This library provides a set of convenient hook objects for defining
custom expansion workflows (using e.g. the
hook_flows library) and
for debugging. They are usable and useful as-is but should also be
regarded as term- and goal-expansion examples that you can learn from,
clone, and change to fit your application requirements.
Open the ../../docs/library_index.html#hook-objects link in a web browser.
To load all hook objects in this library, load the
| ?- logtalk_load(hook_objects(loader)).
To load a specific hook object, e.g. the
| ?- logtalk_load(hook_objects(backend_adapter_hook)).
To test this library hook objects, load the
| ?- logtalk_load(hook_objects(tester)).
The provided hook objects cover different expansion scenarios as follows.
Using the Prolog backend adapter file expansion rules¶
Useful when defining a custom expansion workflow. This can be
accomplished by loading the
backend_adapter_hook.lgt file, which
backend_adapter_hook hook object that can be used as a
Restoring the default compiler expansion workflow¶
In this case, load the
default_workflow_hook.lgt file, which defines
default_workflow_hook hook object, and use the following goal
to set the default hook object:
| ?- set_logtalk_flag(hook, default_workflow_hook).
Preventing applying any (other) user-defined expansion rules¶
When compiling a source file, we sometimes want to prevent applying
expansion rules. This can be accomplished by simply loading the
identity_hook.lgt file, which defines the
object, whose expansion rules simply succeed without changing the terms
and goals, and setting it as the file specific hook object writing as
the first term in the file the directive:
:- set_logtalk_flag(hook, identity_hook).
Note that the compiler will always convert any grammar rules defined in the file into clauses. Although this conversion can also be performed as an expansion, grammar rules are part of the Logtalk language. If you to preserve the grammar rules, use the hook objects described below to write them to a stream.
Expanding grammar rules into clauses independently of the compiler¶
grammar_rules_hook.lgt and use the term-expansion rules in
grammar_rules_hook object. For example:
| ?- grammar_rules_hook::term_expansion((a --> [b],c), Clause). Clause = (a([b|T], C) :- c(T, C)) yes
Using the expansion rules defined in a Prolog module¶
prolog_module_hook.lgt, which defines the parametric hook
prolog_module_hook(Module). To use this hook object, you need
to instantiate the parameter to the name of the module. For example:
:- set_logtalk_flag(hook, prolog_module_hook(user)).
Wrap the contents of a plain Prolog file as an object¶
object_wrapper_hook.lgt, which defines the
object_wrapper_hook/0-2 hook objects. Use them to wrap the contents
of a plain Prolog file as an object named after the file (optionally
implementing a protocol) or an object with the given name and object
relations. Can be used to apply Logtalk developer tools to plain Prolog
code or when porting Prolog application to Logtalk. For example:
| ?- logtalk_load('plain.pl', [hook(object_wrapper_hook)]). ... | ?- current_object(plain). yes
| ?- logtalk_load('world_1.pl', [hook(object_wrapper_hook(some_protocol))]). ... | ?- current_object(world_1). yes | ?- implements_protocol(world_1, Protocol). Protocol = some_protocol yes
| ?- logtalk_load('foo.pl', [hook(object_wrapper_hook(bar,[imports(some_category))]). ... | ?- current_object(bar). yes | ?- imports_category(bar, Category). Category = some_category yes
object_wrapper_hook object sets the
allow for the generated object. This enables calling the
predicates using the
(<<)/2 context-switching control construct. But
it’s usually better to define a protocol for the predicates being
encapsulated and use instead the
Outputting term-expansion results to a stream or a file¶
write_to_stream_hook.lgt file and using the
write_to_stream_hook(Stream, Options) hook objects. Alternatively,
you can load the
write_to_file_hook.lgt file and use the
hook objects. The terms are not modified and thus these hook objects may
be used at any point in an expansion workflow. The terms are written
followed by a period and a new line.
For example, assume that we want to expand all terms in a
source file, writing the resulting terms to a
output.pl file, using
the expansion rules defined in a
expansions hook object. Taking
advantage of the
hook_pipeline/1 object, we
| ?- logtalk_compile( 'input.pl', [hook(hook_pipeline([ expansions, write_to_file_hook('output.pl') ]))] ).
Printing entity predicate goals before or after calling them¶
This is helpful for quick debugging. Load the
file and use the
print_goal_hook hook object. For example, we can
set this hook object as the default hook:
| ?- set_logtalk_flag(hook, print_goal_hook).
Then, edit the entity source code to print selected goals:
foo :- - bar, % print goal before calling it + baz, % print goal after calling it * quux. % print goal before and after calling it
suppress_goal_hook.lgt file provides the
hook object that supports suppressing a goal in a clause body by
prefixing it using the
-- operator. We can set this hook object as
the default hook using the goal:
| ?- set_logtalk_flag(hook, suppress_goal_hook).
If the expansion is only to be used in a single file, use instead the source file directive:
:- set_logtalk_flag(hook, suppress_goal_hook).
Then, edit entity predicates to suppress goals. For example:
foo :- bar, -- baz, quux.
The suppressed goals are replaced by calls to