directive

# `if/1`

## Description

    if(Goal)

Starts 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 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.

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.

See also

elif/1, else/0, endif/0