# `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 ../../apis/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 use 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`.