HLint
A tool for suggesting possible improvements to Haskell code, such as using alternative functions, simplifying code, and spotting redundancies.
What is HLint?
HLint is a tool for suggesting possible improvements to Haskell code. It analyzes Haskell source files and provides hints about alternative functions, simplifications, and redundancies. The tool helps developers write more idiomatic and maintainable Haskell by highlighting areas where code can be cleaned up or optimized for readability.
Haskell developers of all levels, from beginners learning best practices to experienced programmers maintaining large codebases. It is particularly useful for teams enforcing consistent coding standards.
Developers choose HLint because it integrates seamlessly into their workflow, offers extensive customization, and focuses on teaching better Haskell style rather than just enforcing rules. Its ability to work with editors, CI systems, and refactoring tools makes it a versatile addition to any Haskell project.
Overview
Haskell source code suggestions
Use Cases
Best For
- Enforcing consistent coding style across a Haskell codebase
- Learning Haskell best practices and idiomatic patterns
- Automatically identifying code simplifications during code reviews
- Integrating static analysis into continuous integration pipelines
- Refactoring legacy Haskell code to improve readability
- Customizing linting rules for project-specific conventions
Not Ideal For
- Projects relying on custom preprocessors like markdown-unlit or language extensions such as RebindableSyntax that break HLint's parsing
- Codebases where semantic correctness depends on type information or fixities from imports, which HLint cannot infer due to its modular analysis
- Teams focused primarily on performance optimization over code clarity, as HLint's hints target stylistic improvements rather than speed
- Environments requiring real-time, whole-project linting with full context, since HLint processes files in isolation without resolving imports
Pros & Cons
Pros
Allows users to ignore, modify, or add project-specific hints via .hlint.yaml files, with examples like banning unsafePerformIO or enabling extra hint groups.
Works with editors, IDEs, CI systems, and refactoring tools, including Emacs plugins, GitHub actions, and automatic application of suggestions using --refactor.
Classifies hints as warnings (always desirable) or suggestions (require judgment), encouraging developers to think critically about code style rather than blindly following rules.
Supports fast analysis on multi-core systems with flags like -j4, and can fetch pre-compiled binaries for CI to avoid compilation overhead.
Cons
Analyzes each module in isolation without type information or knowledge of imports, leading to potential issues with fixity declarations or scoped names, as admitted in the bugs section.
Fails to parse code using custom preprocessors or certain GHC extensions like Strict or RebindableSyntax, requiring manual disabling and causing parse errors.
Some hints, such as eta reduction, can change program semantics in the presence of seq or type defaults, and the tool warns about increased laziness or removed errors.
Frequently Asked Questions
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