# `if/1`

## Description

    if(Goal)

Starts conditional compilation. The code following the directive is compiled iff `Goal` is true. The goal is subjected to goal expansion 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.

See also

elif/1, else/0, endif/0