BSD-3-Clause licensed and maintained by Neil Mitchell
This version can be pinned in stack with:hlint-3.8@sha256:c12a46452e85fc74fb7b0e25aa7653256716f5dc2f8168385b69c79c763d4a0d,4455

Module documentation for 3.8

HLint Hackage version Stackage version Build status

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. This document is structured as follows:

Bugs and limitations

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. This decision is deliberate, allowing HLint to parallelise and be used incrementally on code that may not type-check. If fixities are required to parse the code properly, they can be supplied.
  • 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. In rare cases, there might be nowhere to write the required type signature.
  • Sometimes HLint will change the code in a way that causes values to default to different types, which may change the behaviour.
  • HLint assumes duplicate identical expressions within in a single expression are used at the same type.
  • The RebindableSyntax extension can cause HLint to suggest incorrect changes.
  • HLint can be configured with knowledge of C Pre Processor flags, but it can only see one conditional set of code at a time.
  • 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.
  • Some hints, like Use const, don’t work for non-lifted (i.e. unlifted and unboxed) types.
  • Some language extensions like Strict can cause certain hints (e.g. eta reduction) to be incorrect.

Installing and running HLint

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
    concat $ map escapeC s
    concatMap escapeC s

darcs-2.1.2\src\CommandLine.lhs:103:1: Suggestion: Use fewer brackets
    ftable ++ (map (\ (c, x) -> (toUpper c, urlEncode x)) ftable)
    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
    mapM (delete_line (fn2fp f) line) old
    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.

Any configuration can be done via .hlint.yaml file. Any other file name, such as .hlint.yml, can only be used explicitly with the --hint=FILE option.

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

Suggested usage

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.

Running with Continuous Integration

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 | 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 Haskell IDEs, haskell-language-server, ghc-mod and Intero.
  • HLint Source Plugin makes HLint available as a GHC plugin.
  • Splint is another source plugin that doesn’t require reparsing the GHC source if you are on the latest GHC version.
  • 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.
  • hlint-test helps you write a small test runner with HLint.
  • hint-man automatically submits reviews to opened pull requests in your repositories with inline hints.
  • CircleCI has a plugin to run HLint more easily.
  • haskell/actions has hlint-setup and hlint-run actions for GitHub.
  • haskell-actions/hlint-scan is a GitHub action using HLint for code scanning.

Automatically Applying Hints

HLint can automatically apply some suggestions using the --refactor flag. If passed, instead of printing out the hints, HLint will output the refactored file on stdout. For --refactor to work it is necessary to have the refactor executable from the apply-refact package on your $PATH. HLint uses that tool to perform the refactoring.

When using --refactor you can pass additional options to the refactor binary using --refactor-options flag. Some useful flags include -i (which replaces the original file) and -s (which asks for confirmation before performing a hint). The --with-refactor flag can be used to specify an alternative location for the refactor binary. Simple bindings for Vim, Emacs and Atom are available.

While the --refactor flag is useful, not all hints support refactoring. See for which hints don’t support refactoring.


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.

Language Extensions

HLint enables most Haskell extensions, disabling only those which steal too much syntax (e.g. Arrows, TransformListComp and TypeApplications). 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: [-XArrows].

Emacs Integration

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

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.

Parallel Operation

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"

C preprocessor support

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.



Why doesn’t the compiler automatically apply the optimisations?

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.

Why do I sometimes get a “Note” with my hint?

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.

What is the difference between error/warning/suggestion?

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.

Why do I get a parse error?

HLint enables/disables a set of extensions designed to allow as many files to parse as possible, but sometimes you’ll need to enable an additional extension (e.g. Arrows, QuasiQuotes, …), or disable some (e.g. MagicHash) to enable your code to parse.

You can enable extensions by specifying additional command line arguments in .hlint.yaml, e.g.: - arguments: [-XQuasiQuotes].

How do I only run hlint on changed files?

If you’re using git, it may be helpful to only run hlint on changed files. This can be a considerable speedup on very large codebases.

{ git diff --diff-filter=d --name-only $(git merge-base HEAD origin/master) -- "***.hs" && git ls-files -o --exclude-standard -- "***.hs"; } | xargs hlint

Because hlint’s --refactor option only works when you pass a single file, this approach is also helpful to enable refactoring many files in a single command:

{ git diff --diff-filter=d --name-only $(git merge-base HEAD origin/master) -- "***.hs" && git ls-files -o --exclude-standard -- "***.hs"; } | xargs -I file hlint file --refactor --refactor-options="--inplace --step"


Why doesn’t HLint know the fixity for my custom !@%$ operator?

HLint knows the fixities for all the operators in the base library, as well as operators whose fixities are declared in the module being linted, 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 named .hlint.yaml with the syntax - fixity: "infixr 5 !@%$". You can also use --find to automatically produce a list of fixity declarations in a file.

Which hints are ignored?

Some hints are off-by-default. Some are ignored by the configuration settings. To see all hints pass --show. This feature is often useful in conjunction with --report which shows the hints in an interactive web page, allowing them to be browsed broken down by hint.

Which hints are used?

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

Are there any extra hints available?

There are a few groups of hints that are shipped with HLint, but disabled by default. These are:

  • future, which suggests switching return for pure.
  • extra, which suggests replacements which introduce a dependency on the extra library.
  • use-lens, which suggests replacements which introduce a dependency on the lens library.
  • use-th-quotes, which suggests using [| x |] where possible.
  • generalise, which suggests more generic methods, e.g. fmap instead of map.
  • generalise-for-conciseness, which suggests more generic methods, but only when they are shorter, e.g. maybe True becomes all.
  • dollar which suggests a $ b $ c is replaced with a . b $ c.
  • teaching which encourages a simple beginner friendly style, learning about related functions.

These can be enabled by passing --with-group=future or adding the following to your .hlint.yaml file:

- group: {name: future, enabled: true}


Why are hints not applied recursively?


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.

Is it possible to use pragma annotations in code that is read by ghci (conflicts with OverloadedStrings)?

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.

Customizing the hints

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 shows lots of examples (as # comments) of how to:

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

You can see the output of --default for a clean lint here but for a dirty project --default output includes an extra warnings section that counts and ignores any hints it finds:

# Warnings currently triggered by your code
- ignore: {name: "Redundant $"} # 20 hints
- ignore: {name: "Unused LANGUAGE pragma"} # 29 hints

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

Finding the name of a hint

Hints are named with the string they display in their help message

For example, if hlints outputs a warning like

./backend/tests/api-tests/src/Main.hs:116:51: Warning: Redundant ==
  regIsEnabled rr == True
  regIsEnabled rr

the name of the lint is Redundant ==.

You can use that name to refer to the lint in the configuration file and ANN pragmas, see the following sections.

Ignoring hints

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 myDef "HLint: ignore" #-} or {-# HLINT ignore myDef #-} or {- HLINT ignore myDef -} - don’t give any hints in the definition myDef. This may be combined with hint names, {- HLINT ignore myDef "Eta reduce" -}, to only ignore that hint in that definition.
  • {-# ANN myDef "HLint: error" #-} or {-# HLINT error myDef #-} or {- HLINT error myDef -} - any hint in the definition myDef 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 myDef ("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.myDef} - don’t give any hints in the definition MyModule.myDef.
  • - error: {within: MyModule.myDef} - any hint in the definition MyModule.myDef 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.

You can choose to ignore all hints with - ignore: {} then selectively enable the ones you want (e.g. - warn: {name: Use const}), but it isn’t a totally smooth experience (see #747 and #748).

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

Adding hints

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.

Restricting items

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. Function names can be given qualified, e.g. Data.List.head, but note that functions available through multiple exports (e.g. head is also available from Prelude) should be listed through all paths they are likely to be obtained, as the HLint qualified matching is unaware of re-exports.

Finally, we can restrict the use of modules with:

- modules:
  - {name: [Data.Set, Data.HashSet], as: Set}
  - {name: Control.Arrow, within: []}
  - {name: Control.Monad.State, badidents: [modify, get, put], message: "Use Control.Monad.State.Class instead"}
  - {name: Control.Exception, only: [Exception], message: "Use UnliftIO.Exception instead"}

This fragment adds the following hints:

  • Requires that all imports of Set must be qualified Data.Set as Set, enforcing consistency
  • Ensures the module Control.Arrow can’t be used anywhere
  • Prevents explicit imports of the given identifiers from Control.Monad.State (e.g. to prevent people from importing reexported identifiers).
  • Prevents all imports from Control.Exception, except Exception

You can customize the Note: for restricted modules, functions and extensions, by providing a message field (default: may break the code).

Other options are available:

  • asRequired: boolean.

    If true, as alias is required. Ignored if as is empty.

  • importStyle: one of 'qualified', 'unqualified', 'explicit', 'explicitOrQualified', 'unrestricted'.

    The preferred import style.

    explicitOrQualified accepts both import Foo (a,b,c) and import qualified Foo, but not import Foo or import Foo hiding (x).

    explicit is basically the same, but doesn’t accept import qualified.

    qualified and unqualified do not care about the import list at all.

  • qualifiedStyle: either 'pre', 'post' or 'unrestricted'; how should the module be qualified? This option also affects how suggestions are formatted.

For example:

- modules:
  - {name: [Data.Set, Data.HashSet], as: Set, asRequired: true}
  - {name: Debug, importStyle: explicitOrQualified}
  - {name: Unsafe, importStyle: qualified, qualifiedStyle: post, as: Unsafe}
  - {name: Prelude, importStyle: unqualified}


  • Requires Data.Set and Data.HashSet to be imported with alias Set; if imported without alias, a warning is generated.
  • Says that Debug must be imported either qualified with post-qualification, i.e. import Debug qualified, or with an explicit import list, e.g. Debug (debugPrint).
  • Requires that Unsafe must always be imported qualified, and can’t be aliased.
  • Forbids import qualified Prelude and import Prelude qualified (with or without explicit import list).

You can match on module names using glob-style wildcards. Module names are treated like file paths, except that periods in module names are like directory separators in file paths. So **.*Spec will match Spec, PreludeSpec, Data.ListSpec, and many more. But *Spec won’t match Data.ListSpec because of the separator. See the filepattern library for a more thorough description of the matching.

Restrictions are unified between wildcard and specific matches. With asRequired, importStyle and qualifiedStyle fields, the more specific option takes precedence. The list fields are merged. With multiple wildcard matches, the precedence between them is not guaranteed (but in practice, names are sorted in the reverse lexicograpic order, and the first one wins – which hopefully means the more specific one more often than not)

If the same module is specified multiple times, for asRequired, importStyle and qualifiedStyle fields, only the first definition will take effect.

- modules:
  - {name: [Data.Map, Data.Map.*], as: Map}
  - {name: Test.Hspec, within: **.*Spec }
  - {name: '**', importStyle: post}

Hacking HLint

Contributions to HLint are most welcome, following my standard contribution guidelines.

How to run tests

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 run -- hlint --test. After changing hints, you will need to regenerate the file with hlint --generate-summary.

How to add tests

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

    Tests to check the zipFrom hint works

zip [1..length x] x -- zipFrom 1 x
zip [1..length y] x
zip [1..length x] x -- ??? @Warning

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.

Printing abstract syntax

Getting started on problems in HLint often means wanting to inspect a GHC parse tree to get a sense of what it looks like (to see how to match on it for example). Given a source program Foo.hs (say), you can get GHC to print a textual representation of Foo’s AST via the -ddump-parsed-ast flag e.g. ghc -fforce-recomp -ddump-parsed-ast -c Foo.hs.

When you have an HsSyn term in your program, it’s quite common to want to print it (e.g. via Debug.Trace.trace). Types in HsSyn aren’t in Show. Not all types in HsSyn are Outputable but when they are you can call ppr to get SDocs. This idiom is common enough that there exists unsafePrettyPrint. The function showAstData can be called on any HsSyn term to get output like with the dump-parsed-ast flag. The showAstData approach is preferable to ppr when both choices exist in that two ASTs that differ only in fixity arrangements will render differently with the former.

Generating the hints summary

The hints summary is an auto-generated list of hlint’s builtin hints. This can be generated with hlint --generate-summary, which will output the summary to


Many improvements to this program 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.


Changelog for HLint (* = breaking change)

3.8, released 2024-01-15
#1552, make --git and --ignore-glob work nicely together
#1502, fix incorrect free variable calculation in some cases
#1555, slightly more efficient concatMap usages (e.g. pull filter out)
#1500, suggest avoiding NonEmpty.unzip (use Functor.unzip)
* #1544, upgrade to GHC 9.8
#1540, correct Functor law hint, was missing brackets
3.6.1, released 2023-07-03
Attempt to make a binary release
3.6, released 2023-06-26
#1522, define __GLASGOW_HASKELL__ for cmdCpp
#1519, don't suggest removing brackets that result in a parse error
#1512, add hints to convert for [1..n] to replicateM n
#1430, add ignore-suggestions flag
#1517, support NO_COLOR
#1505, don't suggest redundant brackets around constraints
#1478, Fix no warn on (x.y) by treating x.y as atom
#1481, more hints about concat/concatMap
#1485, add more hints about using a Functor on a literal tuple
#1479, add specialised not hints for notElem/notNull
#1437, don't add/remove Monad constraints in replacements
#1475, rules for (elem False) and (notElem True)
#1476, generalise map-consuming rules from lists to Foldables/Traversables
#1480, add hints about notElem and notNull
#1482, support SARIF output
#1470, drop support for GHC 9.0
* #1470, move to GHC 9.6 parser
#1449, add some more generalize hints that suggest fmap instead of map
Never suggest concatForM_, because it doesn't exist
#1467, and groups for warning on partial functions
#1465, improve the timing information with -v flag
#1454, filter Just . map f ==> map Just . mapMaybe f
#1452, add more evaluate rules, e.g. fromJust (Just x) ==> x
#1488, add <$>/<&> hints for Either
#1447, make <$> hints also work on <&> (dualized)
#1450, suggest nub/sort ==> nubSort with the extra hints
#1451, isJust y && (x == fromJust y) ==> Just x == y
#578, don't suggest numeric underscores for < 5 digits
#1428, rename "Use map" hints that introduce tuple sections
#1424, don't suggest sortOn; suggest avoiding `reverse . sort`
3.5, released 2022-09-20
#1421, change zip/repeat to map with tuple section
#1418, add more QuickCheck hints
#1420, suggest use of Data.Tuple.Extra.both in the extra hints
#1407, fix some list-comp hints that applied too broadly
#1407, suggest [ f x | x <- [y] ] to [f y]
#1417, add some more isAlpha/isDigit suggestions
#1411, add some empty list equivalent to "" hints
#1416, add hints for (== True) and other bool/section values
#1410, remove support for building with GHC 8.10
* #1410, always default to using ghc-parser instead of the GHC API
* #1410, upgrade to the GHC 9.4 parse tree
#1408, evaluate calls of map with empty/singleton lists
#1409, add notNull hints, e.g. notNull . concat ==> any notNull
#1406, spot list comprehension that should be lefts/rights
#1404, add more if/then/else to min or max hints
#1403, add last . reverse ==> head
#1397, evaluation rules for minimum/maximum on singleton lists
3.4.1, released 2022-07-10
#1345, add --generate-config to generate a complete config
#1345, add --generate-summary-json
#1377, make anyM/allM on [] produce pure, rather than return
#1377, add a pure rule for every return variant
#1380, add counts as comments for --default
#1372, remove unnecessary brackets when suggesting forM_
#1372, suggest void (forM x y) to forM_ without the void
#1394, replace maximum [a, b] ==> max a b (and for min)
#1393, for QuickCheck, join . elements ==> oneOf
#1387, bypass camelCase hint for new tasty_... custom test prefix
3.4, released 2022-04-24
#1360, make -XHaskell2010 still obey CPP pragmas
#1368, make TH quotation bracket rule off by default
#1367, add brackets on refactoring templates when needed
#1348, make module restrict hints more powerful
#1363, add more hints for <$>
#1362, add support for language specifier GHC2021
#1342, make module wildcards work with `within` restrictions
#1340, include Restriction hints in splitSettings and argsSettings output
#1330, make ignore: {} not ignore subsequent hints
#1317, evaluating all/any/allM/anyM on simple lists
#1303, allow more matches for modules doing `import Prelude ()`
#1324, add createModuleExWithFixities
#1336, warn on unused OverloadedRecordDot
#1334, don't remove TypeApplications if there are type-level type applications
#1332, suggest using iterate instead of scanl/foldl
#1331, suggest using min/max instead of if
* #1247, move to GHC 9.2
3.3.6, released 2021-12-29
#1326, produce release binaries
3.3.5, released 2021-12-12
#1304, support aeson-2.0
#1309, suggest `either Left f x` becomes `f =<< x`
#1295, suggest TemplateHaskell to TemplateHaskellQuotes if it works
#1292, don't say redundant bracket around pattern splices
#1289, suggest expanding tuple sections in some cases
#1289, suggest length [1..n] ==> max 0 n
#1279, suggest using NumericUnderscores more if it is enabled
#1290, move reverse out of filter
3.3.4, released 2021-08-30
#1288, fix generation of Linux binaries
3.3.3, released 2021-08-29
#1286, compatibility with extra-1.7.10
#114, if OverloadedLists are enabled, don't suggest list literals
3.3.2, released 2021-08-28
#1244, add `only` restriction to modules
#1278, make --ignore-glob patterns also ignore directories
#1268, move nub/sort/reverse over catMaybes/lefts/rights
#1276, fix some incorrect unused LANGUAGE warnings
#1271, suggest foldr (<>) mempty ==> fold (not mconcat)
#1274, make the (& f) ==> f hint apply more
#1264, suggest eta reduction under a where
#1266, suggest () <$ x ==> void x
#1223, add some traverse laws
#1254, suggest null [x] ==> False
#1253, suggest reverse . init ==> tail . reverse
#1253, suggest null . concat ==> all null
#1255, suggest filter instead of list comprehension in teaching
3.3.1, released 2021-04-26
#1221, allow restrictions to use wildcards
#1225, treat A{} as not-atomic for bracket hints
#1233, -XHaskell98 and -XHaskell2010 now disable extensions too
#1226, don't warn on top-level splices with brackets
#1230, disable LexicalNegation by default
#1219, suggest uncurry f (a, b) ==> f a b
#1227, remove some excess brackets generated by refactoring
3.3, released 2021-03-14
#1212, don't suggest redundant brackets on $(x)
#1215, suggest varE 'foo ==> [|foo|]
#1215, allow matching on Template Haskell variables
#1216, require apply-refact
* #1209, switch to the GHC 9.0.1 parse tree
Drop GHC 8.6 support
#1206, require apply-refact or higher
#1205, generalize hints which were with '&' form
3.2.8, released 2021-12-27
#1304, support aeson-2.0
#1286, compatibility with extra-1.7.10
3.2.7, released 2021-01-16
#1202, add missing parentheses for Avoid Lambda
#1201, allow matching through the & operator (similar to $)
#1196, fix removed parens with operator sections in some cases
3.2.6, released 2020-12-30
Fixes to the release generation script
3.2.5, released 2020-12-30
Fixes to the release generation script
3.2.4, released 2020-12-30
#1193, add warnings for redundant flip
#1183, allow matches where users specify unnecessary brackets
#1177, remove suggestions for fromMaybe False/fromMaybe True
#1176, suggest use of unzip
#1159, spot redundant brackets due to fixities, default ignored
#1172, suggest reusing fromLeft/fromRight
3.2.3, released 2020-11-23
#1160, never consult the .hscolour file for color preferences
#1171, do not refactor redundant lambda with case
#1169, default to -A32 (doubles speed for 4 CPU)
#1169, make -j actually use parallelism
#1167, enable refactoring for "Use lambda"
3.2.2, released 2020-11-15
#1166, detect more unboxed data to avoid suggesting newtype
#1153, fix incorrect redundant bracket with @($foo)
#1163, do not suggest "Use lambda" when there are guards
#1160, add showIdeaANSI to show Idea values with escape codes
3.2.1, released 2020-10-15
#1150, remove the Duplicate hint (was slow)
#1149, allow within to use module wildcards, e.g. **.Foo
#1141, make redundant return highlight just the return
#1142, suggest newtype instead of data for data family instances
#1138, show allowed fields in YAML error message
#1131, fix potential variable capture in zipWith/repeat hint
#1129, add hints to use const and \_ x -> x where appropriate
3.2, released 2020-09-14
#75, make Windows 10 use color terminals
Make sure the extension removed matches what you called it
* #1124, make test into a flag rather than a mode, use --test
#1073, add LHS/RHS hints to the summary
* Remove test --proof, --quickcheck and --typecheck, --tempdir
* #1123, delete grep mode
#1122, show the --refactor command line with --loud
#1120, enable refactoring for use section
#1114, enable refactoring for redundant as-pattern
#1116, enable refactoring for redundant section
#1115, more TVar/TMVar hints
#1113, suggest x >>= pure . f becomes x <&> f
#1111, add hint for \x y -> (x,y) to (,), and for (,,)
#1108, enable refactoring for redundant variable capture
#1105, enable refactoring for more redundant return hints
#1103, enable refactoring for Redundant void
#1083, add hints for mempty as a function
#1097, enable refactoring for more avoid lambda hints
#1094, error on `-XFoo` if `Foo` is not a known extension
#1090, improve --verbose to print more information
#1085, support eta-reduce refactoring
#780, ignore dist and dist-newstyle directories
#1076, fix -XNoCpp and CPP around LANGUAGE extensions
#1077, warn on unused StandaloneKindSignatures
#1070, warn on unused ImportQualifiedPost
#1067, require apply-refact (fixes lots of bugs)
#1064, don't remove OverloadedLists if there is a []
3.1.6, released 2020-06-24
#1062, make sure matching inserts brackets if required
#1058, weaken the self-definition check to match more things
#1060, suggest [] ++ x and [] ++ x to x
3.1.5, released 2020-06-19
#1049, suggest or/and from True `elem` xs and False `notElem` xs
#1055, avoid incorrect hints with nested (.)'s
#1054, make isLitString work again
#1038, make -XNoTemplateHaskell imply -XNoTemplateHaskellQuotes
#970, require an arg to suggest fromMaybe True ==> (Just True ==)
#1047, suggest pushing take outside zip
#1041, fix language/pragma ordering in refactor
#1040, fix refactoring for "Avoid lambda"
#1042, fix redundant lambda refactoring
#1039, don't suggest move map inside list comp repeatedly
#1035, fix refactoring for "Use fewer imports" in some cases
#1036, disable refactoring for Use camelCase
#766, match quasi quotes properly in rules
Ignore [Char] to String hints by default
#1034, remove suggestions to use heirarchical module names
#1032, fix refactoring for "Use :"
#1028, add hints around sequenceA/traverse
#1027, pass enabled and disabled extensions to apply-refact
#1024, make the redundant bracket hints cover just the bracket
#1024, make redundant $ display more context on the command line
Suggest removing OverloadedLabels if there are no labels
#367, suggest removing OverloadedLists if there are no lists
#1023, speed up checking on large files (up to 12%)
3.1.4, released 2020-05-31
#1018, stop --cross being quadratic
#1019, more rules suggesting even/odd
3.1.3, released 2020-05-25
#1016, check scopes of restricted functions
3.1.2, released 2020-05-24
#1014, don't error on empty do blocks
#1008, make redundant do ignored by default
#1012, add CodeWorld hints around pictures
#1003, enable refactoring for (v1 . v2) <$> v3
#1002, warn on unused NumericUnderscores
3.1.1, released 2020-05-13
#993, deal with infix declarations in the module they occur
#993, make createModuleEx use the default HLint fixities
#995, add unpackSrcSpan to the API
3.1, released 2020-05-07
#979, suggest removing flip only for simple final variables
#978, do is not redundant with non-decreasing indentation
#969, wrong redundant bracket suggestion with BlockArguments
#970, detect redundant sections, (a +) b ==> a + b
* #974, split ParseFlags.extensions into enabled/disabled
#971, add support for -XNoFoo command line flags
#976, run refactor even if no hints
#971, add support for NoFoo language pragmas
3.0.4, released 2020-05-03
#968, fail on all parse errors
#967, enable TypeApplications by default
3.0.3, released 2020-05-03
#965, fix incorrect avoid lambda suggestion
3.0.2, released 2020-05-03
#963, don't generate use-section hints for tuples
#745, fix up free variables for A{x}, fixes list comp hints
3.0.1, released 2020-05-02
#961, don't crash on non-extension LANGUAGE pragmas, e.g. Safe
3.0, released 2020-05-02
Be more permissive with names'with'quotes'in
#953, fix incorrect suggestions with free variables and \case
#955, make --find generate fixity: not infix:
#952, improve refactorings with qualified imports
#945, suggest Map.fromList [] ==> Map.empty
#949, warn about redundant fmaps with binds
#950, reduce the span of "Redundant $" to only cover the "$"
#944, reduce the span of "Use let" to only cover the "let" line
#669, don't suggest replacing reverse . sort (it's quite fast)
#939, reduce the span of "Redundant where" to only cover the "where"
Remove support for GHC 8.4
Remove support for _eval,
#926, fix refactoring when the hint contains _noParen_
#933, improve the output for Redundant do hints
* Merge ParseMode into ParseFlags
* Rename Language.Haskell.HLint3,HLint4 to Language.Haskell.HLint
* Delete the old Language.Haskell.HLint
#881, add a monomorphic group of hints
#837, don't suggest redundant do if its being used for brackets
#923, don't suggest eta reducing infix definitions
#931, disable StaticPointers extension by default
#924, remove dependency on haskell-src-exts
#922, reduce the span of "Redundant do" to only cover the "do"
#919, more specific names for foldMap fusion rules
#918, warn on unused TypeOperators
#916, warn on unused InstanceSigs
Improve parse error context messages
* Most parse errors are fixed
#881, disable hints about maybe that are sometimes wrong
#909, be more careful about redundant bracket warnings
#905, match hints even if there is composition to the left
#904, suggest map/fromMaybe[] becomes maybe [] map
* Remove the hse command line argument, to parse a file with HSE
#901, warn on unused MultiWayIf
Don't raise a parse error if haskell-src-exts can't parse code
#899, warn on unused PatternSynonyms
#898, don't suggest removing NamedFieldPuns with record updates
* Make any --hint flag disable implicit .hlint.yaml search
* Delete the --with flag
* Haskell hint definitions are no longer supported (use YAML)
* Report hints with src-span information, e.g. file:1:1-10
* Delete resolveHints (it was the identity)
* Change to GHC types in the API
Add --with-group=future to add return ==> pure hint
#888, suggest foldr from (.) to ($) in some cases
#884, add more >=> operator hints
#875, fix the extension implication information
Add --with-group=extra to give extra library hints
#873, add more Applicative hints
#872, fix refactoring in hints to use lists
#871, warn when fmapping the result of gets or asks
#869, improve hints for maybe/fromMaybe on Bool
2.2.11, released 2020-02-09
#868, fix some brackets in refactoring suggestions
#865, suggest biList if generalise-for-conciseness is turned on
#859, suggest regular if instead of a simple multi-way if
#860, improve the sortBy/sortOn hints
#862, only suggest TupleSections for 2-tuples once
#854, add more generalise-for-conciseness hints for Either/Maybe
#852, change maybe to fromMaybe, when the function is duplicated
#851, add a rule for maybe Nothing Just
2.2.10, released 2020-02-02
#846, add splitAt warnings
#774, don't warn about 'Redundant compare' in == and /=
2.2.9, released 2020-01-27
Add any/map and all/map fusion hints
#837, don't warn about redundant do for BlockArguments
#842, fix parsing of <% operators in hlint.yaml files
#839, match hints inside instances
#833, UnboxedTuples can be necessary from newtype deriving
#817, add the ability to blacklist identifiers from a module
#834, move not out of any and all
2.2.8, released 2020-01-22
#802, suggest lambda instead of lambda-case for single alts
#811, add some foldMap/map hints
#822, generalise the map/zipWith hint
#824, embed HLint data files using TemplateHaskell
#826, remove curry/uncurry on lambdas
#820, make some hints work in more situations
Reenable PackageImport unused extension detectection
#821, warn on unless/not
#821, avoid curry/uncurry and vice versa
#819, fix a lot of bifunctor hints
#812, add some rules for generalised and/or/any/all
2.2.7, released 2020-01-11
#818, fix incorrect unused LANGUAGE BangPatterns hint
2.2.6, released 2020-01-09
#813, remove any/all with const predicates
Allow haskell-src-exts-1.23
#814, suggest find instead of listToMaybe/filter
Allow ghc-lib-parser-8.8.*
2.2.5, released 2019-12-06
#803, allow newer ghc-lib-8.8.1
#792, note that reverse/sort changes sort stability
#793, don't incorrectly suggest foldr
2.2.4, released 2019-11-02
Allow haskell-src-exts-1.22
#788, give less redundant context on unused variable capture
#334, add --ignore=glob flag
2.2.3, released 2019-09-29
#766, turn on more extensions when parsing config files
#255, don't match variables with type application
Switch to ghc-parser-8.8.1
Slightly restrict the replace case with fromMaybe hint
#701, add hints for replacing case with maybe
#724, suggest Data.Bifunctor in some places
#725, allow custom message for restricted items
2.2.2, released 2019-07-23
#716, upgrade to ghc-lib-parser
2.2.1, released 2019-07-22
#713, make sure -XNoPatternSynonyms works (fix regression)
#700, add some Monoid and Alternative hints
Add createModuleEx to the API
#698, don't suggest a replacement for DerivingStrategies
2.2, released 2019-06-26
* Remove functions and make some things abstract in HLint3 API
2.1.26, released 2019-06-26
Make sure unknown extensions don't cause errors
2.1.25, released 2019-06-26
#681, fix for extensions on the command line not being used
#686, suggest head (drop n x) ==> x !! max 0 n
#683, add Use DerivingStrategies hint, ignored by default
#685, skip running refactoring tool if there are no hints
#675, warn about redundant fmaps on Eithers and Maybes
Add back two $ hints removed in error
2.1.24, released 2019-06-10
Add Language.Haskell.HLint4
#658, ignore the previously undocumented {- LINT -} comments
#658, force parsing of all pragmas and comments eagerly
#665, make different fromMaybe hints have different names
#664, better name for the Use uncurry hint
#659, make hints with brackets at the root work
2.1.23, released 2019-06-09
Make it an error if your code does not parse with GHC
#662, don't warn on ($x), since it might not really be TH
#660, suggest tuple sections for \y -> (x,y) and similar
#667, warn on return x >> m and similar
#653, add symmetric versions of some == hints
#650, add a group of teaching hints
#651, warn on unused NamedFieldPuns
#646, switch to an HTML doctype
2.1.22, released 2019-05-25
#634, suggest modifyIORef ==> writeIORef when applicable
#642, suggest null in more places
#640, reenable GHC parsing
2.1.21, released 2019-05-19
#637, temporary workaround for GHC parser segfaults
2.1.20, released 2019-05-15
* Fix a dumb break in the API on parseModuleEx
2.1.19, released 2019-05-14
* Revert PVP breakage
2.1.18, released 2019-05-13
* Change parseModuleEx/ParseError by accident
#633, don't suggest changes inside RULES
#631, suggest typeOf ==> typeRep
Add matching on type variables
#627, restrict to GHC 8.4 and above
2.1.17, released 2019-04-17
#626, add operator wildcards with ?, ??, ??? etc
#625, fix an rnf/rhs typo
#562, make test --verbose show a list of matched hints
2.1.16, released 2019-04-15
Make `seq` and `rem` hints apply to prefix functions
#604, suggest rnf x `seq` () ==> rnf x
#619, require haskell-src-exts-util-0.2.5
#619, fix move guards forward with record puns
#618, add pure x <*> y ==> x <$> y
#611, add == and subst for more advanced match conditions
#612, add: Suggest f =<< instead of maybe Nothing f
#609, add code smells
#614, adds refactorings for camelCase and some list suggestions
#605, make command line arguments override the .yaml file
#603, QuasiQuotes can programatically use any extension
2.1.15, released 2019-02-27
#593, reorder guards in list comps where possible
#597, suggest pushing a map over a list comp inside
Say redundant pure, when the thing you are removing is pure
#554, add more verbosity
Don't test with GHC 7.4 to 7.8
#590, say which extensions should be deleted
#591, be careful about encoding on stdin
2.1.14, released 2019-01-28
#587, fix extensions implied by ImplicitParams
#588, suggest optional from attoparsec
2.1.13, released 2019-01-23
#583, suggest left sections to avoid lambdas
#580, remove redundant LANGUAGE pragmas which are implied by others
#575, add fixities for lattice
#564, fix hint around withFile with AppendMode
2.1.12, released 2018-12-10
Require haskell-src-exts-1.21
2.1.11, released 2018-12-02
#553, define __HLINT__=1 for the C preprocessor
#546, suggest `x $> y` for `const x <$> y`, `pure x <$> y`, and `return x <$> y`
#546, suggest `x <$ y` for `x <&> const y`, `x <&> pure y`, and `x <&> return y`
#556, disable a few incorrect lens hints
#545, don't suggest turning type applications into sections
#466, avoid false positives for Esqueleto
#535, more lens hints
Allow {-# HLINT #-} and {- HLINT -} pragmas
#532, generate requested report files even if there are no hints
#524, don't suggest newtype for existentials
#521, add a hint for f x@_ = ... ==> f x = ...
2.1.10, released 2018-08-16
#516, don't require a .hlint.yaml when running tests
Prefer .hlint.yaml to HLint.hs for settings
#513, add section links in the HTML report
2.1.9, released 2018-08-08
Add QuickCheck fixities
Warn on redundant EmptyCase extension
2.1.8, released 2018-07-06
#509, remove incorrect suggestions around sequence/pure
2.1.7, released 2018-07-03
#483, don't break quasi quotes when suggesting const
#404, remove the "Unnecessary hiding" hint introduced in #338
#162, make avoiding lambda with `infix` give a different name
#507, rename id x ==> x to redundant id
#286, improve the duplicate pragma message
#399, suggest (& f) ==> f
#136, don't suggest eta-reducing runST
#345, add catMaybes/fmap ==> mapMaybe
#345, add foldMap id ==> fold
#364, suggest >> instead of >>= \_ ->
#502, DeriveTraversable implies DeriveFoldable and DeriveFunctor
Add hints about fusing traverse/map
Better names for mapM/map fusion hints and others
#498, change the output to say "Perhaps:" rather than "Why not:"
2.1.6, released 2018-06-16
Match on explicit brackets at the root of a match expression
#470, suggest TupleSections
#496, suggest sequence/fmap ==> mapM
#473, warn on redundant void, _ <- and return ()
Make use of <$> more general, but in simpler cases
Warn about returns in the middle of do blocks
#471, suggest readTVarIO
#468, suggest using sortOn/Down
#458, document the restriction feature
#494, don't suggest newtype for unboxed tuples
#488, avoid warning about more test prefixes
2.1.5, released 2018-05-05
#478, take account of deriving strategies for extension use
#477, don't warn about unit_ as tasty-discover recommends it
2.1.4, released 2018-05-01
Don't warn about redundant $ for a $ b{c=d}
2.1.3, released 2018-04-18
Improve the performance of the camelCase hint
Don't suggest camelCase for record fields
Add a --timing flag to detect what is slow
2.1.2, released 2018-04-16
#407, don't error on unknown extensions on the command line
Require extra-1.6.6
#464, add more hints for concatMap
#462, ignore home directory when it isn't present
2.1.1, released 2018-03-24
#457, suggest turning on LambdaCase if necessary
#457, add RequiresExtension note
#454, add fixities for the HSpec `should*` functions
#455, add some more sequence hints
#453, allow pure in a few Monad hints as well as return
#451, add --with-group command line option
#424, suggest Foldable.forM_ in a few more places
#445, add suggestions for reader/state monad
#443, suggest join (x <$> y) ==> x =<< y
2.1, released 2018-02-07
* #433, make ideas span multiple modules/declarations
#433, allow ignoring statement-level duplication hint
#439, add more fixities for new base operators
#437, --json output should be finite
#425, avoid misparsing use of Gtk2Hs `on` function
#353, detect unused results from for/traverse/sequenceA
#428, add a few rules for the lens package
#429, spot restricted functions in infix operators
#427, don't eta reduce variables in the presence of quasi-quotes
Improve the HTML slightly
#416, add lens package fixities
2.0.15, released 2018-01-18
#426, don't suggest removing brackets for "x . (x +? x . x)"
#426, better results with haskell-src-exts-util-0.2.2
2.0.14, released 2018-01-14
#376, apply the "use fmap" hint in fewer places
#421, binaries available for OS X
2.0.13, released 2018-01-12
#376, suggest <$> instead of x <- foo; return $ f x
#401, suggest removing brackets for (f . g) <$> x
Add Semigroup instances
2.0.12, released 2017-12-12
Don't suggest Control.Arrow
Upgrade to haskell-src-exts-1.20
2.0.11, released 2017-11-30
#411, parse the YAML file with lots of HSE extensions turned on
#408, use the same config file logic in argsSettings as in hlint
Don't use unexported type synonyms in the public API
#405, fix false positives on MagicHash due to unboxed literals
2.0.10, released 2017-11-03
#377, suggest lambda case
Add CodeClimate support
#378, suggest map for degenerate foldr
#395, suggest x $> y from x *> pure y and x *> return y
#395, suggest x <$ y from pure x <* y and return x <* y
#393, suggest f <$> m from m >>= pure . f and pure . f =<<
#366, avoid the github API for prebuilt hlint, is rate limited
#352, suggest maybe for fromMaybe d (f <$> x)
#338, warn about things imported hidden but not used
#337, add --git flag to additionally check files in git
#353, suggest _ <- mapM to mapM_
#357, warn on unnecessary use of MagicHash
2.0.9, released 2017-06-13
#346, don't suggest explicit export lists
#344, fix the API so it works with hlint.yaml by default
2.0.8, released 2017-05-21
#342, add back support for - to mean stdin
2.0.7, released 2017-05-16
#340, fix for directory arguments in the .hlint.yaml
2.0.6, released 2017-05-08
Do statements are not redundant if they change an operator parse
#333, simplify labels on Parse error, makes it easier to ignore
2.0.5, released 2017-04-19
If the datadir is missing use data/ relative to the executable
Fix test mode to obey --datadir
2.0.4, released 2017-04-17
--default adds ignores for any warnings it finds
2.0.3, released 2017-04-12
#312, suggest removing the DeriveAnyClass extension
Suggest removing the DeriveLift extension
Remove redundant parts from list comprehensions, e.g. [a | True]
#326, fix up the bounds on the eta-reduce hint
2.0.2, released 2017-04-10
#323, try and avoid malformatted JSON
#324, use `backticks` in notes
#324, remove double escaping in note for --json
#322, fix the YAML syntax in default.yaml
2.0.1, released 2017-04-07
#320, still read ./HLint.hs if it exists
2.0, released 2017-04-06
#319, add a hint \x -> f <$> g x ==> fmap f . g
Don't say how many hints were ignored
Add a --default flag
#314, allow arguments in YAML configuration files
Add maybe False (== x) ==> (== Just x) hint, plus for /=
Remove the ability to pass the file on stdin using - as the file
Remove encoding from ParseFlags
Remove the --encoding/--utf8 options, always use UTF8
#311, suggest writeFile instead of withFile/hPutStr
#288, add configurable restrictions
Suggest using an export list on modules
Look for nearby .hlint.yaml files to supply configuration
Support YAML configuration files
#308, don't suggest newtype for unboxed types
Remove the import "hint" feature for hint inclusion
Builtin hints do not need to be imported, can only be ignored
Delete HLint2 API
#290, add hints suggesting traverse/traverse_
#303, detect unused OverloadedStrings extension
#302, detect unused TupleSections extension
1.9.41, released 2017-02-09
#299, warn in some cases when NumDecimals extension is unused
#300, warn when LambdaCase extension is unused
#301, when suggesting newtype remove strictness annotations
#297, better testing that there isn't a performance regression
#167, add -j flag for number of threads
#292, add fst/snd . unzip ==> map fst/snd
Don't suggest module export trick, breaks Haddock
1.9.40, released 2017-01-22
#293, fix the JSON format of the output
1.9.39, released 2016-12-04
#287, don't incorrectly suggest newtype
1.9.38, released 2016-11-24
#279, suggest newtype instead of data
#262, add rules to detect redundant castPtr calls
Detect unused TypeApplications extension
#277, don't enable TypeApplications extension by default
Allow haskell-src-exts-1.19
#276, remove multiple redundant parens in one go
#160, add a --only CLI option
#237, fix incorrect quasi quotes extension warning
#257, better bang pattern hints
1.9.37, released 2016-08-08
#255, don't suggest id @Int ==> @Int
#252, avoid clashes with GHCJS in the interim
1.9.36, released 2016-07-25
Require haskell-src-exts-1.18
#249, suggest avoiding elem on singletons
1.9.35, released 2016-06-10
#245, fix parse error reports
#243, update hlint.ghci to work with modern GHC
Require extra-1.4.9
1.9.34, released 2016-06-01
#154, fix some incorrect line numbers in literate Haskell
#161, fix literate Haskell and CPP
1.9.33, released 2016-05-30
#240, remove type-incorrect "on" hint
#234, warn about join seq
#232, suggest <|> instead of mplus in a few cases
1.9.32, released 2016-03-23
#53, require cpphs-1.20.1, has important fixes
#224, treat select $ specially, as per esqueleto conventions
#231, don't modify qualification on substitutions
#229, add void/mapM_/forM_ hints
1.9.31, released 2016-03-01
#222, don't suggest removing ~ if the Strict extension is on
1.9.30, released 2016-02-26
#220, fix incorrect hints of foldr/foldl on a tuple accumulator
1.9.29, released 2016-02-25
#219, add warnings about foldable methods on tuple
Put warnings before suggestions in the HTML report
1.9.28, released 2016-02-04
#215, spot newtype deriving inside classes
1.9.27, released 2016-02-01
#203, avoid recompiling everything twice
#213, don't suggest removing bang patterns on let
Rename HintStructure to HintPattern
#208, add an hlint function to the HLint3 API
#1, warn about unused DefaultSignatures extension
#137, add -XHaskell2010 and fix -XHaskell98
Allow checking even if a function has different arities
#193, don't warn on a -> (b -> c), it's sometimes sensible
#182, make parse errors severity Error
#181, warn on otherwise in a pattern variable
#163, eta reduce fun x = f $ x
#132, don't ever suggest liftM
#99, downgrade built in hints, Error => Warning => Suggestion
#99, add a Suggestion level severity
#207, make sure you close file handles
#205, add hint compare x y == EQ and /=
#204, add hint concatMap id ==> concat
#202, include refactorings is --json output
1.9.26, released 2016-01-02
#200, fix all lint warnings
#143, expose argsSettings
1.9.25, released 2015-11-24
#192, fix stdin output and --refactor
1.9.24, released 2015-11-22
#188, improve spotting redundant brackets around patterns
#138, reenable redundant where hint
1.9.23, released 2015-11-19
#184, require haskell-src-exts-1.17
#183, allow test_ as a prefix
1.9.22, released 2015-10-28
Don't suggest redundant lambda on view patterns
Add --no-exit-code flag
#174, don't suggest string literals
#175, disable 'rec' stealing extensions by default
#170, add hints for eta-reduced operators
#149, integrate a --refactor flag
#147, fix the -fglasgow-exts hint
#140, better name for moving brackets to eliminate $
Extra hints for <$>
Remove a redundant fmap hint
#131, add =<< rules in addition to >>=
1.9.21, released 2015-05-26
#130, ignore a BOM if it exists
#128, don't find files starting with . when searching directories
Suggest concat even if the [] is written ""
1.9.20, released 2015-04-21
#122, fix the zipWith/repeat hint
1.9.19, released 2015-03-26
#119, don't remove RecursiveDo if they use the rec statement
Add a suggestion concatMap/map ==> concatMap
1.9.18, released 2015-03-17
More GHC 7.10 warnings and build support
1.9.17, released 2015-02-25
#116, support hscolour-1.21
1.9.16, released 2015-01-09
#108, make "hlint ." work again
1.9.15, released 2015-01-03
#106, avoid warnings with GHC 7.10
#105, build with GHC 7.10
1.9.14, released 2014-12-24
#649, don't suggest const for values using RecordWildCards
1.9.13, released 2014-11-30
#97, remove the encoding bits of the API
#98, add an HLint3 prototype API
#93, make the --quickcheck tests work on GHC 7.8
Add --tempdir flag to the test mode
1.9.12, released 2014-11-09
#96, fix the --utf8 flag
Make Encoding an alias for TextEncoding
Default to UTF8 encoding
1.9.11, released 2014-11-07
#95, don't suggest camel case for names containing digits
Add a dependency on the extra package
#92, use a new way for determining the color default
Add a dependency on ansi-terminal
1.9.10, released 2014-10-19
Spot unsafePerformIO without NOINLINE
1.9.9, released 2014-10-13
#89, fix compiling the executable with --flag=-gpl
1.9.8, released 2014-10-08
#82, don't crash on XmlHybrid modules
#88, allow avoiding HsColour, as it is GPL licensed
#87, don't push if down, since it can be type incorrect
1.9.7, released 2014-10-02
#86, don't use color unless $TERM claims to support it
1.9.6, released 2014-09-30
#85, fix the free variable matching check for lambda
#84, suggest fmap for Either
Make --json put each hint on a different line
Support -X for extensions to the hse mode
1.9.5, released 2014-09-14
Remove support for GHC 7.2
Upgrade to haskell-src-exts-1.16
1.9.4, released 2014-08-27
#81, fixes for GHC 7.9
#78, add hints for list patterns
#72, make --color the default on Linux
1.9.3, released 2014-07-28
#73, fix multithreading and exceptions
1.9.2, released 2014-07-23
#68, add --no-summary
1.9.1, released 2014-07-21
#65, add flip (>>=) ==> (=<<) and the reverse
#61, add --json flag
1.9, released 2014-06-30
Remove not (isControl x) ==> isPrint (not true for '\173')
#57, warn on invalid pragmas
Make the API pass and require comments
#59, make sure qualified operators match properly
Rename notTypeSafe annotation to noTypeCheck
Remove an invalid rule suggesting tanh
#13, add a --quickcheck flag to test the hints
Add --typecheck flag to test mode to type check the hints
Remove incorrect for intercalate to unlines
#37, remove incorrect hint for isAlphaNum
#45, add mapMaybe id ==> catMaybes
#42, add some repeat hints
1.8.61, released 2014-04-14
#40, allow haskell-src-exts-1.15
Don't detect redundant Generics extension
1.8.60, released 2014-04-02
#33, add --cpp-file to preinclude a file
#34, add back --quiet flag
#639, don't suggest evaluate, because not all Monad's are IO
#31, delete the elem/notElem hints
#30, remove weird "free module" matching
#15, add prototype grep mode
Change to make test a separate mode
#12, more list based suggestions
#637, turn off QuasiQuotes by default
1.8.59, released 2014-03-13
#27, fix up directory file searching
1.8.58, released 2014-03-11
Move the API to Language.Haskell.HLint2
#638, ensure $! doesn't break strictness with strict fields
#24, don't remove DeriveFunctor even when only on a newtype
#22, turn off UnboxedTuples by default
#21, strip /* C style */ comments
#635 and #18, require cpphs-1.18.1
Switch to CmdArgs for command line parsing
Remove -x as a synonym for --cross
1.8.57, released 2014-02-04
#6, add a preview of an API
#331, improve parse error locations for literate Haskell
1.8.56, released 2014-01-30
Remove support for GHC 6.12 and below
#317, tone down the void hint
#16, match not . not (and reverse . reverse etc)
Suggest <$> instead of fmap f $ ...
Tweak some priorities, make >=> a warn and void an error
#3, make top of the file ANN pragmas work
#10, add a suggestion to use unlines
#11, add a few hints about characters
#8, add CHANGES.txt to the Cabal package
1.8.55, released 2013-11-29
#627, fix the UnboxedTuples extension warning
1.8.54, released 2013-11-28
Fix a bug when suggesting const
1.8.53, released 2013-09-24
Fix some corner cases when suggesting foldr etc.
#517, don't introduce new free variables in a replacement
1.8.52, released 2013-09-24
#2, Generic is not newtype derivable
1.8.51, released 2013-08-20
Upgrade to haskell-src-exts-1.14
1.8.50, released 2013-08-18
Eliminate upper bounds on all dependencies
#617, fix up notIn to take account of Template Haskell variables
#573, suggest removing various deriving language extensions
1.8.49, released 2013-07-23
Remove ^^ ==> ** hint
Remove a duplicate sqrt hint
Ensure that --test failures throws an error
Fix up the copyright year in --help
1.8.48, released 2013-07-16
Brackets at the root of annotations are fine
Reduce a few more lambda expressions
1.8.47, released 2013-06-28
#613, compatibility with base-4.7
1.8.46, released 2013-06-06
Remove incorrect isPrefixOf hints
#586, add span/break/takeWhile/dropWhile hints
#588, add sort/reverse hints
#601, add replicate/map/repeat hints
Add a hint about reverse/reverse
Add side as an alias for _
Add hint as an alias for error
1.8.45, released 2013-05-12
#600, hints for unnecessary lazy annotations
1.8.44, released 2013-04-21
#598, warn on unnecessary bang patterns
1.8.43, released 2013-01-27
Change some hint error/warning levels
1.8.42, released 2013-01-23
Allow cpphs-1.16
1.8.41, released 2013-01-19
#586, add a rule for takeWhile/dropWhile ==> span
#522, add hints for the state monad
#499, fix up the test suite
Fix the side conditions for the `isPrefixOf` hint
Add hints about take/drop on non positive numbers
Add isNat/isPos/isNeg/isNegPos as notes
Make the notes a structured type
Add --proof feature
Retire the Prelude.catch hint
Additional boolean equality hints
1.8.40, released 2013-01-06
#585, lots of additional list based hints
1.8.39, released 2012-12-06
#582, don't suggest renaming with trailingHashes#
#578, treat _ bindings differently in lambdas
1.8.37, released 2012-12-01
#575, allow cpphs-1.15
1.8.36, released 2012-11-27
Make --with imply no default Hint files
1.8.35, released 2012-11-17
#567, avoid duplicate hints around (.) hints
1.8.34, released 2012-11-06
Switch license from GPL to BSD3
1.8.33, released 2012-10-23
Lots more hints on laziness, foldable and a few others
Use mapM_ etc in more situations, when using explicit >>=
1.8.32, released 2012-10-23
Add notes about how to deal with imported fixites
Add a --with flag for passing settings on the command line
#563, make sure TypeSig hints get the right function name
Update the copyright year to 2012
#564, allow brackets and type signatures on annotations
Add a note that about using !! if the index is negative
1.8.31, released 2012-08-18
Avoid incomplete patterns when reading ANN pragmas
#555, top-level expressions require TemplateHaskell
1.8.30, released 2012-07-11
Add elemIndex/elemIndices hints
Allow cpphs-1.14
#551, allow case_ as a name with an underscore
1.8.29, released 2012-06-01
Allow hscolor-1.20.*
#574, add a hint to for mapM/zip ==> zipWithM
1.8.28, released 2012-04-01
Fix a bug, >=> hint was missing check about removal of free var
1.8.27, released 2012-03-30
Allow haskell-src-exts-1.13.*
1.8.26, released 2012-03-27
Allow haskell-src-exts-1.12.*
Don't suggest redundant brackets when turning ++ into :
Add hints suggesting >=> and <=<
1.8.25, released 2012-03-25
Update the copyright year in the Cabal file
Allow transformers-0.3.*
1.8.24, released 2012-02-20
#531, Make hlint.ghci well formed again
1.8.23, released 2012-02-05
Add hints for redundant seq/evaluate using isWHNF
#526, don't hint for return $! (x :: Int)
1.8.22, released 2012-02-04
Add hint for $! where the RHS is not a variable
1.8.21, released 2012-01-26
#508, add lots of hints from the base library
#317, add hints for a >> return () to void
Add a fromMaybe/fmap ==> maybe hint
#304, don't backet tuple sections
Add foldl (++) [] ==> concat
#512, detect unnecessary case construct
When finding hints, don't abort on a parse error
#507, add exitSuccess hint
#505, suggest record patterns
1.8.20, released 2011-11-29
#500, make sure eta reduction has position information
1.8.19, released 2011-11-27
#498, eta reduce even if there is a where block
#497, don't produce an incorrect lambda when suggesting flip
1.8.18, released 2011-11-05
#438, use Foo.Bar to mean Foo/Bar.hs
Add a --path command line option to say where files live
#441, avoid bad matches due to automatically eta reducing rules
#489, import Foo as Foo is redundant
#481, suggest liftM instead of fmap when using the Monad laws
1.8.17, released 2011-10-01
#479, allow - as the file to specify using stdin
1.8.16, released 2011-09-28
#478, allow cpphs-1.13.1
Never suggest view patterns (they aren't sufficiently better)
Don't suggest use of Data.Ord.comparing, using `on` is better
Only suggest elem/notElem on 3 or more items
1.8.15, released 2011-08-13
Add --cpp-ansi to turn on ANSI compat in cpphs
1.8.14, released 2011-08-12
#455, GHC 7.2 compatibility
Add lots of hints from Lennart Augustsson
1.8.13, released 2011-07-05
#302, add a backup fixity analysis, if the HSE one fails
Fix x /= y || x /= z ==> x `notElem` [y,z], should be &&
1.8.12, released 2011-07-03
Allow cpphs-1.12
1.8.11, released 2011-06-18
#440, suggest removing redundant brackets under do
#439, don't add redundant brackets under do
1.8.10, released 2011-06-12
Upgrade to hscolour-1.19
1.8.9, released 2011-05-26
#436, add a hint about mapMaybe/map
Upgrade to haskell-src-exts-1.11.1
Add a --cross flag, to detect hints between multiple modules
#428, don't suggest using String in an instance head
1.8.8, released 2011-04-03
#384, suggest collapsing multiple imports/exports
#374, don't suggest the removal of necessary brackets
#337, suggest Control.Exception.catch instead of Prelude.catch
#412, add hints based on Control.Exception
#378, suggest removing fromInteger/fromIntegral on literals
#369, add notes to a few hints about possible pitfalls
#409, fix a few cases where definitions suggested themselves
#410, Support test* as ignored items in settings files
#414, add isLit* pattern, and hint about ^^ ==> **
#420, make the suggestion to use let a warning
#408, rework the when/unless hints, don't suggest on itself
Add duplicate detector, for copy/pasted code
#285, don't show duplicate filepath separators
If the user enters directories containing no files then say
Make suggesting curry/uncurry a warning instead of an error
1.8.7, released 2011-01-31
Relax the transformers dependency, works with 0.0.* and 0.1.*
1.8.6, released 2011-01-27
Export suggestionSeverity/Severity from the API
Allow hint imports with "hlint", as well as the existing "hint"
1.8.5, released 2011-01-23
Update the copyright year to 2011
#400, support more encoding strings, give useful errors
#401, rename the report template to report_template.html
Replace filter f x /= [] with any f x, and 2 more similar
1.8.4, released 2011-01-12
#308, allow haskell-src-exts-1.10.1, which parses Unicode better
import qualified Char ==> import qualified Data.Char as Char
#393, fix suggestion for import IO, requires more than System.IO
#376, note that RecordWildCards implies DisambiguateRecordFields
1.8.3, released 2010-11-10
Allow uniplate-1.6
Switch from mtl to transformers
#373, require haskell-src-exts-1.9.6
Add a type signature for GHC 7
Suggest [x | x <- xs] ==> xs, if x is a variable
1.8.2, released 2010-10-23
#371, foo (\x -> y :: Int -> Int) is not a redundant bracket
Add a hint to use just equality rather than isJust/fromJust
1.8.1, released 2010-10-15
Massive speed up for files with many naming hints
#361, keep module names when suggesting infix
Add support for wildcard matching on module names
#357, don't camel case suggest on FOO_A
#370, fix building with GHC 6.10.4
#313, upgrade to haskell-src-exts-1.9.4
Workaround for #358, disable empty where hints
#355, make "--ignore=Parse error" work
Add --cpp-simple to run a simple CPP to strip lines begining #
Add bracketing information if the parent is a case
Suggest intercalate
1.8, released 2010-09-11
Make --test --hint=file typecheck a file for valid hints
#347, Suggest use of otherwise, instead of True, in pattern guards
Add hints about redundant where statements
Suggest removal of redundant guards
Make hints about guards work on patterns/infix matches/case alts
Make finding guards look a child functions
Correctly collapse functions and lambdas using the same patterns
Suggest promoting patterns bound to lambdas to functions
Allow collapsing lambdas sharing pattern variables correctly
#344, only give one warning for multiple collapsable lambdas
#300, substantially improve module name resolution with imports
BREAKING: imports in hint files require import "hint" HintFile
#335, redundant id should only generate one warning
Add a hint for using map (f &&& g)
#328, for foo'bar suggest the naming fooBar
#323, detect redundant brackets in field declarations
#321, force the whole file before displaying a parse error
Make --find more robust, fixes a potential parse error
1.7.3, released 2010-07-25
Upgrade to hscolour-1.17
1.7.2, released 2010-06-11
#318, match rules by expanding out (.)
#319, don't remove lambdas on the right of infix operators
1.7.1, released 2010-06-07
Add a --quiet flag, to supress stdout (mainly for API users)
1.7, released 2010-06-06
Add support for HLint.Builtin.All
Fix crash on (\x -> x)
Make the library correctly honour the data directory
Improve the manual, mainly language changes and hyperlinking
Fix a bug in ListRec, could have _recursive_ in the result
#315, spot list rec hints through $ and let
Add hints based on (f $) ==> f, and change in ListRec hints
Changes to the lambda suggestions, now gives a few more hints
Don't suggest importing modules in old-locale/old-time
Make the API return the suggestions, rather than just the count
#278, add -XNoCpp to disable the C preprocessor
#279, add -XExt/-XNoExt to choose extensions
Remove some redundant brackets in type replacements
#286, remove redundant brackets in match
Additional bracket removal, application under sections
#299, rework hints to use flip (suggest infix in some cases)
Add some fromMaybe hints
Fix bug where hints didn't always get names
#306, make --find use the hints if there are files specified
Upgrade to haskell-src-exts-1.9
#303, allow fixities to be specified in hint files
1.6.21, released 2010-04-07
#287, warn about Haskell 98 imports
#297, add a hint to use mplus
#288, detect redundant brackets under a lambda
#302, remove error about ambiguous fixities
#281, enhance the redundant monad return warnings
#293, eliminate _noParen_ from the result
#284, eliminate ViewPatterns from FindHints, hits compiler bug
#283, don't suggest removal of RecordWildCards
Add some hints about concat and (++)
#273, require haskell-src-exts >= 1.8.2
1.6.20, released 2010-02-10
#275, add more acknowledgements (still very incomplete)
#254, remove the foldr1/map hint
Compress nested lambdas, \x -> \y -> ... ==> \x y -> ...
Fix minor bug on \x -> \x -> foo x x
#274, add redundant bracket inside record update/construct
#272, don't mess up creating sections from qualified names
Add some hints to suggest elem
Add Paths_hlint to the .cabal file, or the library doesn't link
#271, rewrite the match engine in terms of SYB
1.6.19, released 2010-02-06
#251, add automatic definition hunting with --find
#268, rewrite the (.) expansion in hints to fix various bugs
#269, replacing a case with an if should generate one hint
Document the ANN pragmas
Require haskell-src-exts-1.8.1
1.6.18, released 2010-02-02
Remove a hint replacing do x <- foo; bar x with foo >>= bar
#263, support CPP files more fully
Upgrade to hscolour-1.16
Upgrade to cpphs-1.11
1.6.17, released 2010-02-01
Force cpphs-1.10, since 1.11 breaks the interface
More hints from the Data.Maybe module
#262, add support for the TupleSections extension
#264, upgrade to haskell-src-exts-1.8.*, fixes QuasiQuote pos
Upgrade to cpphs 1.10
#266, don't match hints that appear to be the definitions
#248, tone down the eta reduction hints
Add support for WARNING pragma's to reclassify hints
Support ignoring hints on types
Give better error messages on incorrect settings files
Add temporary haskell-src-exts 1.5/1.6 compatibility
#327, add hints to use expressions infix
#240, if a then True else False no longer suggests a || False
Upgrade to haskell-src-exts-1.7.*
#236, support changing the text encoding with --encoding/--utf8
#260, generate nicer lambdas for (($) . f)
Add the hint (($) . f) ==> (f $)
1.6.16, released 2010-01-23
Further performance enhancements (for details see my blog)
Update to uniplate 1.5.* (fixes performance bug)
Improve speed based on profiling (roughly twice as fast)
#245, add hints for excess brackets in types and patterns
Make 100% redundant brackets an error
Fix bug where qualified names did not match
Remove dependency on SYB
#234, allow TH top-level splices for ignore
#110, add tests for ignoring commands
1.6.15, released 2010-01-12
Upgrade to uniplate 1.4.* (fixes performance bug)
#192, make HLint into a fairly basic library
Add --datadir to allow running with a different data directory
#254, eliminate foldl/map fusion rules (which were untrue)
Fix a few typos in the hint rules
Upgrade to uniplate 1.3.*
Upgrade to haskell-src-exts 1.6.*
Add a .ghci file snippet
#247, Fix bug matching expressions containing position info
1.6.14, released 2010-01-05
Upgrade to haskell-src-exts 1.5.*
1.6.13, released 2010-01-05
#246, redundant brackets in [(...)]
Add fold/map fusion hints
Don't suggest namings that are already used in the module
#239, Add suggestions of and/or on foldl
Add --extension flag, to find files not named .hs/.lhs
Only activate the builtin hints when they are imported
Fix matching bug, said "Use flip" on "\v -> f v . g"
Suggest changing some pattern guards to view patterns
1.6.12, released 2009-11-06
Fix a bug with ignored hints being written to reports
Upgrade to haskell-src-exts 1.3.*
#228, suggest let instead of <- return in do statements
#229, suggest comparing
Qualify all non-Prelude function suggestions
#225, Add redundant flip hint
#226, Add ((+) x) ==> (x +)
#223, TemplateHaskell may allow other extensions via code
Fix incorrect suggestion on do x <- f ; g x x
A few small additional hints (use flip, redundant id)
1.6.11, released 2009-09-13
Don't perform type eta reduction
1.6.10, released 2009-09-13
Fix bug, eta reduction on chained infix operators, i.e. x#y#z
1.6.9, released 2009-09-12
#217, don't suggest eta reduction on - or +
Fix bug, PatternGuards under case alternatives were ignored
1.6.8, released 2009-09-07
#213, upgrade to cpphs 1.9
Add suggestion to replace lambda with operator sections
Fix bug, ''Name decided TemplateHaskell was unnecessary
HPC statistics, and increase in test coverage
Fix bug, import A as Y; import A gave import A, missing the as Y
Fix bug, type Foo a = Bar a a incorrectly suggested eta reduce
1.6.7, released 2009-08-31
NOTE: #213 has not been fixed, cpphs can cause hangs
Add threaded flag to Cabal to disable -threaded mode
#212, fix crash
Fix bug, incorrectly decided TemplateHaskell was unnecessary
1.6.6, released 2009-08-29
Upgrade to hscolour 1.15
Add a hint for using unless
#211, add hints for unused extensions
#188, add pragma hints
Add a few additional hints (Functor laws)
#137, add cpphs support
#189, give hints for redundant imports
Upgrade to haskell-src-exts 1.1.*
1.6.5, released 2009-08-02
#206, better presentation of parse errors
#208, give the correct precedence to ==> in source files
1.6.4, released 2009-07-12
Start of changelog