# `hook_objects`

This library provides a set of convenient hook objects for defining custom expansion workflows (using e.g. the `hook_flows` library).

## 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:

1. Using the Prolog backend adapter file expansion rules 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.

2. Restore 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).

3. Prevent applying any (other) user-defined expansion rules when compiling a source file. Simply load the `identity_hook.lgt` file, which defines the `identity_hook` hook object, whose expansion rules simply succeed without changing the terms and goals, and set 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.

4. Expand grammar rules into clauses independently of the compiler by loading the `grammar_rules_hook.lgt` and using 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

5. Use the expansion rules defined in a Prolog module (which can be `user`) by loading 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)).

6. Output term-expansion results to a stream by loading 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.