Ormolu
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