# `linter`

Logtalk provides a built-in linter tool that runs automatically when compiling and loading source files. The lint warnings are controlled by a set of flags. The default values for these flags are defined in the backend Prolog compiler adapter files and can be overriden from a settings file or from a source file (e.g. a loader file). These flags can be set globally using the set_logtalk_flag/2 built-in predicate. For (source file or entity) local scope, use instead the set_logtalk_flag/2 directive. Some lint checks are turned off by default, specially when computationally expensive.

Note that, in some cases, the linter may generate false warnings due to source code analysis limitations or special cases that, while valid when intended, usually result from programming issues. When a code rewrite is not a sensible solution to avoid the warning, the workaround is to turn off locally the flag that controls the warning.

## Main linter checks

Lint checks include:

- Missing directives (including scope, meta-predicate, dynamic, discontiguous, and multifile directives)

- Duplicated directives, clauses, and grammar rules

- Missing predicates (calls to non-declared and non-defined predicates)

- Calls to declared but not defined static predicates

- Non-portable predicate calls, predicate options, arithmetic function calls, directives, flags, and flag values

- Suspicious calls (syntactically valid calls that are likely semantic errors; e.g. float comparisons using the standard arithmetic comparison operators)

- Deprecated directives, predicates, control constructs, and flags

- References to unknown entities (objects, protocols, categories, or modules)

- Top-level shortcuts used as directives

- Unification goals that will succeed without binding any variables

- Unification goals that will succeed creating a cyclic term

- Goals that are always true or always false

- Trivial goal fails (due to no matching predicate clause)

- Redefined built-in predicates

- Redefined standard operators

- Lambda expression unclassified variables and mixed up variables

- Lambda expression with parameter variables used elsewhere in a clause

- Singleton variables

- If-then-else and soft cut control constructs without an else part

- Cuts in clauses for multifile predicates

- Missing cut in repeat loops

- Possible non-steadfast predicate definitions

- Non-tail recursive predicate definitions

- Redundant calls to control constructs and built-in predicates

- Calls to all-solutions predicates with existentially qualified variables not occurring in the qualified goal

- Calls to all-solutions predicates with no shared variables between template and goal

- Calls to `bagof/3` and `setof/3` where the goal argument contains singleton variables

- Calls to `findall/3` used to backtrack over all solutions of a goal

- Calls to `catch/3` that catch all exceptions

- Calls to standard predicates that have more efficient alternatives

- Entity, predicate, and variable names not following official coding guidelines

- Variable names that differ only on case

- Clauses whose body is a disjunction

Additional checks are provided by the `make` and `dead_code_scanner` tools. For large projects, the data generated by the `code_metrics` tool may also be relevant in accessing the quality of your code.

## Help on linter warnings

By loading the `tutor` tool, most lint warnings are expanded with explanations and suggestions on how to fix the reported issues.

## Extending the linter

Experimental support for extending the linter with user-defined warnings is available using the logtalk_linter_hook/7 multifile hook predicate.

## Linting 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.

## Linting 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:

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

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

    | ?- logtalk_load([os(loader), hook_objects(object_wrapper_hook)]).
    ...

    | ?- logtalk_load(code, [hook(object_wrapper_hook), source_data(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 checked when wrapped by an object.