HLint Manual

by Neil Mitchell

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:

  1. Installing and running HLint
  2. Adding additional hints
  3. Ignoring particular hints


This program has only been made possible by the presence of the haskell-src-exts package, and many useful improvements have been made by Niklas Broberg in response to feature requests.

Bugs and limitations

A complete bug list is kept at my bug tracker.

Installing and running HLint

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

Once HLint is installed, simply run hlint source where source is either a Haskell file or a directory containing some Haskell files. Any 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: Error: Use concatMap
  concat $ map escapeC s
Why not:
  concatMap escapeC s

darcs-2.1.2\src\CommandLine.lhs:103:1: Warning: Use fewer brackets
  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: Error: Use a more efficient monadic variant
  mapM (delete_line (fn2fp f) line) old
Why not:
  mapM_ (delete_line (fn2fp f) line) old

... lots more suggestions ...

Each suggestion says which file/line the suggestion relates to, how serious the issue is, a description of the issue, 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 suggestion is marked as an error, 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 warning include requiring an additional import, something not everyone agrees on, and functions only available in more recent versions of the base library.

Disclaimer: While these hints are meant to be correct, they aren't guaranteed to be. Please report any non equivalent hints (ignoring effects of seq).


HLint can generate a lot of information, and often searching for either the errors specific to a file, or a specific class of errors, is difficult. Using the --report flag HLint will produce a report file in HTML, which can be viewed interactively. It is recommended that if investigating more than a handlful of hints, a report is used.

Recursive Suggestions

Suggestions are 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:

Adding additional hints

The majority of hints are contained in a Hints.hs file which will be installed in the appropriate data directory by Cabal. This file may be freely edited, to add library specific knowledge, or to include hints that may have been missed. In addition, any Hints.hs in the current directory, or specified with the --hint flag, will be used. As an example of a hint file, the line specifying concatMap is:

error = concat (map f x) ==> concatMap f x

The line can be read as replace concat (map f x) with concat (map f x). Anything with a 1-letter variable is treated as a substitution parameter. For examples of more complex hints see the supplied hints file. In general, hints should not be given in point free style, as this reduces the power of the matching. Hints may start with error or warn to denote how severe they are by default.

If you come up with interesting hints, please submit them. For example, some of the hints about last were supplied by Henning Thielemann.

Ignoring particular 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 supressing certain hints. Ignore directives are picked up from the hint files. Some example directives are:

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