SWI-Prolog -- if/1

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Predicate if/1

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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.

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ifdef

LogicalCaptain said (2021-05-02T13:50:24):

This is like a flexible version of C's #ifdef ...

An example from file statistics.pl ( library(statistics) )

If thread_statistics/3 is not currently defined, we assume we are in the single-threaded SWI-Prolog version and define our own thread_statistics/3

:- if(\+current_predicate(thread_statistics/3)).
thread_statistics(_Thread, Key, Value) :-       % single threaded version
    statistics(Key, Value).
:- endif.