hlint

Source code suggestions https://github.com/ndmitchell/hlint#readme

Version on this page:1.9.21
LTS Haskell 9.18:2.0.11
Stackage Nightly 2017-12-13:2.0.11
Latest on Hackage:2.0.12
BSD3 licensed and maintained by Neil Mitchell

Module documentation for 1.9.21

HLint Hackage version Stackage version Linux Build Status Windows 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. You can try HLint online at lpaste.net - suggestions are shown at the bottom. This document is structured as follows:

Acknowledgements

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.

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.
  • 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. These extensions can be disabled with -XNoMagicHash.

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
Found:
  concat $ map escapeC s
Why not:
  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)
Why not:
  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
Why not:
  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.

Running with Continuous Integration

Before running HLint on your continuous integration (CI) server, you should first ensure there are no existing hints. One way to achieve that is to ignore existing hints by running hlint . --default > .hlint.yaml and checking in the resulting .hlint.yaml.

On the CI you should then 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 certain CI environments there are helper scripts to do that.

Travis: Execute the following command:

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

The arguments after -s are passed to hlint, so modify the final . if you want other arguments.

Appveyor: Add the following statement to .appveyor.yml:

- ps: Invoke-Command ([Scriptblock]::Create((Invoke-WebRequest 'https://raw.githubusercontent.com/ndmitchell/hlint/master/misc/appveyor.ps1').Content)) -ArgumentList @('.')

The arguments inside @() are passed to hlint, so add new arguments surrounded by ', space separated - e.g. @('.' '--report').

Integrations

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.
  • Code Climate is a CI for analysis which integrates HLint.
  • Danger can be used to automatically comment on pull requests with HLint suggestions.

Automatically Applying Hints

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.

Reports

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 (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.

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.

FAQ

Why are hints not applied recursively?

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.

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

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 file and pass --hint=fixities.hs. You can also use --find to automatically produce a list of fixity declarations in a file.

Which hints are used?

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.

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.

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 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.

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 Eta reduce" #-} - ignore all eta reduction suggestions in this module (use module literally, not the name of the module). Put this annotation after the import statements.
  • {-# ANN myFunction "HLint: ignore" #-} - don't give any hints in the function myFunction.
  • {-# ANN myFunction "HLint: error" #-} - any hint in the function myFunction is an error.
  • {-# ANN module "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).

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) #-}.

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.

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.

More Advanced Hints

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.

Hacking HLint

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 stack exec 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.

Changes

Changelog for HLint

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
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