if in prolog?

if in prolog?

Yes, there is such a control construct in ISO Prolog, called ->. You use it like this:

( condition -> then_clause ; else_clause )

Here is an example that uses a chain of else-if-clauses:

(   X < 0 ->
    writeln(X is negative.  Thats weird!  Failing now.),
;   X =:= 0 ->
    writeln(X is zero.)
;   writeln(X is positive.)

Note that if you omit the else-clause, the condition failing will mean that the whole if-statement will fail. Therefore, I recommend always including the else-clause (even if it is just true).

A standard prolog predicate will do this.


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.

To add another example.


if in prolog?

Prolog predicates unify –

So, in an imperative langauge Id write

function bazoo(integer foo)
   if(foo == 5)

In Prolog Id write

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

which, when you understand both styles, is actually a lot clearer.
Im bazoo for the special case when foo is 5
Im bazoo for the normal case when foo isnt 5

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