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From: ok@goanna.cs.rmit.oz.au (Richard A. O'Keefe)
Newsgroups: comp.lang.prolog
Subject: Re: Having trouble with disjunction
Message-ID: <3567@goanna.cs.rmit.oz.au>
Date: 15 Aug 90 04:59:42 GMT
References: <90226.091749F0O@psuvm.psu.edu>
Organization: Comp Sci, RMIT, Melbourne, Australia
Lines: 44

In article <90226.091749F0O@psuvm.psu.edu>, F0O@psuvm.psu.edu writes:
>          gofirst(Player) :-
>              random(RandNum),
>              RandNum < 0.5, Player = computer ;
>              RandNum >= 0.5, Player = opponent.
> 
>     When I tried to run the program, I got the message 'Free variables
> not allowed here', at Player = opponent.
>     Is this a quirk of PDC prolog, or don't I fully understand disjunction?

This is easily the best thing I have ever heard about PDC Prolog.
Your problem is that you have a terrible layout convention.
Let's put your code through a Prolog pretty-printer and see what comes out:

	gofirst(Player) :-
		(   random(RandNum),
		    RandNum < 0.5,
		    Player = computer
		;   RandNum >= 0.5,
		    Player = opponent
		).

See what's going on?
Use this indentation rule:  always write disjunctions as
		(   <disj1>
		;   <disj2>
		...
		;   <disjn>
		)
exactly like you'd write an Ada if statement, with "(" acting like "if",
"->" [when using it] acting like "then", ";" acting like "else if" or
"else", and ")" acting like "end if".  Following that rule, and writing
what you meant, we get

	gofirst(Player) :-
		random(RandNum),
		(   RandNum < 0.5,
		    Player = computer
		;   RandNum >= 0.5,
		    Player = opponent
		).

-- 
The taxonomy of Pleistocene equids is in a state of confusion.