rule/2

Documentation
- Reference manual
  - SWI-Prolog extensions
    - Single Sided Unification rules
      - Consequences of => single sided unification rules
        
        rule/2
        
        rule/3
- Packages

Availability:built-in

rule(:Head, -Rule)

rule(:Head, -Rule, -Ref)

True when Rule is a rule/clause that implements Head. Rule is a complete rule term. For a normal clause this is a term Head :- Body and for a single sided unification rule it is a term Head => Body.

History

Fixed in 8.5.8 (👉00bfdc5) rule/3 for => rules that have the unification moved into the head and listing/1 for such SSU clauses.