4.3.1.2 Conditional compilation

Conditional compilation builds on the same principle as term_expansion/2, goal_expansion/2 and the expansion of grammar rules to compile sections of the source code conditionally. One of the reasons for introducing conditional compilation is to simplify writing portable code. See section C for more information. Here is a simple example:

:- if(\+source_exports(library(lists), suffix/2)).

suffix(Suffix, List) :-
        append(_, Suffix, List).

:- endif.

Note that these directives can only appear as separate terms in the input. Typical usage scenarios include:

• Load different libraries on different dialects.
• Define a predicate if it is missing as a system predicate.
• Realise totally different implementations for a particular part of the code due to different capabilities.
• Realise different configuration options for your software.

:- 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.

:- elif(:Goal)

Equivalent to :- else. :-if(Goal). ... :- endif. In a sequence as below, the section below the first matching elif is processed. If no test succeeds, the else branch is processed.

:- if(test1).
section_1.
:- elif(test2).
section_2.
:- elif(test3).
section_3.
:- else.
section_else.
:- endif.

:- else

Start `else' branch.

:- endif

End of conditional compilation.