
+Condition -> +Then ; +Else

   Conditional construct - succeeds if Then succeeds for the first solution of Condition,
or if Condition fails and Else succeeds.

Arguments
   Condition           Atom or compound term.
   Then                Atom or compound term.
   Else                Atom or compound term.

Type
   Control

Description

   The conditional (if-then-else) construct.  First Condition is called
   and if this succeeds any further solutions of Condition are cut and
   Then is called.  Else is never executed in this case regardless of the
   outcome of Then.

   If Condition fails, Else is called.  In this case, Then is never executed.

   It is allowed, but not recommended to use ->/2 without ;/2 as
   
   ( Condition -> Then )
   
   because this is equivalent to
   
   ( Condition -> Then ; fail )
   
   which is sometimes considered unintuitive. If this behaviour is really
   wanted, it can be expressed more clearly by
   
   once Condition , Then
   

   The more common idiom, where nothing is to be done in the else-case,
   must be written like this, using true/0:
   
   ( Condition -> Then ; true )
   

   Also note that a !/0 inside Condition only has a local effect in Condition.
   If a !/0 appears in Then or Else, it cuts through the whole construct.


   Since ->/2 and ;/2 have a lower precedence than ,/2, the whole construct
   should always be enclosed in parentheses:
    
    ( Condition ->
        Then
    ;
        Else
    )
    


Modules
   This predicate is sensitive to its module context (tool predicate, see @/2).

Fail Conditions
   Fails if Condition succeeds and Then fails, or if Condition and Else fail

Resatisfiable
   Resatisfiable if Condition succeeds and Then is resatisfiable, or Condition fails and Else is resatisfiable

Exceptions
     4 --- One of the arguments is not instantiated.
     5 --- One of the arguments is neither an atom nor a compound term.

Examples
   
Success:
      % Then-branch executed
      ?- X = 1, ( X == 1 -> write(a) ; write(b) ).
      a
      X = 1
      Yes (0.00s cpu)

      % Else-branch executed
      ?- X = 2, ( X == 1 -> write(a) ; write(b) ).
      b
      No (0.00s cpu)

      % the Condition is cut, the Then-branch isn't
      ?- ( member(X,[1,2]) -> member(Y,[a,b]) ; member(Y,[c,d]) ).
      X = 1
      Y = a
      Yes (0.00s cpu, solution 1, maybe more)
      X = 1
      Y = b
      Yes (0.03s cpu, solution 2)


See Also
   ; / 2, ! / 0, *-> / 2
