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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:
:- else. :-if(Goal)....
:- endif.In a sequence as below, the section below the first matching
elifis 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.