:- module play.

:- interface.

% synopsis: "The play is the thing ..."  This module reflects an
%           exercise at the end of section 3.3 of Bratko's
%           /Prolog Programming for Artificial Intelligence, 3rd ed./,
%           enhanced with type information.  op/3 is working nicely in
%           Mercury.
% author:   Douglas M. Auclair (DMA)
% date:     January 5, 2006
%
% Just one question ... who is this 'geophf' person, anyway?

:- import_module io.

:- pred main(state::di, state::uo) is det.

:- implementation.

:- import_module list, int, string.

:- type person ---> geophf; susan; jimmy; biff.
:- type sport ---> go; rugby; basketball; football;
	           volleyball; squash; halo; cricket.

:- type sports ---> and(sport, sport).

:- typeclass showable(T) where [ func show(T) = string ].
:- instance showable(person) where [
  show(geophf) = "geophf",
  show(susan) = "Susan",
  show(jimmy) = "Jimmy",
  show(biff) = "Biff"
].

:- op(800, xfx, plays).
% N.B.: op(720, xfy, and) is predefined in the Mercury language

% Below are some "interesting" facts.

jimmy plays football and squash.       % 'Merkans call it "soccor" ...
susan plays volleyball and basketball. % ... so the assertation:
geophf plays halo and rugby.           % "Boys play football; Men play Rugby"
biff plays go and cricket.             % can have different implications for
				       % different people...

main -->
	{ jimmy plays What,
	  Who plays halo and _,
	  Susan plays _ and basketball },
        print("Jimmy plays "), write(What), nl,

	% instead of the below, we could have discarded the typeclass
	% and simply wrote: 'write(Who), print(" plays halo\n").' &c.

	format("%s plays halo.\n  ...huh? '%s'? Who's that?\n",
               [s(show(Who)), s(show(Who))]),
        format("%s plays basketball.\n", [s(show(Susan))]),

        % and one query
	({ Biff plays go and cricket } ->
	   format("%s plays go; which isn't exactly cricket, now, is it!",
                  [s(show(Biff))])
         ;
	   print("What?  No go?  Is it tea time, then?")),
	nl.