Industrial strength refactoring tools are something that the Haskell community as a whole has desired for a number of years. As a result a number of projects have spawned to provide varying levels of abstraction to the refactoring process. Recently, ghc-exactprint has promised to provide a robust foundation for refactoring. This summer I propose to use ghc-exactprint and HaRe to add a --refactor flag to HLint which will automatically apply relevant suggestions.


I'd really like HLint to have an "automatically replace" flag, but I want to do it preserving whitespace and style, which is quite hard.

Neil Mitchell

Whilst the eye-catching end-goal of any refactoring tool is the transformation, another key tenet of refactoring is that the changes don't affect any irrelevant parts of the program. It follows that if no transformation is applied then one should expect no changes to the source file. This is the challenge which ghc-exactprint attempts to solve.

ghc-exactprint works in two phases. The first transforms the GHC AST such that all absolute positions are replaced with relative positions. The second printing phase performs the inverse transformation and prints the desired output. Now to perform transformations, you only need to specify where your elements should be located relative to each other and the preceding element. This makes performing transformations whilst retaining layout easier than ever before.

API Annotations

This approach is only feasible now1 because, before GHC 7.10 the parser discarded all information about the location of keywords. This made tools such as haskell-src-exts necessary if you wanted to precisely print a source file. Now, whilst parsing, the GHC parser records the information of these keywords in a map indexed by the annotation type (AnnKeywordId) and the SrcSpan of the element it came from. This information can then be used to recreate the original document with sufficient care.

Using this new and powerful machinery, writing robust mechanical transformations is much easier that before.


HLint is a very widely used linting tool. It suggests a wide array of refactorings, most of which are trivial for a programmer to perform. The README on the HLint homepage talks briefly about the possibility of refactoring.

If you want to automatically apply suggestions, ... , there are a number of reasons that HLint itself doesn't have an option to automatically apply suggestions:

The underlying Haskell parser library makes it hard to modify the code, then print it similarly to the original. Sometimes multiple transformations may apply. After applying one transformation, others that were otherwise suggested may become inappropriate. I am intending to develop such a feature, but the above reasons mean it is likely to take some time.

I will firstly address the feasibility of using HaRe to perform these refactorings and then the possibility that refactorings may interfere with each other.

Flavours of refactoring

I categorise the types of refactorings found in HLint into three different categories. A full classification is provided in Appendix A.

  1. Direct substitutions - these are listed in a source file and then a naive search of the AST is performed to check if there are any matches. Below is a simple example from the default definitions.

    error = putStrLn (show x) ==> print x
    ^       ^                     ^
    | Severity                    |
            | Expression to replace
                                  | Expression to insert
  2. Simple transformations - Other simple transformations are achieved by traversing over the HSE AST. The simplest is the one which checks for a redundant dollar sign.2

    [msg x y | InfixApp _ a d b <- [x], opExp d ~= "$"
              ,let y = App an a b, not $ needBracket 0 y a
              , not $ needBracket 1 y b]
  3. Complex type sensitive transformations - The most complicated transformation in HLint is the check for duplicated expressions. This is performed by maintaining a map of previously seen expressions and checking whether the current expression appears in the map. Details can be seen in src/Hint/Duplicate.hs.

I predict that automating both (1) and (2) are easy to achieve with further development of ghc-exactprint and HaRe. It has already been proved that a combination of these programs can be used to perform these kinds of simple substitutions.

(3) requires more care to avoid name clashes but would be achievable using a type-aware refactoring. Additionally, HLint does not suggest how to avoid the code duplication (as it does not know any semantic information) which means that implementing this suggestion would require additional machinery only found in HaRe.3

Conflicting Suggestions

There are certain situations where HLint suggests refactorings which would conflict with each other. Consider the following simple example4

example = f $ (x y)

which generates the following warnings.

example.hs:1:11: Warning: Redundant bracket
  f $ (x y)
Why not:
  f $ x y

example.hs:1:11: Warning: Redundant $
  f $ (x y)
Why not:
  f (x y)

2 suggestions

Clearly, as the suggestions overlap each other, choosing to apply either one first results in the other becoming invalid but both reduce to an equivalent normal form.

One simple way to deal with this is to detect when two suggestions are overlapping. If so, then it can only be safe to perform one of the transformations. In general it would seem sensible to prioritise errors rather than the warnings but it would be possible explore different heuristics for this rare occurrence. Another possibility would be to provide an interface for the user to decide which suggestion they would like to apply.

Connecting HLint and HaRe

So far we have discussed how it is both possible to perform source transformations which preserve layout with ghc-exactprint as well as the possibility of applying HLint suggestions. The final piece of the puzzle is how to link the two together.

Depending directly on HaRe

These transformations could easily be achieved by depending directly on HaRe and calling API methods in order to perform the necessary transformations. This approach would perhaps be simple but is undesirable due to HaRe's dependency on the GHC API. Said dependency is rather undesirable as it ties your users to a particular version of GHC. For a project in as wide use as HLint, this is unacceptable.

Intermediate specification format

Much like pandoc-types separates document synthesis from document generation. Perhaps the cleanest solution would be to separate refactoring specification from transformation. Designing a DSL for specifying refactorings without a dependency on HaRe would mean that external libraries could specify transformations before invoking HaRe and piping in their desired transformation. A typical invocation might then be as follows;

hlint --refactor | ghc-hare

this could be folded inside HLint as a system call.

An obvious problem with this approach is how to specify Haskell AST elements to use in the transformations. For variable renaming, one only need provide a new name for the variable. But, say we want to inject more complicated expressions the problem becomes more difficult. A solution is to use the parseExpression endpoint exposed by GHC 7.105 and rely on your user to be able to at least give you a valid string representation of the construct they wanted to insert.

For example, if a user wanted to replace concat . map f with concatMap then the program may look as follows:

  1. The types of refactorings allowed are specified in a separate module.

    -- In a separate library, not depending on GHC.
    data Refactor = Replace SrcSpan String | ...
  2. The refactoring is specified inside HLint.

    -- In HLint
    findReplacement :: ... -> Refactor
    findReplacement expr = Replace sspan "concatMap"
        sspan :: GHC.SrcSpan
        sspan = findReplacementLocation expr
  3. The refactoring is interpreted inside HaRe.

    -- In HaRe
    replaceExpr :: GHC.SrcSpan -> GHC.LHsExpr -> ...
    refactor :: Refactor -> ...
    refactor (Replace span expr) =
      let parsedExpr = parseExpression expr
        in replaceExpr span parsedExpr

Initially, designing the intermediate library will be difficult. It isn't clear which refactoring operations should be atomic and even then there are clear overheads to performing composite refactorings without informed optimisation. For that reason I propose a very naive intermediate layer which is only intended to work between HLint and HaRe. Constructors should directly correspond to refactorings offered by HaRe as to simulate calling the API without causing a dependency on GHC.

Higher-level DSL

Coming full circle, after HaRe, Thompson and Li went on to design a second refactoring tool, this time for Erlang. The community uptake was much greater and as a result they designed such a DSL for scripting refactorings. (Li and Thompson 2012) If time permitted it would be productive to extend my proposed specification format to a more fully featured DSL which enabled users to specify their refactorings. Using Li and Thompson's work could provide a useful starting point. This design work is most likely outside the scope of this project.

Longer term vision

Propagating API changes

With the capability to read a serialisation format, library authors could distribute a changes file which would then be used by HaRe to perform the necessary changes. This is similar to the idea that Roman Cheplyaka suggested at HIW 2012.

Name resolution would be an important and difficult challenge to overcome. Under the influence of NoImplicitPrelude, HLint already provides erroneous suggestions as the substitution mechanism performs syntactic rather than semantic matching. I think that extending HLints matching mechanism to perform semantic matching would be difficult. haskell-src-exts doesn't immediately resolve names, and so it would be necessary to use haskell-names to provide name resolution. This in turn relies on the interface files generated by hs-gen-interface. These kinds of changes can lead you down a rabbit hole and so are not something I plan to pursue in detail this summer.

Caching a loaded module

Applying many small disjoint operations individually could get very expensive as the source file is reloaded by HaRe on each separate transformation. A better approach would be to inspect each transformation and apply disjoint transformations without reloading the file.

Updated vim and Emacs bindings

Most Haskell refactoring tools provide bindings to both vim and Emacs, it is also important that HaRe provides such bindings. Once the HaRe API stabilises in the next year this is something I could provide towards the tail end of the summer.


It is well-known that the secret to the success of any library is a mixture of solving a cool problem and a suitable level of promotion. HaRe has been around for a number of years but has not gained very much traction. Once it is easy to write your own transformations then I would look to write a series of blog posts demonstrating how easy Haskell program synthesis can be.6

About Me

Since successfully completing a project working on Pandoc last year I have remained involved in the Haskell community. I have continued to contribute to Pandoc through triaging bug reports and contributing infrastructure patches.

Over the winter I completed an internship at the social media startup Borders where I worked on writing slack-api and contributing to internal admin tools.

I am already intimately familiar with ghc-exactprint after recently completing a significant refactoring of the core machinery. This work has made me well aware of how fiddly these kind of foundational tools can be! Something to be conscious of when committing a summer to a project.


Due to university exams, I plan to start a month late and finish a month late. This arrangement worked well last summer.

Start End Activity
25th May 20th June Inactive due to university exams.
20th June 20th July Work on ghc-exactprint and HaRe to provide robust transformations for simple refactorings like those found in HLint.
20th July 20th August Design a simple intermediate library which can be used to bridge between HLint and HaRe.
20th August 20th September Fine tune the intermediate library with experience gained in the first two months.


  1. Solidify the relationship between HaRe and ghc-exactprint by providing primitive refactorings in ghc-exactprint.
  2. A standalone refactoring DSL (provided by a separate library) based on expression replacements.
  3. Modifying HLint to produce suggestions in this format.
  4. Modify HaRe to understand this DSL and perform the replacements.

Appendix A - HLint refactorings classified

Kind Module Type Comments
Substituting malformed pragmas Comment.hs (2) Pragmas and comments are treated differently in the AST so it may be a little bit fiddly to transform one from the other.
Removing redundant parentheses/dollars Bracket.hs (2) This should be straightforward.
Duplicated code blocks Duplicate.hs (3) As previously discussed this will be difficult to get right without additional work to HaRe.
Remove unused language extensions Extensions.hs (2)
Combining import declarations Import.hs (2) Can be viewed as a deletion followed by an insertion.
Replace lambda functions with common library definitions Lambda.hs (2)/(1) Very much the same flavour as straight substitution.
Replace expressions built with list constructors with sugar List.hs (2) Care needed to correctly manage annotations.
Replace recursive functions with higher-order functions ListRec.hs (2) Best to deal with by insertion and deletion rather than trying very hard to maintain formatting.
Direct substitutions given by a file. Match.hs (1) Direct substitution.
Replace monadic expressions with more idiomatic counterparts. Monad.hs (2) Direct substitution.
Check for camel case variable name Naming.hs (3) Renaming requires knowledge of types but HLint currently doesn't suggest names which will clash with definitions in the module.
Checks for OPTIONS and LANGUAGE pragmas which should be expressed differently. Pragma.hs (2) Almost direct substitution.
Structural refactorings Structural.hs (2)
Checks to see if every usage of unsafePerformIO has a {-# NOINLINE #-} pragma Unsafe.hs (2) Care will be needed to make sure to attach the pragma to the correct location.


Li, Huiqing. 2006. “Refactoring Haskell Programs.” PhD thesis.

Li, Huiqing, and Simon Thompson. 2012. “A domain-specific language for scripting refactorings in Erlang.” Lecture Notes in Computer Science (Including Subseries Lecture Notes in Artificial Intelligence and Lecture Notes in Bioinformatics) 7212 LNCS: 501–15. doi:10.1007/978-3-642-28872-2\_34.

  1. Previously HaRe used an extremely brittle mechanism which performed transformations on the AST but printed using the token stream produced by the lexer. This meant that performing any transformation it was necessary to update both. (see Li 2006, 57 - 59)

  2. A dollar sign ($) is redundant when both arguments do not require parentheses.

  3. Such as a fresh supply of variable names and the ability to check for name capture.

  4. As suggested by Neil Mitchell.

  5. A full list of newly exposed endpoints is: parseModule, parseImport, parseStatement, parseDeclaration, parseExpression, parseTypeSignature, parseFullStmt, parseStmt, parseIdentifier, parseType and parseHeader.

  6. Or maybe just how you can now simultaneously change you names of all your cost centres at once. The reader can decide which she finds more appealing.