SWI-Prolog -- send

Documentation
- Reference manual
  - Built-in Predicates
    - Control Predicates
      - fail/0
      - false/0
      - true/0
      - repeat/0
      - !/0
      - ,/2
      - ;/2
      - |/2
      - send_arrow/2
      - *->/2
      - \+/1
- Packages

[ISO]:Condition -> :Action

If-then and If-Then-Else. The ->/2 construct commits to the choices made at its left-hand side, destroying choice points created inside the clause (by ;/2), or by goals called by this clause. Unlike !/0, the choice point of the predicate as a whole (due to multiple clauses) is not destroyed. The combination ;/2 and ->/2 acts as if defined as:

If -> Then; _Else :- If, !, Then.
If -> _Then; Else :- !, Else.
If -> Then :- If, !, Then.

Please note that (If -> Then) acts as (If -> Then ; fail), making the construct fail if the condition fails. This unusual semantics is part of the ISO and all de-facto Prolog standards.

Please note that (if->then;else) is read as ((if->then);else) and that the combined semantics of this syntactic construct as defined above is different from the simple nesting of the two individual constructs, i.e., the semantics of ->/2 changes when embedded in ;/2. See also once/1.