Rewriting to Haskell–Automatic Formatting

Posted on March 19, 2020 by Riccardo.

Rewriting to Haskell (Series)

Rewriting to Haskell–Intro Rewriting to Haskell–Project Setup Rewriting to Haskell–Deployment Rewriting to Haskell–Automatic Formatting Rewriting to Haskell–Configuration Rewriting to Haskell–Standing on the shoulders of Rails Rewriting to Haskell–Making GHC More Nitpicky Rewriting to Haskell–Testing Rewriting to Haskell–Linting Rewriting to Haskell–Parsing Query Params Rewriting to Haskell–Parsing Query Params, Again Rewriting to Haskell–Errors

One comment in a pull request about code formatting was enough to convince us we needed a tool to solve the problem once for all!

We decided to go with Ormolu because it’s really similar to how Elm Format behaves. In particular, from the readme, among other things:

Implementing one “true” formatting style which admits no configuration.

and

Let some whitespace be programmable. The layout of the input influences the layout choices in the output. This means that the choices between single-line/multi-line layouts in each particular situation are made by the user, not by an algorithm. This makes the implementation simpler and leaves some control to the user while still guaranteeing that the formatted code is stylistically consistent.

To install Ormolu it’s enough to run the following:

stack install ormolu

Then formatting can be checked in CI with:

ormolu -m check **/*.hs

Or formatted with:

ormolu -m inplace **/*.hs

Also, plugins are available to automate formatting whenever a Haskell file is saved! In VIM it works really well:

Gif showing Ormolu formatting a Haskell file in a VIM buffer

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