Dhall to HLint: Using Dhall to generate HLint rules

Date: September 9, 2018
Updated: October 2, 2019
Author: Veronika Romashkina

This short blog post covers the process of creating the .hlint.yaml file from the Dhall configuration. We are using dhall-json-1.4.0 in all following code snippets.


Let’s first figure out why one could need custom HLint rules. The default HLint settings mostly cover base library. However, nowadays, many people are using alternative preludes. Having .hlint.yaml file specific to the prelude library can help to migrate to the alternative prelude and use it more efficiently.

In Kowainik, we also have our custom prelude – relude. relude persues lots of goals such as minimalism, convenience, user-friendliness, etc. We are working a lot to make it pleasant to those who are using it. relude functionality has a few differences from what standard Prelude gives us, which means that not all of the out-of-the-box HLint rules are suitable for us. That’s why we need our own rules to provide more information about relude just by running hlint on your project that uses relude as a custom prelude. To show how helpful it could be, I list some use cases for the custom HLint rules below.

  • Reexports

    relude brings common types and functions (from packages like containers and bytestring) into the scope because they are used in almost every application. So we need a way to assist users in getting rid of the redundant imports since GHC can’t warn about such imports. hlint lets us do it with a warn rule. For example:

    - warn:
        name: Use 'ByteString' from Relude
        lhs: Data.ByteString.ByteString
        rhs: ByteString
        note: '''ByteString'' is already exported from Relude'
  • Lifted functions

    In relude a lot of IO functions are lifted to MonadIO for the convenience. All similar HLint suggestions have the following structure:

    - warn:
        name: '''liftIO'' is not needed'
        lhs: liftIO (print x)
        rhs: print x
        note: If you import 'print' from Relude, it's already lifted
  • Relude specific rules

    We also would like to encourage people to make the code more efficient and clear. In relude we introduced some functions with the improved performance comparing to the base analogues. For instance, with the following rule, you can perform much faster nubbing:

    - warn:
        lhs: nub
        rhs: ordNub
        note: '''nub'' is O(n^2), ''ordNub'' is O(n log n)'
  • Ignoring

    Also, as we are constructing a renewed interface, not all base rules are suitable. Consequently, we need to ignore some of them:

    # `relude` doesn't export untotal `head`.
    - ignore:
        name: Use head
  • Hints

    Users might not want to perform all changes we suggest, so instead of warns we can offer hints on some less significant rules:

    - hint:
        lhs: (fmap and (sequence s))
        rhs: andM s
        note: Applying this hint would mean that some actions that were being
              executed previously would no longer be executed.

To summarise everything said before, we have to write a lot of boilerplate to cover all these rules which we (as lazybones) would like to avoid at all costs. Moreover, the maintenance price is quite high for thousands of lines of yaml.

Why Dhall🔗

As the tool that can help us with removing boilerplate, we have chosen Dhall language. As reference:

Dhall is a programmable configuration language that is not Turing-complete. You can think of Dhall as: JSON + functions + types + imports

This sounds like pretty much what we need.

You may wonder why we are not using Haskell for such purposes (though we love it so much ♥). The answer is that we don’t need IO capabilities for our problem; totality and safety of Dhall are enough here. Changing the configuration in Haskell requires to recompile the whole program before generating config, but with Dhall there’s no such extra step. Not to mention fantastic string interpolation that Dhall has. Also, we wanted to familiarise ourselves with the new technologies, and this seemed like an excellent opportunity to dive into Dhall.


Basically, HLint file is just a list of different kind of rules. List in Dhall should consist of the elements of the same type (because Dhall is a typed configuration language), but, as you’ve seen, we need to use warn, ignore, hint and other rules. To unify different rule types with the same type, we can create a sum type in Dhall. Here how you do it (this is the hlint/Rule.dhall]Rule file):

< Arguments :
    { arguments : List Text }
| Ignore :
    { ignore : {name : Text} }
| Warn :
    { warn :
        { name : Optional Text
        , lhs  : Text
        , rhs  : Text
        , note : Optional Text
| Hint :
    { hint :
        { lhs  : Text
        , rhs  : Text
        , note : Optional Text

This type might look like this in Haskell:

data Rule
    = RuleArguments Arguments
    | RuleIgnore Ignore
    | RuleWarn Warn
    | RuleHint Hint

newtype Arguments = Args
    { arguments :: [Text]

newtype Ignore = Ignore
    { ignore :: Name

newtype Name = Name
    { name :: Text

newtype Warn = Warn
    { warn :: Wrn

data Wrn = Wrn
    { name :: Maybe Text
    , lhs  :: Text
    , rhs  :: Text
    , note :: Maybe Text

newtype Hint = Hint
    { hint :: Hnt

data Hnt =
    { lhs  :: Text
    , rhs  :: Text
    , note :: Maybe Text

Since we’ve introduced the main Rule type, we should create functions for adding rules. I’m going to explain it on one example for the reexport warnings. The rest can be found in hlint/warn.dhall.

We need to implement a Dhall function that takes a type name for which we are applying the rule and the module from which people can export it because they don’t know that it’s already in relude, and we expect this function to output the HLint warning about the redundant import.

Let’s try to do this. Talking in Haskell syntax, we need a function like warnReexport :: Text -> Text -> Rule (in our case, the rule is RuleWarn). Let’s write the similar one in Dhall.

-- import Rule type
let Rule = ./Rule.dhall
-- get all constructors of `Rule`, so we can refer to it as `rule.Warn` etc.
let warnReexport
    : Text -> Text -> Rule
    = \(f : Text) -> \(mod : Text) ->
        -- using our constructor to create the `Warn`
        { warn =
            { name = Some "Use '${f}' from Relude"
            , lhs = "${mod}.${f}"
            , rhs = "${f}"
            , note = Some "'${f}' is already exported from Relude"
    { warnReexport = warnReexport
    , ... -- here we have other functions

And, the most important part is how actually this function is used. Let’s look at hlint/hlint.dhall.

-- import functions from previous file
let warn           = ./warn.dhall
-- reassign function
let warnReexport   = warn.warnReexport
    [ warnReexport "ByteString" "Data.ByteString"
    , warnReexport "Text" "Data.Text"

So with such rules, users would be getting a warning when they import ByteString from Data.ByteString module of base instead of omitting it in favour of the relude exports.

And in such manner, we could create functions and build all other rules, pretty easy.

Now, the only thing left is to generate .hlint.yaml from hlint.dhall. To do so, you can run the following command (you need to have dhall-json installed):

$  dhall-to-yaml --omitNull <<< './hlint/hlint.dhall' > .hlint.yaml


The process of creating Dhall modules was not very painful, so we can consider the experiment successful, and we are going to maintain the .hlint.yaml using our hlint.dhall configuration. Now, adding additional rule is just one line, and it’s much easier to refactor configuration!

Here is the table of file size comparison in different formats to show you how many keystrokes we managed to avoid.

.hlint.yaml hlint.dhall
Lines 3086 1019