# `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 or different predicate implementations per operating-system.

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 the 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)).

## Testing

To test this tool, load the `tester.lgt` file:

    | ?- logtalk_load(dead_code_scanner(tester)).

## 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 from 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 are also `rlibrary/1-2` predicates that perform 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.

## Excluding code from analysis

A set of options is available to specify code that should be excluded when looking for unused predicates (and non-terminals):

- `exclude_directories(Directories)`
  list of directories to exclude (default is `[]`); all sub-directories of the excluded directories are also excluded; directories may be listed by full or relative path
- `exclude_files(Files)`
  list of source files to exclude (default is `[]`); files may be listed by full path or basename, with or without extension
- `exclude_libraries(Libraries)`
  list of libraries to exclude (default is `[startup, scratch_directory]`)
- `exclude_entities(Entities)`
  list of entities to exclude (default is `[]`)

## 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(loader)).
    ...

    | ?- 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.