# `dead_code_scanner`

This tool detects likely dead code in Logtalk entities and in Prolog modules compiled as objects. Predicates (and non-terminals) are classified as dead code when:

- There is no scope directive for them and they are not called, directly or indirectly, by any predicate with a (local or inherited) scope directive.

- They are listed in `uses/2` and `use_module/2` directives but not called.

Besides dead code, this tool can also help detect other problems in the code that often result in reporting false positives. For example, typos in `alias/2` directives, missing scope directives, and missing `meta_non_terminal/1` and `meta_predicate/1` directives.

Given the possibility of false positives, care must be taken before deleting reported dead code to ensure that it’s, in fact, code that is not used. A common cause of false positives is the use of conditional compilation directives to provide implementations for predicates missing in some systems.

The `dead_code_scanner.lgt` source file implements the scanning predicates for finding dead code in entities, libraries, and directories. The source file `dead_code_scanner_messages.lgt` defines the default translations for the messages printed when scanning for dead code. These messages can be intercepted to customize output, e.g. to make it less verbose, or for integration with e.g. GUI IDEs and continuous integration servers.

## API documentation

This tool API documentation is available at:

../../docs/library_index.html#dead-code-scanner

For sample queries, please see the `SCRIPT.txt` file in the tool directory.

## Loading

This tool can be loaded using the query:

    | ?- logtalk_load(dead_code_scanner(loader)).

## Usage

This tool provides a set of predicates that allows scanning entities, libraries, files, and directories. See the tool API documentation for details. The source code to be analyzed should be loaded with the `source_data` and `optimize` flags turned on (possibly set in a loader file).

As an example, assume that we want to scan an application with a library alias `my_app`. The following goals could be used:

    | ?- set_logtalk_flag(source_data, on),
         set_logtalk_flag(optimize, on).
    yes

    | ?- logtalk_load(my_app(loader)).
    ...
    yes

    | ?- dead_code_scanner::library(my_app).
    ...

For complex applications that make use of sub-libraries, there is also a `rlibrary/1` predicate that performs a recursive scan of a library and all its sub-libraries. Conversely, we may be interested in scanning a single entity:

    | ?- dead_code_scanner::entity(some_object).
    ...

For other usage examples, see the `SCRIPT.txt` file in the tool directory.

## Integration with the `make` tool

The `loader.lgt` file sets a make target action that will call the `dead_code_scanner::all` goal whenever the `logtalk_make(check)` goal (or its top-level abbreviation, `{?}`) is called.

## Caveats

Use of local meta-calls with goal arguments only known at runtime can result in false positives. When using library or user-defined meta-predicates, compilation of the source files with the `optimize` flag turned on may allow meta-calls to be resolved at compile time and thus allow calling information for the meta-arguments to be recorded, avoiding false positives for predicates that are only meta-called.

## Scanning Prolog modules

This tool can also be applied to Prolog modules that Logtalk is able to compile as objects. For example, if the Prolog module file is named `module.pl`, try:

    | ?- logtalk_load(module, [source_data(on)]).

Due to the lack of standardization of module systems and the abundance of proprietary extensions, this solution is not expected to work for all cases.

## Scanning plain Prolog files

This tool can also be applied to plain Prolog code. For example, if the Prolog file is named `code.pl`, simply define an object including its code:

    :- object(code).
        :- include('code.pl').
    :- end_object.

Save the object to an e.g. `code.lgt` file in the same directory as the Prolog file and then load it in debug mode:

    | ?- logtalk_load(code, [source_data(on), optimize(on)]).

In alternative, use the `object_wrapper_hook` provided by the `hook_objects` library:

    | ?- logtalk_load(hook_objects(object_wrapper_hook)).
    ...

    | ?- logtalk_load(code, [hook(object_wrapper_hook), source_data(on), optimize(on)]).

With either wrapping solution, pay special attention to any compilation warnings that may signal issues that could prevent the plain Prolog from being fully analyzed when wrapped by an object.

## Other notes

All source files are indented using tabs (a common setting is a tab width equivalent to 4 spaces).