SWI-Prolog -- ($)/1

Documentation
- Reference manual
  - Built-in Predicates
    - Debugging and declaring determinism
      - det/1
      - $/0
      - $/1
- Packages

Availability:built-in

[experimental]$(:Goal)

Verify that Goal succeeds deterministically. This predicate has no effect if Goal succeeds without a choicepoint. Otherwise the result depends on the Prolog flag determinism_error:

silent: Act as once/1.
warning: Print a warning and act as once/1.
error: Raise a determinism_error exception.

Note that if $/1 is used for the last call, last call optimization is not effective. This behaviour ensures consistent errors or warnings. Last call optimization with determinism checking can be realised using ..., $, Last., i.e. by executing $/0 before the last call rather than wrapping the last call in $/1.

Violations throw an error/2 exception determinism_error(Pred, Declared, Observed, goal).