ormolu

A formatter for Haskell source code

https://github.com/tweag/ormolu

Version on this page:0.0.3.1@rev:1
LTS Haskell 19.8:0.3.1.0@rev:1
Stackage Nightly 2022-05-28:0.5.0.0
Latest on Hackage:0.5.0.0

See all snapshots ormolu appears in

Ormolu

License BSD3 Hackage Stackage Nightly Stackage LTS CircleCI

Ormolu is a formatter for Haskell source code. The project was created with the following goals in mind:

  • Using GHC’s own parser to avoid parsing problems caused by haskell-src-exts.
  • 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.
  • Writing code in such a way so it’s easy to modify and maintain.
  • Implementing one “true” formatting style which admits no configuration.
  • That formatting style aims to result in minimal diffs while still remaining very close to “conventional” Haskell formatting people use.
  • Choose a style compatible with modern dialects of Haskell. As new Haskell extensions enter broad use, we may change the style to accomodate them.
  • Idempotence: formatting already formatted code doesn’t change it.
  • Be well-tested and robust to the point that it can be used in large projects without exposing unfortunate, disappointing bugs here and there.

Building

The easiest way to build the project is with Nix:

$ nix-build -A ormolu

Or with cabal-install from the Nix shell:

$ nix-shell --run "cabal new-build"

Alternatively, stack could be used with a stack.yaml file as follows.

$ cat stack.yaml
resolver: lts-14.3
packages:
- '.'

$ stack build

To use Ormolu directly from GitHub with Nix, this snippet may come in handy:

# This overlay adds Ormolu straight from GitHub.
self: super:

let source = super.fetchFromGitHub {
      owner = "tweag";
      repo = "ormolu";
      rev = "de279d80122b287374d4ed87c7b630db1f157642"; # update as necessary
      sha256 = "0qrxfk62ww6b60ha9sqcgl4nb2n5fhf66a65wszjngwkybwlzmrv"; # as well
    };
    ormolu = import source { pkgs = self; };
in {
  haskell = super.haskell // {
    packages = super.haskell.packages // {
      "${ormolu.ormoluCompiler}" = super.haskell.packages.${ormolu.ormoluCompiler}.override {
        overrides = ormolu.ormoluOverlay;
      };
    };
  };
}

Usage

The following will print the formatted output to the standard output.

$ ormolu Module.hs

Add --mode inplace to replace the contents of the input file with the formatted output.

$ ormolu --mode inplace Module.hs

Current limitations

  • Does not handle CPP (wontfix, see the design document).
  • Input modules should be parsable by Haddock, which is a bit stricter criterion than just being valid Haskell modules.
  • Various minor idempotence issues, most of them are related to comments.

Editor integration

We know of the following editor integrations:

Running on Hackage

It’s possible to try Ormolu on arbitrary packages from Hackage. For that execute (from the root of the cloned repo):

$ nix-build -A hackage.<package>

Then inspect result/log.txt for possible problems. The derivation will also contain formatted .hs files for inspection and original inputs with .hs-original extension (those are with CPP dropped, exactly what is fed into Ormolu).

Contributing

See CONTRIBUTING.md.

License

See LICENSE.md.

Copyright © 2018–present Tweag I/O

Changes

Ormolu 0.0.3.1

  • Fixed rendering of record updates with the record dot preprocessor syntax Issue 498.

Ormolu 0.0.3.0

  • Fixed an issue related to unnecessary use of curly braces. Issue 473.

  • Fixed the issue with formatting multi-way if when it happens to be a function applied to arguments Issue 488. This changed the way multi-line if is formatted in general.

  • Added support for record dot pre-processor when used via the plugin. Issue 486.

  • Stopped hanging record constructors and improved placing potentially-hanging consturctions in the presence of comments. Issue 447.

  • Fixed indentation in presence of type applications. Issue 493.

  • Class and instance declarations now do not have a blank line after where. Grouping of methods from the original input is also preserved with some normalizations. Issue 431.

Ormolu 0.0.2.0

  • Switched to ghc-lib-parser instead of depending on the ghc package directly. This should allow us to use newest features of GHC while not necessarily depending on the newest version of the compiler. In addition to that Ormolu is now GHCJS-compatible.

  • Now unrecognized GHC options passed with --ghc-opt cause Ormolu to fail (exit code 7).

  • Fixed formatting of result type in closed type families. See issue 420.

  • Fixed a minor inconsistency between formatting of normal and foreign type signatures. See issue 408.

  • Fixed a bug when comment before module header with Haddock was moved inside the export list. See issue 430.

  • Empty foralls are now correctly preserved. See issue 429.

  • Fixed issue 446, which involved braces and operators.

  • When there are comments between preceding Haddock (pipe-style) and its corresponding declaration they are preserved like this in the output instead of being shifted. To be clear, this is not a very good idea to have comments in that position because the Haddock will end up not being associated with the declarations. Issues 440 and 448.

  • Implemented correct handling of shebangs. Issue 377.

  • Implemented correct handling of stack headers. Issue 393.

  • Sorting language pragmas cannot not change meaning of the input program anymore. Issue 404.

  • Fixed formatting of applications where function is a complex expression. Issue 444.

Ormolu 0.0.1.0

  • Initial release.