4.3.1 Conditional compilation and program transformation

ISO Prolog defines no way for program transformations such as macro expansion or conditional compilation. Expansion through term_expansion/2 and expand_term/2 can be seen as part of the de-facto standard. This mechanism can do arbitrary translation between valid Prolog terms read from the source file to Prolog terms handed to the compiler. As term_expansion/2 can return a list, the transformation does not need to be term-to-term.

Various Prolog dialects provide the analogous goal_expansion/2 and expand_goal/2 that allow for translation of individual body terms, freeing the user of the task to disassemble each clause.

term_expansion(+Term1, -Term2)

Dynamic and multifile predicate, normally not defined. When defined by the user all terms read during consulting are given to this predicate. If the predicate succeeds Prolog will assert Term2 in the database rather than the read term (Term1). Term2 may be a term of the form ?- Goal. or :- Goal. Goal is then treated as a directive. If Term2 is a list, all terms of the list are stored in the database or called (for directives). If Term2 is of the form below, the system will assert Clause and record the indicated source location with it:

'$source_location'(<File>, <Line>):<Clause>

When compiling a module (see chapter 6 and the directive module/2), expand_term/2 will first try term_expansion/2 in the module being compiled to allow for term expansion rules that are local to a module. If there is no local definition, or the local definition fails to translate the term, expand_term/2 will try term_expansion/2 in module user. For compatibility with SICStus and Quintus Prolog, this feature should not be used. See also expand_term/2, goal_expansion/2 and expand_goal/2.

expand_term(+Term1, -Term2)

This predicate is normally called by the compiler on terms read from the input to perform preprocessing. It consists of four steps, where each step processes the output of the previous step.

1. Test conditional compilation directives and translate all input to [] if we are in a `false branch' of the conditional compilation. See section 4.3.1.2.

2. Call term_expansion/2. This predicate is first tried in the module that is being compiled and then in the module user.

3. Call DCG expansion (dcg_translate_rule/2).

4. Call expand_goal/2 on each body term that appears in the output of the previous steps.

goal_expansion(+Goal1, -Goal2)

Like term_expansion/2, goal_expansion/2 provides for macro expansion of Prolog source code. Between expand_term/2 and the actual compilation, the body of clauses analysed and the goals are handed to expand_goal/2, which uses the goal_expansion/2 hook to do user-defined expansion.

The predicate goal_expansion/2 is first called in the module that is being compiled, and then follows the module inheritance path as defined by default_module/2, i.e., by default user and system. If Goal is of the form Module:Goal where Module is instantiated, goal_expansion/2 is called on Goal using rules from module Module followed by default modules for Module.

Only goals appearing in the body of clauses when reading a source file are expanded using this mechanism, and only if they appear literally in the clause, or as an argument to a defined meta-predicate that is annotated using `0' (see meta_predicate/1). Other cases need a real predicate definition.

The expansion hook can use prolog_load_context/2 to obtain information about the context in which the goal is exanded such as the module, variable names or the encapsulating term.

expand_goal(+Goal1, -Goal2)

This predicate is normally called by the compiler to perform preprocessing using goal_expansion/2. The predicate computes a fixed-point by applying transformations until there are no more changes. If optimisation is enabled (see -O and optimise), expand_goal/2 simplifies the result by removing unneeded calls to true/0 and fail/0 as well as unreachable branches.

compile_aux_clauses(+Clauses)

Compile clauses on behalf of goal_expansion/2. This predicate compiles the argument clauses into static predicates, associating the predicates with the current file but avoids changing the notion of current predicate and therefore discontiguous warnings.

Note that in some cases multiple expansions of similar goals can share the same compiled auxiliary predicate. In such cases, the implementation of goal_expansion/2 can use predicate_property/2 using the property defined to test whether the predicate is already defined in the current context.

dcg_translate_rule(+In, -Out)

This predicate performs the translation of a term Head-->Body into a normal Prolog clause. Normally this functionality should be accessed using expand_term/2.

var_property(+Var, ?Property)

True when Property is a property of Var. These properties are available during goal- and term-expansion. Defined properties are below. Future versions are likely to provide more properties, such as whether the variable is a singleton or whether the variable is referenced in the remainder of the term. See also goal_expansion/2.

fresh(Bool)

Bool has the value true if the variable is guaranteed to be unbound at entry of the goal, otherwise its value is false. This implies that the variable first appears in this goal or a previous appearance was in a negation (\+/1) or a different branch of a disjunction.

name(Name)

True when variable appears with the given name in the source.

4.3.1.1 Program transformation with source layout info

This sections documents extended versions of the program transformation predicates that also transform the source layout information. Extended layout information is currently processed, but unused. Future versions will use for the following enhancements:

• More precise locations of warnings and errors
• More reliable setting of breakpoints
• More reliable source layout information in the graphical debugger.

expand_goal(+Goal1, ?Layout1, -Goal2, -Layout2)

goal_expansion(+Goal1, ?Layout1, -Goal2, -Layout2)

expand_term(+Term1, ?Layout1, -Term2, -Layout2)

term_expansion(+Term1, ?Layout1, -Term2, -Layout2)

dcg_translate_rule(+In, ?LayoutIn, -Out, -LayoutOut)

These versions are called before their 2-argument counterparts. The input layout term is either a variable (if no layout information is available) or a term carrying detailed layout information as returned by the subterm_positions of read_term/2.

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.