hook_objects

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.

API documentation

Open the ../../docs/library_index.html#hook_objects link in a web browser.

Loading

To load all hook objects in this library, load the loader.lgt file:

| ?- logtalk_load(hook_objects(loader)).

To load a specific hook object, e.g. the backend_adapter_hook object:

| ?- logtalk_load(hook_objects(backend_adapter_hook)).

Testing

To test this library hook objects, load the tester.lgt file:

| ?- logtalk_load(hook_objects(tester)).

Usage

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 defines a backend_adapter_hook hook object that can be used as a workflow step.

Restoring the default compiler expansion workflow

In this case, load the default_workflow_hook.lgt file, which defines a 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 identity_hook hook 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

Load the grammar_rules_hook.lgt and use the term-expansion rules in the 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

Load the prolog_module_hook.lgt, which defines the parametric hook object 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

Load the object_wrapper_hook.lgt, which defines the parametric hook object object_wrapper_hook. Use it to wrap the contents of a plain Prolog file as an object named after the file. 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

Outputting term-expansion results to a stream

Load the write_to_stream_hook.lgt file and using the write_to_stream_hook(Stream) or write_to_stream_hook(Stream, Options) hook object. The terms are not modified and thus these hook objects may be used at any point in an expansion workflow.

Printing entity predicate goals before or after calling them

This is helpful for quick debugging. Load the print_goal_hook.lgt 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

Suppressing goals

The suppress_goal_hook.lgt file provides the suppress_goal_hook 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 true/0.