Starts conditional compilation. The code following the directive is
Goal is true. The goal is subjected to
goal expansion when the directive occurs
in a source file. Conditional compilation directives can be nested.
Conditional compilation goals cannot depend on predicate definitions contained in the same source file that contains the conditional compilation directives (as those predicates only become available after the file is fully compiled and loaded).
Template and modes¶
A common example is checking if a built-in predicate exists and providing a definition when the predicate is absent:
:- if(\+ predicate_property(length(_,_), built_in)). length(List, Length) :- ... :- endif.
Another common example is conditionally including code for a specific backend Prolog compiler:
:- if(current_logtalk_flag(prolog_dialect, swi)). % SWI-Prolog specific code :- set_prolog_flag(double_quotes, codes). :- endif.