How to use the command hlint (with examples)

The hlint command is a tool for suggesting improvements to Haskell code. It provides static code analysis and offers suggestions to make code more readable and efficient. It can be used to check individual files or entire directories, generate reports, automatically apply suggestions, display additional options, and generate settings files.

Use case 1: Display suggestions for a given file

Code:

hlint path/to/file options

Motivation: When working on a specific Haskell file, you may want to get suggestions for improving the code. By using hlint with the file path, you can receive recommendations on how to make the code better.

Explanation:

• hlint: The command itself.
• path/to/file: The path to the file on which you want to receive suggestions.
• options: Any additional options or flags you want to pass to the hlint command.

Example output:

Suggestions for path/to/file:
- Use <$> instead of fmap
- Replace 'head . reverse' with 'last'
- Use pattern matching instead of 'case' expression

Use case 2: Check all Haskell files and generate a report

Code:

hlint path/to/directory --report

Motivation: To ensure consistency across all Haskell files in a directory and generate a detailed report that highlights areas for improvement, you can use the hlint command with the --report flag.

Explanation:

• hlint: The command itself.
• path/to/directory: The path to the directory containing the Haskell files you want to check.
• --report: A flag indicating that a report should be generated.

Example output:

HLint report for path/to/directory:
- File1.hs: 3 suggestions
- File2.hs: 5 suggestions
- File3.hs: 0 suggestions

Total suggestions: 8

Use case 3: Automatically apply most suggestions

Code:

hlint path/to/file --refactor

Motivation: If you want to automatically apply most of the suggestions to a specific Haskell file, using the hlint command with the --refactor flag can save you time by automatically applying common improvements.

Explanation:

• hlint: The command itself.
• path/to/file: The path to the file on which you want to apply suggestions.
• --refactor: A flag indicating that most suggestions should be automatically applied.

Example output:

Successfully applied suggestions to path/to/file.
Suggestions remaining: 0

Use case 4: Display additional options

Code:

hlint path/to/file --refactor-options

Motivation: When using the hlint command with the --refactor flag, you may want to know what additional options are available for customizing the suggestion application process. The --refactor-options flag can be used to display these options.

Explanation:

• hlint: The command itself.
• path/to/file: The path to the file on which you want to display additional options.
• --refactor-options: A flag indicating that additional options should be displayed.

Example output:

Additional options for --refactor:
- --in-place: Apply suggestions directly to the file, modifying it.
- --backup: Create a backup of the original file before applying suggestions.
- --no-actions: Display the suggestions without actually applying them.

Use case 5: Generate a settings file ignoring all outstanding hints

Code:

hlint path/to/file --default > .hlint.yaml

Motivation: If you want to generate a settings file for hlint that ignores all outstanding hints, you can use the hlint command with the --default flag and redirect the output to a file (e.g., .hlint.yaml).

Explanation:

• hlint: The command itself.
• path/to/file: The path to the file on which you want to generate the settings file.
• --default: A flag indicating that a default settings file should be created.
• > .hlint.yaml: Redirect the output of the hlint command to a file named .hlint.yaml.

Example output:

Default settings file generated: .hlint.yaml
The generated settings file ignores all outstanding hints.