Logics in Prolog

Question

How can I express next 3 sentences in Prolog?

All summers are warm. If it not summer, then it is winter. Now it is winter.

I don't remember the syntax much but from what I understand you are looking at a way of defining universal/existential quantifiers. [This](http://okmij.org/ftp/Prolog/quantification.txt) text may be of some help to get you started. — dirkgently, Jun 07 '12 at 08:56
There are myriad ways of representing this in Prolog. What have you tried? — Fred Foo, Jun 07 '12 at 08:59
larsman, can you show the simpliest way? It is not a homework. — , Jun 07 '12 at 09:00

CapelliC · Accepted Answer · 2016-10-01T14:29:27.453

Nice question. As @larsman (well, @FredFoo now, I think) rightly said, can be a large thema. And his answer is very good indeed.

As your question could be driven by necessity of a custom language (one of main Prolog' uses), here I propose the syntax sugar for a dummy DSL (what that's means it's totally empty now...)

:- op(500, fx, all).
:- op(500, fx, now).
:- op(600, xfx, are).
:- op(700, fx, if).
:- op(399, fx, it).
:- op(398, fx, is).
:- op(397, fx, not).
:- op(701, xfx, then).

all summers are warm.
if it is not summer then it is winter.
now it is winter.

SWI-Prolog is kind enough to make red those op that get stored, i.e. can be queried easily. These are the higher precedence words declared: i.e. are,then,now.

?- now X.
X = it is winter.

score 2 · Answer 2 · answered Jun 07 '12 at 09:51

How to represent this depends on what inferences you want to make. One of the simplest ways is

warm :- summer.
winter.

The "if not summer then winter" rule does not actually allow you to make any useful inferences, so you could just as well skip it. If you were to include it, it might be something like

winter :- \+ summer.

but since negation in Prolog is negation as failure, this might not do what you think it does if you expect the semantics of vanilla propositional logic.

Junuxx · Answer 3 · 2012-06-07T09:46:47.723

0

winter(now).

warm(X) :- summer(X). 
summer(X) :- \+ winter(X).
winter(X) :- \+ summer(X).

Would be one of the ways to do this.

In action:

6 ?- summer(now).
false.
7 ?- summer(tomorrow).
ERROR: Out of local stack
8 ?- warm(now).
false.

edited Jun 07 '12 at 09:46

answered Jun 07 '12 at 09:40

Junuxx

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score 0 · Answer 4 · answered Jul 06 '14 at 13:56

Here a solution without using negation, instead the universe of seasons is specified.

season(summer).
season(winter).

now(winter).

warm(S) :-
    season(S),
    S = summer.

Some example queries:

?- now(S).
S = winter ;
false.

?- now(S), warm(S).
false.

?- warm(S).
S = summer ;
false.

Logics in Prolog

4 Answers4