SWI-Prolog -- if/1

Documentation
- Reference manual
  - Built-in Predicates
    - Loading Prolog source files
      - Conditional compilation and program transformation
        
        Conditional compilation
        
        if/1
        
        elif/1
        
        else/0
        
        endif/0
- Packages

:- if(:Goal)

Compile subsequent code only if Goal succeeds. For enhanced portability, Goal is processed by expand_goal/2 before execution. If an error occurs, the error is printed and processing proceeds as if Goal has failed.