.wu-wien.ac.at>; from Markus Mottl on Mon, Jan 11, 1999 at 07:52:57PM +0100
Hello,
> My specific problem is: I have a program with classes for "terminal"
> and "nonterminal" symbols. Both inherit from a base class "symbol". The
> symbol class inherits from a class "ord", which defines a virtual method
> "compare". This allows me to compare terminals with terminals and
> nonterminals with nonterminals, but it also allows comparison between
> symbols (all three classes redefine "compare").
>
> Look at this graph:
>
> ord -> defines virtual method "compare"
> |
> symbol -> redefines "compare"
> / \
> terminal nonterminal -> both redefine "compare"
>
> But the problem is: I will never be able to compare terminals and
> nonterminals with each other, because the appropriate comparison method
> as defined in "symbol" has been "forgotten" - it is redefined in all
> child classes. Thus, I am not able to coerce them to "symbol".
>
> I have worked around this with the following inheritance scheme:
>
> ---- ord ----- -> defines virtual method "compare"
> | |
> | symbol | -> has a method "compare_symbol"
> | / \ |
> terminal nonterminal -> both redefine "compare"
>
> Although it is now possible to coerce terminals and nonterminals
> to symbols and have them thus compared via "compare_symbol", I am not
> content with this scheme: I would really like to have all methods from
> "ord" in "symbol", which all make use of the virtual "compare" method.
I think the method "compare" in class "ord" need not be a binary method: its
argument type can probably instead be a type parameter of the class.
class virtual ['a] ord = object
method virtual compare : 'a -> bool
(* ... *)
end;;
Then, the method "compare" can be given the type "symbol -> bool" in
class "symbol" (and its subclasses) :
class symbol = object (self)
inherit [symbol] ord
method x = "x"
method compare other = self#x = other#x
end;;
class terminal = object
inherit symbol
method x = "a"
method y = "y"
end;;
This way, you can compare objects from the class "symbol" and any of
its subclasses.
# new terminal#compare new symbol;;
- : bool = false
# new terminal#compare (new terminal :> symbol);;
- : bool = true
Regards,
-- Jerome
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