'if' in prolog?

Question

Probably a stupid question, but I can't find any documentation anywhere for it. Is there a way to do an if in prolog, e.g. if a variable is 0, then to do some actions (write text to the terminal). An else isn't even needed, but I can't find any implementation of if.

Answer 1

Prolog predicates 'unify' -

So, in an imperative langauge I'd write

function bazoo(integer foo)
{
   if(foo == 5)
       doSomething();
   else
       doSomeOtherThing();
}

In Prolog I'd write

bazoo(5) :-  doSomething.
bazoo(Foo) :- Foo =/= 5, doSomeOtherThing.

which, when you understand both styles, is actually a lot clearer.
"I'm bazoo for the special case when foo is 5"
"I'm bazoo for the normal case when foo isn't 5"

Answer 2

Prolog program actually is big condition for "if" with "then" which prints "Goal is reached" and "else" which prints "No sloutions was found". A, Bmeans "A is true and B is true", most of prolog systems will not try to satisfy "B" if "A" is not reachable (i.e. X=3, write('X is 3'),nl will print 'X is 3' when X=3, and will do nothing if X=2).

Answer 3

A standard prolog predicate will do this.

   isfive(5). 

will evaluate to true if you call it with 5 and fail(return false) if you run it with anything else. For not equal you use \=

isNotEqual(A,B):- A\=B.

Technically it is does not unify, but it is similar to not equal.

Learn Prolog Now is a good website for learning prolog.

Edit: To add another example.

isEqual(A,A). 

Answer 4

There are essentially three different ways how to express something like if-then-else in Prolog. To compare them consider char_class/2. For a and b the class should be ab and other for all other terms. One could write this clumsily like so:

char_class(a, ab).
char_class(b, ab).
char_class(X, other) :-
   dif(X, a),
   dif(X, b).

?- char_class(Ch, Class).
   Ch = a, Class = ab
;  Ch = b, Class = ab
;  Class = other,
   dif(Ch, a), dif(Ch, b).

To write things more compactly, an if-then-else construct is needed. Prolog has a built-in one:

?- ( ( Ch = a ; Ch = b ) -> Class = ab ; Class = other ).
   Ch = a, Class = ab.

While this answer is sound, it is incomplete. Just the first answer from ( Ch = a ; Ch = b ) is given. The other answers are chopped away. Not very relational, indeed.

A better construct, often called a "soft cut" (don't believe the name, a cut is a cut is a cut), gives slightly better results (this is in YAP):

?- ( ( Ch = a ; Ch = b ) *-> Class = ab ; Class = other ).
   Ch = a, Class = ab
;  Ch = b, Class = ab.

Alternatively, SICStus has if/3 with very similar semantics:

?- if( ( Ch = a ; Ch = b ), Class = ab , Class = other ).
   Ch = a, Class = ab
;  Ch = b, Class = ab.

So the last answer is still suppressed. Now enter library(reif) for SICStus, YAP, and SWI. Install it and say:

?- use_module(library(reif)).

?- if_( ( Ch = a ; Ch = b ), Class = ab , Class = other ).
   Ch = a, Class = ab
;  Ch = b, Class = ab
;  Class = other,
   dif(Ch, a), dif(Ch, b).

Note that all the if_/3 is compiled away to a wildly nested if-then-else for

char_class(Ch, Class) :-
   if_( ( Ch = a ; Ch = b ), Class = ab , Class = other ).

which expands in YAP 6.3.4 to:

char_class(A,B) :-
   ( A\=a
   ->
     ( A\=b
     ->
       B=other
     ;
       ( A==b
       ->
         B=ab
       )
     ;
       A=b,
       B=ab
     ;
       dif(A,b),
       B=other
     )
   ;
     ( A==a
     ->
       B=ab
     )
   ;
     A=a,
     B=ab
   ;
     dif(A,a),
     ( A\=b
     ->
       B=other
     ;
       ( A==b
       ->
         B=ab
       )
     ;
       A=b,
       B=ab
     ;
       dif(A,b),
       B=other
     )
   ).