HLint is a tool for suggesting possible improvements to Haskell code. These suggestions include ideas such as using alternative functions, simplifying code and spotting redundancies. You can try HLint online at lpaste.net - suggestions are shown at the bottom. This document is structured as follows:

• Installing and running HLint
• FAQ
• Customizing the hints
• Hacking HLint

Bugs can be reported on the bug tracker. There are some issues that I do not intend to fix:

• HLint operates on each module at a time in isolation, as a result HLint does not know about types or which names are in scope.
• The presence of seq may cause some hints (i.e. eta-reduction) to change the semantics of a program.
• Some transformed programs may require additional type signatures, particularly if the transformations trigger the monomorphism restriction or involve rank-2 types.
• The RebindableSyntax extension can cause HLint to suggest incorrect changes.
• HLint turns on many language extensions so it can parse more documents, occasionally some break otherwise legal syntax - e.g. {-#INLINE foo#-} doesn't work with MagicHash, foo $bar means something different with TemplateHaskell. These extensions can be disabled with -XNoMagicHash or -XNoTemplateHaskell etc.
• HLint doesn't run any custom preprocessors, e.g. markdown-unlit or record-dot-preprocessor, so code making use of them will usually fail to parse.

Installation follows the standard pattern of any Haskell library or program: type cabal update to update your local hackage database, then cabal install hlint to install HLint.

Once HLint is installed, run hlint source where source is either a Haskell file, or a directory containing Haskell files. A directory will be searched recursively for any files ending with .hs or .lhs. For example, running HLint over darcs would give:

$ hlint darcs-2.1.2

darcs-2.1.2\src\CommandLine.lhs:94:1: Warning: Use concatMap
Found:
    concat $ map escapeC s
Perhaps:
    concatMap escapeC s

darcs-2.1.2\src\CommandLine.lhs:103:1: Suggestion: Use fewer brackets
Found:
    ftable ++ (map (\ (c, x) -> (toUpper c, urlEncode x)) ftable)
Perhaps:
    ftable ++ map (\ (c, x) -> (toUpper c, urlEncode x)) ftable

darcs-2.1.2\src\Darcs\Patch\Test.lhs:306:1: Warning: Use a more efficient monadic variant
Found:
    mapM (delete_line (fn2fp f) line) old
Perhaps:
    mapM_ (delete_line (fn2fp f) line) old

... lots more hints ...

Each hint says which file/line the hint relates to, how serious an issue it is, a description of the hint, what it found, and what you might want to replace it with. In the case of the first hint, it has suggested that instead of applying concat and map separately, it would be better to use the combination function concatMap.

The first hint is marked as an warning, because using concatMap in preference to the two separate functions is always desirable. In contrast, the removal of brackets is probably a good idea, but not always. Reasons that a hint might be a suggestion include requiring an additional import, something not everyone agrees on, and functions only available in more recent versions of the base library.

Bug reports: The suggested replacement should be equivalent - please report all incorrect suggestions not mentioned as known limitations.

HLint usage tends to proceed in three distinct phases:

1. Initially, run hlint . --report to generate report.html containing a list of all issues HLint has found. Fix those you think are worth fixing and keep repeating.
2. Once you are happy, run hlint . --default > .hlint.yaml, which will generate a settings file ignoring all the hints currently outstanding. Over time you may wish to edit the list.
3. For larger projects, add custom hints or rules.

Most hints are intended to be a good idea in most circumstances, but not universally - judgement is required. When contributing to someone else's project, HLint can identify pieces of code to look at, but only make changes you consider improvements - not merely to adhere to HLint rules.

On CI you might wish to run hlint . (or hlint src if you only want to check the src directory). To avoid the cost of compilation you may wish to fetch the latest HLint binary release.

For the CI systems Travis, Appveyor and Azure Pipelines add the line:

curl -sSL https://raw.github.com/ndmitchell/hlint/master/misc/run.sh | sh -s .

The arguments after -s are passed to hlint, so modify the final . if you want other arguments. This command works on Windows, Mac and Linux.

HLint is integrated into lots of places:

• Lots of editors have HLint plugins (quite a few have more than one HLint plugin).
• HLint is part of the multiple editor plugins ghc-mod and Intero.
• HLint Source Plugin makes HLint available as a GHC plugin.
• Code Climate is a CI for analysis which integrates HLint.
• Danger can be used to automatically comment on pull requests with HLint suggestions.
• Restyled includes an HLint Restyler to automatically run hlint --refactor on files changed in GitHub Pull Requests.

By supplying the --refactor flag hlint can automatically apply most suggestions. Instead of a list of hints, hlint will instead output the refactored file on stdout. In order to do this, it is necessary to have the refactor executable on you path. refactor is provided by the apply-refact package, it uses the GHC API in order to transform source files given a list of refactorings to apply. Hlint directly calls the executable to apply the suggestions.

Additional configuration can be passed to refactor with the --refactor-options flag. Some useful flags include -i which replaces the original file and -s which asks for confirmation before performing a hint.

An alternative location for refactor can be specified with the --with-refactor flag.

Simple bindings for vim, emacs and atom are provided.

There are no plans to support the duplication nor the renaming hints.

HLint can generate a lot of information, making it difficult to search for particular types of errors. The --report flag will cause HLint to generate a report file in HTML, which can be viewed interactively. Reports are recommended when there are more than a handful of hints.

HLint enables most Haskell extensions, disabling only those which steal too much syntax (currently Arrows, TransformListComp, XmlSyntax and RegularPatterns). Individual extensions can be enabled or disabled with, for instance, -XArrows, or -XNoMagicHash. The flag -XHaskell2010 selects Haskell 2010 compatibility. You can also pass them via .hlint.yaml file. For example: - arguments: [-XQuasiQuotes].

Emacs integration has been provided by Alex Ott. The integration is similar to compilation-mode, allowing navigation between errors. The script is at hs-lint.el, and a copy is installed locally in the data directory. To use, add the following code to the Emacs init file:

(require 'hs-lint)
(defun my-haskell-mode-hook ()
    (local-set-key "\C-cl" 'hs-lint))
(add-hook 'haskell-mode-hook 'my-haskell-mode-hook)

GHCi integration has been provided by Gwern Branwen. The integration allows running :hlint from the GHCi prompt. The script is at hlint.ghci, and a copy is installed locally in the data directory. To use, add the contents to your GHCi startup file.

To run HLint on 4 processors append the flags -j4. HLint will usually perform fastest if n is equal to the number of physical processors, which can be done with -j alone.

If your version of GHC does not support the GHC threaded runtime then install with the command: cabal install --flags="-threaded"

HLint runs the cpphs C preprocessor over all input files, by default using the current directory as the include path with no defined macros. These settings can be modified using the flags --cpp-include and --cpp-define. To disable the C preprocessor use the flag -XNoCPP. There are a number of limitations to the C preprocessor support:

• HLint will only check one branch of an #if, based on which macros have been defined.
• Any missing #include files will produce a warning on the console, but no information in the reports.

Consider:

foo xs = concat (map op xs)

This will suggest eta reduction to concat . map op, and then after making that change and running HLint again, will suggest use of concatMap. Many people wonder why HLint doesn't directly suggest concatMap op. There are a number of reasons:

• HLint aims to both improve code, and to teach the author better style. Doing modifications individually helps this process.
• Sometimes the steps are reasonably complex, by automatically composing them the user may become confused.
• Sometimes HLint gets transformations wrong. If suggestions are applied recursively, one error will cascade.
• Some people only make use of some of the suggestions. In the above example using concatMap is a good idea, but sometimes eta reduction isn't. By suggesting them separately, people can pick and choose.
• Sometimes a transformed expression will be large, and a further hint will apply to some small part of the result, which appears confusing.
• Consider f $ (a b). There are two valid hints, either remove the $ or remove the brackets, but only one can be applied.

HLint doesn't suggest optimisations, it suggests code improvements - the intention is to make the code simpler, rather than making the code perform faster. The GHC compiler automatically applies many of the rules suggested by HLint, so HLint suggestions will rarely improve performance.

HLint knows the fixities for all the operators in the base library, but no others. HLint works on a single file at a time, and does not resolve imports, so cannot see fixity declarations from imported modules. You can tell HLint about fixities by putting them in a hint file, or passing them on the command line. For example, pass --with=infixr 5 !@%$, or put all the fixity declarations in a .hlint.yaml file as - fixity: "infixr 5 !@%$". You can also use --find to automatically produce a list of fixity declarations in a file.

HLint uses the hlint.yaml file it ships with by default (containing things like the concatMap hint above), along with with the first .hlint.yaml file it finds in the current directory or any parent thereof. To include other hints, pass --hint=filename.yaml. If you pass any --with hint you will need to explicitly add any --hint flags required.

Most hints are perfect substitutions, and these are displayed without any notes. However, some hints change the semantics of your program - typically in irrelevant ways - but HLint shows a warning note. HLint does not warn when assuming typeclass laws (such as == being symmetric). Some notes you may see include:

• Increases laziness - for example foldl (&&) True suggests and including this note. The new code will work on infinite lists, while the old code would not. Increasing laziness is usually a good idea.
• Decreases laziness - for example (fst a, snd a) suggests a including this note. On evaluation the new code will raise an error if a is an error, while the old code would produce a pair containing two error values. Only a small number of hints decrease laziness, and anyone relying on the laziness of the original code would be advised to include a comment.
• Removes error - for example foldr1 (&&) suggests and including the note Removes error on []. The new code will produce True on the empty list, while the old code would raise an error. Unless you are relying on the exception thrown by the empty list, this hint is safe - and if you do rely on the exception, you would be advised to add a comment.

Every hint has a severity level:

• Error - by default only used for parse errors.
• Warning - for example concat (map f x) suggests concatMap f x as a "warning" severity hint. From a style point of view, you should always replace a combination of concat and map with concatMap.
• Suggestion - for example x !! 0 suggests head x as a "suggestion" severity hint. Typically head is a simpler way of expressing the first element of a list, especially if you are treating the list inductively. However, in the expression f (x !! 4) (x !! 0) (x !! 7), replacing the middle argument with head makes it harder to follow the pattern, and is probably a bad idea. Suggestion hints are often worthwhile, but should not be applied blindly.

The difference between warning and suggestion is one of personal taste, typically my personal taste. If you already have a well developed sense of Haskell style, you should ignore the difference. If you are a beginner Haskell programmer you may wish to focus on warning hints before suggestion hints.

Short answer: yes, it is!

If the language extension OverloadedStrings is enabled, ghci may however report error messages such as:

Ambiguous type variable ‘t0’ arising from an annotation
prevents the constraint ‘(Data.Data.Data t0)’ from being solved.

In this case, a solution is to add the :: String type annotation. For example:

{-# ANN someFunc ("HLint: ignore Use fmap" :: String) #-}

See discussion in issue #372.

To customize the hints given by HLint, create a file .hlint.yaml in the root of your project. For a suitable default run:

hlint --default > .hlint.yaml

This default configuration contains lots of examples, including:

• Adding command line arguments to all runs, e.g. --color or -XNoMagicHash.
• Ignoring certain hints, perhaps within certain modules/functions.
• Restricting use of GHC flags/extensions/functions, e.g. banning Arrows and unsafePerformIO.
• Adding additional project-specific hints.

You can see the output of --default here.

If you wish to use the Dhall configuration language to customize HLint, there is an example and type definition.

Some of the hints are subjective, and some users believe they should be ignored. Some hints are applicable usually, but occasionally don't always make sense. The ignoring mechanism provides features for suppressing certain hints. Ignore directives can either be written as pragmas in the file being analysed, or in the hint files. Examples of pragmas are:

• {-# ANN module "HLint: ignore" #-} or {-# HLINT ignore #-} or {- HLINT ignore -} - ignore all hints in this module (use module literally, not the name of the module).
• {-# ANN module "HLint: ignore Eta reduce" #-} or {-# HLINT ignore "Eta reduce" #-} or {- HLINT ignore "Eta reduce" -} - ignore all eta reduction suggestions in this module.
• {-# ANN myFunction "HLint: ignore" #-} or {-# HLINT ignore myFunction #-} or {- HLINT ignore myFunction -} - don't give any hints in the function myFunction.
• {-# ANN myFunction "HLint: error" #-} or {-# HLINT error myFunction #-} or {- HLINT error myFunction -} - any hint in the function myFunction is an error.
• {-# ANN module "HLint: error Use concatMap" #-} or {-# HLINT error "Use concatMap" #-} or {- HLINT error "Use concatMap" -} - the hint to use concatMap is an error (you may also use warn or suggest in place of error for other severity levels).

For ANN pragmas it is important to put them after any import statements. If you have the OverloadedStrings extension enabled you will need to give an explicit type to the annotation, e.g. {-# ANN myFunction ("HLint: ignore" :: String) #-}. The ANN pragmas can also increase compile times or cause more recompilation than otherwise required, since they are evaluated by TemplateHaskell.

For {-# HLINT #-} pragmas GHC may give a warning about an unrecognised pragma, which can be suppressed with -Wno-unrecognised-pragmas.

For {- HLINT -} comments they are likely to be treated as comments in syntax highlighting, which can lead to them being overlooked.

Ignore directives can also be written in the hint files:

• - ignore: {name: Eta reduce} - suppress all eta reduction suggestions.
• - ignore: {name: Eta reduce, within: [MyModule1, MyModule2]} - suppress eta reduction hints in the MyModule1 and MyModule2 modules.
• - ignore: {within: MyModule.myFunction} - don't give any hints in the function MyModule.myFunction.
• - error: {within: MyModule.myFunction} - any hint in the function MyModule.myFunction is an error.
• - error: {name: Use concatMap} - the hint to use concatMap is an error (you may also use warn or suggest in place of error for other severity levels).

These directives are applied in the order they are given, with later hints overriding earlier ones.

Finally, hlint defines the __HLINT__ preprocessor definition (with value 1), so problematic definitions (including those that don't parse) can be hidden with:

#ifndef __HLINT__
foo = ( -- HLint would fail to parse this
#endif

The hint suggesting concatMap can be defined as:

- warn: {lhs: concat (map f x), rhs: concatMap f x}

This line can be read as replace concat (map f x) with concatMap f x. All single-letter variables are treated as substitution parameters. For examples of more complex hints see the supplied hlint.yaml file in the data directory. This hint will automatically match concat . map f and concat $ map f x, so there is no need to give eta-reduced variants of the hints. Hints may tagged with error, warn or suggest to denote how severe they are by default. In addition, hint is a synonym for suggest. If you come up with interesting hints, please submit them for inclusion.

You can search for possible hints to add from a source file with the --find flag, for example:

$ hlint --find=src/Utils.hs
-- hints found in src/Util.hs
- warn: {lhs: "null (intersect a b)", rhs: "disjoint a b"}
- warn: {lhs: "dropWhile isSpace", rhs: "trimStart"}
- fixity: "infixr 5 !:"

These hints are suitable for inclusion in a custom hint file. You can also include Haskell fixity declarations in a hint file, and these will also be extracted. If you pass only --find flags then the hints will be written out, if you also pass files/folders to check, then the found hints will be automatically used when checking.

Hints can specify more advanced aspects, with names and side conditions. To see examples and descriptions of these features look at the default hint file and the hint interpretation module comments.

HLint can restrict what Haskell code is allowed, which is particularly useful for larger projects which wish to enforce coding standards - there is a short example in the HLint repo itself. As an example of restricting extensions:

- extensions:
  - default: false
  - name: [DeriveDataTypeable, GeneralizedNewtypeDeriving]
  - {name: CPP, within: CompatLayer}

The above block declares that GHC extensions are not allowed by default, apart from DeriveDataTypeable and GeneralizedNewtypeDeriving which are available everywhere. The CPP extension is only allowed in the module CompatLayer. Much like extensions, you can use flags to limit the GHC_OPTIONS flags that are allowed to occur. You can also ban certain functions:

- functions:
  - {name: nub, within: []}
  - {name: unsafePerformIO, within: CompatLayer}

This declares that the nub function can't be used in any modules, and thus is banned from the code. That's probably a good idea, as most people should use an alternative that isn't O(n^2) (e.g. nubOrd). We also whitelist where unsafePerformIO can occur, ensuring that there can be a centrally reviewed location to declare all such instances. Finally, we can restrict the use of modules with:

- modules:
  - {name: [Data.Set, Data.HashSet], as: Set}
  - {name: Control.Arrow, within: []}

This fragment requires that all imports of Set must be qualified Data.Set as Set, enforcing consistency. It also ensures the module Control.Arrow can't be used anywhere.

Contributions to HLint are most welcome, following my standard contribution guidelines. You can run the tests either from within a ghci session by typing :test or by running the standalone binary's tests via cabal run hlint test or stack init && stack run hlint test.

New tests for individual hints can be added directly to source and hint files by adding annotations bracketed in <TEST></TEST> code comment blocks. As some examples:

{-
    Tests to check the zipFrom hint works

<TEST>
zip [1..length x] x -- zipFrom 1 x
zip [1..length y] x
zip [1..length x] x -- ??? @Warning
</TEST>
-}

The general syntax is lhs -- rhs with lhs being the expression you expect to be rewritten as rhs. The absence of rhs means you expect no hints to fire. In addition ??? lets you assert a warning without a particular suggestion, while @ tags require a specific severity -- both these features are used less commonly.

This program has only been made possible by the presence of the haskell-src-exts package, and many improvements have been made by Niklas Broberg in response to feature requests. Additionally, many people have provided help and patches, including Lennart Augustsson, Malcolm Wallace, Henk-Jan van Tuyl, Gwern Branwen, Alex Ott, Andy Stewart, Roman Leshchinskiy, Johannes Lippmann, Iustin Pop, Steve Purcell, Mitchell Rosen and others.