`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 want 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 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 a Prolog application to Logtalk. For example:

| ?- logtalk_load('plain.pl', [hook(object_wrapper_hook)]).
...

| ?- current_object(plain).
yes

Or:

| ?- 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

Or:

| ?- logtalk_load('foo.pl', [hook(object_wrapper_hook(bar,[imports(some_category))]).
...

| ?- current_object(bar).
yes

| ?- imports_category(bar, Category).
Category = some_category
yes

The object_wrapper_hook object sets the context_switching_calls flag to 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 object_wrapper_hook/1-2 objects.

Outputting term-expansion results to a stream or a file

Load the write_to_stream_hook.lgt file and using the write_to_stream_hook(Stream) or write_to_stream_hook(Stream, Options) hook objects. Alternatively, you can load the write_to_file_hook.lgt file and use the write_to_file_hook(File) or write_to_file_hook(File, Options) 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 input.pl 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_flows library hook_pipeline/1 object, we can write:

| ?- 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 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.

hook_objects