directive
if/1
Description
if(Goal)
Starts an if-then branch when performing conditional compilation. The code
following the directive is compiled iff Goal is true. If Goal
throws an error instead of either succeeding or failing, the error is
reported by the compiler and compilation of the enclosing source file
or entity is aborted. The goal is
expanded when the directive occurs
in a source file. Conditional compilation directives can be nested.
Warning
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 successfully compiled and loaded).
Template and modes
if(@callable)
Examples
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.
If necessary, test goal errors can be converted into failures using the
standard catch/3 control construct. For example:
:- if(catch(\+ log(7,_), _, fail)).
% define the legacy log/2 predicate
log(X, Y) :- Y is log(X).
:- endif.