# HLint [](https://hackage.haskell.org/package/hlint) [](https://www.stackage.org/package/hlint) [](https://travis-ci.org/ndmitchell/hlint) [](https://ci.appveyor.com/project/ndmitchell/hlint)
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:
* [Installing and running HLint](#installing-and-running-hlint)
* [FAQ](#faq)
* [Customizing the hints](#customizing-the-hints)
* [Hacking HLint](#hacking-hlint)
### Bugs and limitations
Bugs can be reported [on the bug tracker](https://github.com/ndmitchell/hlint/issues). There are some issues that I do not intend to fix:
* HLint operates on each module at a time in isolation, as a result HLint does not know about types or which names are in scope.
* The presence of `seq` may cause some hints (i.e. eta-reduction) to change the semantics of a program.
* Some transformed programs may require additional type signatures, particularly if the transformations trigger the monomorphism restriction or involve rank-2 types.
* The `RebindableSyntax` extension can cause HLint to suggest incorrect changes.
* HLint turns on many language extensions so it can parse more documents, occasionally some break otherwise legal syntax - e.g. `{-#INLINE foo#-}` doesn't work with `MagicHash`, `foo $bar` means something different with `TemplateHaskell`. These extensions can be disabled with `-XNoMagicHash` or `-XNoTemplateHaskell` etc.
* HLint doesn't run any custom preprocessors, e.g. [markdown-unlit](https://hackage.haskell.org/package/markdown-unlit) or [record-dot-preprocessor](https://hackage.haskell.org/package/record-dot-preprocessor), so code making use of them will usually fail to parse.
## 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:
```console
$ hlint darcs-2.1.2
darcs-2.1.2\src\CommandLine.lhs:94:1: Warning: Use concatMap
Found:
concat $ map escapeC s
Perhaps:
concatMap escapeC s
darcs-2.1.2\src\CommandLine.lhs:103:1: Suggestion: Use fewer brackets
Found:
ftable ++ (map (\ (c, x) -> (toUpper c, urlEncode x)) ftable)
Perhaps:
ftable ++ map (\ (c, x) -> (toUpper c, urlEncode x)) ftable
darcs-2.1.2\src\Darcs\Patch\Test.lhs:306:1: Warning: Use a more efficient monadic variant
Found:
mapM (delete_line (fn2fp f) line) old
Perhaps:
mapM_ (delete_line (fn2fp f) line) old
... lots more hints ...
```
Each hint says which file/line the hint relates to, how serious an issue it is, a description of the hint, what it found, and what you might want to replace it with. In the case of the first hint, it has suggested that instead of applying `concat` and `map` separately, it would be better to use the combination function `concatMap`.
The first hint is marked as an warning, because using `concatMap` in preference to the two separate functions is always desirable. In contrast, the removal of brackets is probably a good idea, but not always. Reasons that a hint might be a suggestion include requiring an additional import, something not everyone agrees on, and functions only available in more recent versions of the base library.
**Bug reports:** The suggested replacement should be equivalent - please report all incorrect suggestions not mentioned as known limitations.
### 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.
1. 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.
1. For larger projects, add [custom hints or rules](#customizing-the-hints).
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](https://github.com/ndmitchell/hlint/releases/latest).
For the CI systems [Travis](https://travis-ci.org/), [Appveyor](https://www.appveyor.com/) and [Azure Pipelines](https://azure.microsoft.com/en-gb/services/devops/pipelines/) add the line:
```sh
curl -sSL https://raw.github.com/ndmitchell/hlint/master/misc/run.sh | sh -s .
```
The arguments after `-s` are passed to `hlint`, so modify the final `.` if you want other arguments. This command works on Windows, Mac and Linux.
### 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](https://hackage.haskell.org/package/ghc-mod) and [Intero](https://github.com/commercialhaskell/intero).
* [HLint Source Plugin](https://github.com/ocharles/hlint-source-plugin) makes HLint available as a GHC plugin.
* [Code Climate](https://docs.codeclimate.com/v1.0/docs/hlint) is a CI for analysis which integrates HLint.
* [Danger](http://allocinit.io/haskell/danger-and-hlint/) can be used to automatically comment on pull requests with HLint suggestions.
* [Restyled](https://restyled.io) includes an HLint Restyler to automatically run `hlint --refactor` on files changed in GitHub Pull Requests.
* [lpaste](http://lpaste.net/) integrates with HLint - suggestions are shown at the bottom.
* [hlint-test](https://hackage.haskell.org/package/hlint-test) helps you write a small test runner with HLint.
### 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`](https://github.com/mpickering/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](https://github.com/mpickering/hlint-refactor-vim),
[emacs](https://github.com/mpickering/hlint-refactor-mode) and [atom](https://github.com/mpickering/hlint-refactor-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. You can also pass them via `.hlint.yaml` file. For example: `- arguments: [-XQuasiQuotes]`.
### Emacs Integration
Emacs integration has been provided by [Alex Ott](http://xtalk.msk.su/~ott/). The integration is similar to compilation-mode, allowing navigation between errors. The script is at [hs-lint.el](https://raw.githubusercontent.com/ndmitchell/hlint/master/data/hs-lint.el), and a copy is installed locally in the data directory. To use, add the following code to the Emacs init file:
```guile
(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](https://raw.githubusercontent.com/ndmitchell/hlint/master/data/hlint.ghci), and a copy is installed locally in the data directory. To use, add the contents to your [GHCi startup file](https://www.haskell.org/ghc/docs/latest/html/users_guide/ghci.html#the-ghci-and-haskeline-files).
### 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](http://hackage.haskell.org/package/cpphs) 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:
```haskell
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](http://haskell.org/ghc/) 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 !@%$`, or put all the fixity declarations in a `.hlint.yaml` file as `- fixity: "infixr 5 !@%$"`. You can also use [--find](https://rawgithub.com/ndmitchell/hlint/master/hlint.htm#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:
```console
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:
```haskell
{-# ANN someFunc ("HLint: ignore Use fmap" :: String) #-}
```
See discussion in [issue #372](https://github.com/ndmitchell/hlint/issues/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:
```console
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](https://github.com/ndmitchell/hlint/blob/master/data/default.yaml).
If you wish to use the [Dhall configuration language](https://github.com/dhall-lang/dhall-lang) to customize HLint, there [is an example](https://kowainik.github.io/posts/2018-09-09-dhall-to-hlint) and [type definition](https://github.com/kowainik/relude/blob/master/hlint/Rule.dhall).
### 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 myFunction "HLint: ignore" #-}` or `{-# HLINT ignore myFunction #-}` or `{- HLINT ignore myFunction -}` - don't give any hints in the function `myFunction`.
* `{-# ANN myFunction "HLint: error" #-}` or `{-# HLINT error myFunction #-}` or `{- HLINT error myFunction -}` - any hint in the function `myFunction` is an error.
* `{-# ANN module "HLint: error Use concatMap" #-}` or `{-# HLINT error "Use concatMap" #-}` or `{- HLINT error "Use concatMap" -}` - the hint to use `concatMap` is an error (you may also use `warn` or `suggest` in place of `error` for other severity levels).
For `ANN` pragmas it is important to put them _after_ any `import` statements. If you have the `OverloadedStrings` extension enabled you will need to give an explicit type to the annotation, e.g. `{-# ANN myFunction ("HLint: ignore" :: String) #-}`. The `ANN` pragmas can also increase compile times or cause more recompilation than otherwise required, since they are evaluated by `TemplateHaskell`.
For `{-# HLINT #-}` pragmas GHC may give a warning about an unrecognised pragma, which can be suppressed with `-Wno-unrecognised-pragmas`.
For `{- HLINT -}` comments they are likely to be treated as comments in syntax highlighting, which can lead to them being overlooked.
Ignore directives can also be written in the hint files:
* `- ignore: {name: Eta reduce}` - suppress all eta reduction suggestions.
* `- ignore: {name: Eta reduce, within: [MyModule1, MyModule2]}` - suppress eta reduction hints in the `MyModule1` and `MyModule2` modules.
* `- ignore: {within: MyModule.myFunction}` - don't give any hints in the function `MyModule.myFunction`.
* `- error: {within: MyModule.myFunction}` - any hint in the function `MyModule.myFunction` is an error.
* `- error: {name: Use concatMap}` - the hint to use `concatMap` is an error (you may also use `warn` or `suggest` in place of `error` for other severity levels).
These directives are applied in the order they are given, with later hints overriding earlier ones.
Finally, `hlint` defines the `__HLINT__` preprocessor definition (with value `1`), so problematic definitions (including those that don't parse) can be hidden with:
```haskell
#ifndef __HLINT__
foo = ( -- HLint would fail to parse this
#endif
```
### Adding hints
The hint suggesting `concatMap` can be defined as:
```yaml
- 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:
```console
$ 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](https://github.com/ndmitchell/hlint/blob/master/data/hlint.yaml) and [the hint interpretation module comments](https://github.com/ndmitchell/hlint/blob/master/src/Hint/Match.hs).
### 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](https://github.com/ndmitchell/hlint/blob/master/.hlint.yaml#L10-L32). As an example of restricting extensions:
```yaml
- 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:
```yaml
- 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`](https://hackage.haskell.org/package/extra/docs/Data-List-Extra.html#v:nubOrd)). We also whitelist where `unsafePerformIO` can occur, ensuring that there can be a centrally reviewed location to declare all such instances. Finally, we can restrict the use of modules with:
```yaml
- modules:
- {name: [Data.Set, Data.HashSet], as: Set}
- {name: Control.Arrow, within: []}
```
This fragment requires that all imports of `Set` must be `qualified Data.Set as Set`, enforcing consistency. It also ensures the module `Control.Arrow` can't be used anywhere.
## Hacking HLint
Contributions to HLint are most welcome, following [my standard contribution guidelines](https://github.com/ndmitchell/neil/blob/master/README.md#contributions). You can run the tests either from within a `ghci` session by typing `:test` or by running the standalone binary's tests via `cabal run hlint test` or `stack init && stack run hlint test`.
New tests for individual hints can be added directly to source and hint files by adding annotations bracketed in `` code comment blocks. As some examples:
```haskell
{-
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.
### Acknowledgements
This program has only been made possible by the presence of the [haskell-src-exts](https://github.com/haskell-suite/haskell-src-exts) package, and many improvements have been made by [Niklas Broberg](http://www.nbroberg.se) 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.