Again on polymorphism.

From: seguso (look_at_in.signature)
Date: 04/19/04

  • Next message: pimko: "Re: A simple (?) graph algorithm" 
    Date: Mon, 19 Apr 2004 20:28:53 GMT

    Hello again, :-)

    At first, it seems you can mimick polymorphism (duck typing) in prolog this
    way:

      % this is a polymorphic predicate. It can be applied to any X for which
      % width/2 and height/2 are defined.
      area( X, A):-
         width(X, W),
         height(X, H),
         A is W*H.

      width(ship( W, _, _Passengers), W).
      height(ship( _, H, _Passengers), H).
      width(plane( W, _), W).
      height(plane( _, H), H).

    Now I can have a list of intermixed planes and ships, and still apply area/2
    to all members of the list:

      maplist(area, [plane(32,3), ship(2,3,4)], Areas).

    But this approach cannot be used when the polymorphic predicate must modify
    the argument! I mean:

      % walk/2: I want walk/2 to be polymorphic, i.e. to apply to any X for
      % which speed/2 and position/2 are defined.
      walk(X, NewX):-
         speed(X, S),
         position(X, P),
         NewPosition is P + S,
         NewX = ????

    I need to call walk/2 both with X = ship(3,4,5) and with X = plane(3,4).
    What I need to say is "let NewX = a copy of X where the position has been
    replaced by NewPosition".

    Is there a way to do that?

    Thanks a lot! ^_^

    Maurizio
       

    -- 
Best Regards,
Maurizio Colucci
Please remove the uppercase letters "S,P,A,M":
seSgPuAsMo.forever@tin.it
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