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. SWI-Prolog accomodates syntax extensions under conditional compilation by silently ignoring syntax errors when in the false branch. This allow, for example, for the code below. With rational number support 1r3 denotes the rational number 1/3 while without it is a syntax error. Note that this only works properly if (1) the syntax error still allows to re-synchronize on the full stop of the invalid clause and (2) the subsequent conditional compilation directive is valid.

:- if(current_prolog_flag(bounded, false)).
one_third(1r3).
:- endif.

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.