Term and goal expansion¶
Logtalk supports the term and goal expansion mechanism also found in some Prolog systems. This macro mechanism is used to define source-to-source transformations. Two common uses are the definition of language extensions and domain-specific languages.
Defining expansions¶
Term and goal expansions are defined using, respectively, the predicates term_expansion/2 and goal_expansion/2, which are declared in the expanding built-in protocol. For example:
:- object(an_object,
implements(expanding)).
term_expansion(ping, pong).
term_expansion(
colors,
[white, yellow, blue, green, read, black]
).
goal_expansion(a, b).
goal_expansion(b, c).
goal_expansion(X is Expression, true) :-
catch(X is Expression, _, fail).
:- end_object.
These predicates can be explicitly called using the expand_term/2 and expand_goal/2 built-in methods.
Clauses for the term_expansion/2
predicate are called until of them
succeeds. The returned expansion can be a single term or a list of terms.
For example:
| ?- an_object::expand_term(ping, Term).
Term = pong
yes
| ?- an_object::expand_term(colors, Colors).
Colors = [white, yellow, blue, green, read, black]
yes
When no term_expansion/2
clause applies, the same term that we are
trying to expand is returned:
| ?- an_object::expand_term(sounds, Sounds).
Sounds = sounds
yes
Clauses for the goal_expansion/2
predicate are recursively called on
the expanded goal until a fixed point is reached. Care must be taken to
avoid compilation loops. For example:
| ?- an_object::expand_goal(a, Goal).
Goal = c
yes
| ?- an_object::expand_goal(X is 3+2*5, Goal).
X = 13,
Goal = true
yes
When no goal_expansion/2
clause applies, the same goal that we are
trying to expand is returned:
| ?- an_object::expand_goal(3 =:= 5, Goal).
Goal = (3=:=5)
yes
Term and goal expansion predicates can also be used when compiling a source file as described below.
Expanding grammar rules¶
A common term expansion is the translation of grammar rules into predicate
clauses. This transformation is performed automatically by the compiler
when a source file entity defines grammar rules. It can also be done
explicitly by calling the expand_term/2
built-in method. For example:
| ?- logtalk::expand_term((a --> b, c), Clause).
Clause = (a(A,B) :- b(A,C), c(C,B))
yes
Note that the default translation of grammar rules can be overriden by defining clauses for the term_expansion/2 predicate.
Hook objects¶
Term and goal expansion of a source file during its compilation is performed by using hook objects. A hook object is simply an object implementing the expanding built-in protocol, defining clauses for the term and goal expansion hook predicates.
To compile a source file using a hook object, we can use the hook compiler flag in the second argument of the logtalk_compile/2 and logtalk_load/2 built-in predicates. For example:
| ?- logtalk_load(source_file, [hook(hook_object)]).
...
In alternative, we can use a set_logtalk_flag/2 directive in the source file itself. For example:
:- set_logtalk_flag(hook, hook_object).
It is also possible to define a default hook object by defining a global
value for the hook
flag by calling the set_logtalk_flag/2
predicate. For example:
| ?- set_logtalk_flag(hook, hook_object).
yes
When compiling a source file, the compiler will first try the source file specific hook object, if defined. If that fails, it tries the default hook object, if defined. If that also fails, the compiler tries the Prolog dialect specific expansion predicate definitions if defined in the adapter file.
Note
Clauses for the term_expansion/2
and goal_expansion/2
predicates
defined within an object or a category are never used in the compilation
of the object or the category itself.
When using an hook object to expand the terms of a source file, two
virtual terms are generated: begin_of_file
and end_of_file
.
These terms allow the user to define term-expansions before and after
the actual source file terms.
Logtalk provides a logtalk_load_context/2 built-in predicate that can be used to access the compilation/loading context when performing expansions. The logtalk built-in object also provides a set of predicates that can be useful, notably when adding Logtalk support for languages extensions originally developed for Prolog.
Bypassing expansions¶
Terms and goals wrapped by the {}/1 control construct are not expanded. For example:
| ?- an_object::expand_term({ping}, Term).
Term = {ping}
yes
| ?- an_object::expand_goal({a}, Goal).
Goal = {a}
yes
This also applies to source file terms and source file goals.
Combining multiple expansions¶
Sometimes we have multiple hook objects that we need to use in the compilation of a source file. The Logtalk library includes support for two basic expansion workflows: a pipeline of hook objects, where the expansion results from a hook object are feed to the next hook object in the pipeline, and a set of hook objects, where expansions are tried until one of them succeeds. These workflows are implemented as parametric objects allowing combining them to implement more sophisticated expansion workflows.
Using Prolog defined expansions¶
In order to use clauses for the term_expansion/2
and goal_expansion/2
predicates defined in plain Prolog, simply specify the pseudo-object user
as the hook object when compiling source files. When using backend Prolog
compilers that support a module system, it can also be specified a module
containing clauses for the expanding predicates as long as the module
name doesn’t coincide with an object name. But note that Prolog module
libraries may provide definitions of the expansion predicates that are
not compatible with the Logtalk compiler. Specially when setting the
hook object to user
, be aware of any Prolog library that is loaded,
possibly by default or implicitly by the Prolog system, that may be
contributing definitions of the expansion predicates. It is usually
safer to define a specific hook object for combining multiple expansions
in a fully controlled way.