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182 lines
5.7 KiB
Markdown
182 lines
5.7 KiB
Markdown
# Ormolu
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[![License BSD3](https://img.shields.io/badge/license-BSD3-brightgreen.svg)](http://opensource.org/licenses/BSD-3-Clause)
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[![Hackage](https://img.shields.io/hackage/v/ormolu.svg?style=flat)](https://hackage.haskell.org/package/ormolu)
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[![Stackage Nightly](http://stackage.org/package/ormolu/badge/nightly)](http://stackage.org/nightly/package/ormolu)
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[![Stackage LTS](http://stackage.org/package/ormolu/badge/lts)](http://stackage.org/lts/package/ormolu)
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[![Build status](https://badge.buildkite.com/8e3b0951f3652b77e1c422b361904136a539b0522029156354.svg?branch=master)](https://buildkite.com/tweag-1/ormolu)
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Ormolu is a formatter for Haskell source code. The project was created with
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the following goals in mind:
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* Using GHC's own parser to avoid parsing problems caused by
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[`haskell-src-exts`][haskell-src-exts].
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* Let some whitespace be programmable. The layout of the input influences
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the layout choices in the output. This means that the choices between
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single-line/multi-line layouts in each particular situation are made by
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the user, not by an algorithm. This makes the implementation simpler and
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leaves some control to the user while still guaranteeing that the
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formatted code is stylistically consistent.
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* Writing code in such a way so it's easy to modify and maintain.
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* Implementing one “true” formatting style which admits no configuration.
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* That formatting style aims to result in minimal diffs while still
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remaining very close to “conventional” Haskell formatting people use.
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* Choose a style compatible with modern dialects of Haskell. As new Haskell
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extensions enter broad use, we may change the style to accommodate them.
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* Idempotence: formatting already formatted code doesn't change it.
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* Be well-tested and robust to the point that it can be used in large
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projects without exposing unfortunate, disappointing bugs here and there.
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## Building
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The easiest way to build the project is with Nix:
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```console
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$ nix-build -A ormolu
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```
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Or with `cabal-install` from the Nix shell:
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```console
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$ nix-shell --run "cabal new-build"
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```
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Alternatively, `stack` could be used with a `stack.yaml` file as follows.
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```console
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$ cat stack.yaml
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resolver: lts-16.0
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packages:
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- '.'
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$ stack build # to build
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$ stack install # to install
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```
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To use Ormolu directly from GitHub with Nix, this snippet may come in handy:
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```nix
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# This overlay adds Ormolu straight from GitHub.
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self: super:
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let source = super.fetchFromGitHub {
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owner = "tweag";
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repo = "ormolu";
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rev = "de279d80122b287374d4ed87c7b630db1f157642"; # update as necessary
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sha256 = "0qrxfk62ww6b60ha9sqcgl4nb2n5fhf66a65wszjngwkybwlzmrv"; # as well
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};
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ormolu = import source { pkgs = self; };
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in {
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haskell = super.haskell // {
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packages = super.haskell.packages // {
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"${ormolu.ormoluCompiler}" = super.haskell.packages.${ormolu.ormoluCompiler}.override {
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overrides = ormolu.ormoluOverlay;
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};
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};
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};
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}
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```
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## Usage
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The following will print the formatted output to the standard output.
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```console
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$ ormolu Module.hs
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```
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Add `--mode inplace` to replace the contents of the input file with the
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formatted output.
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```console
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$ ormolu --mode inplace Module.hs
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```
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Use `find` to format a tree recursively:
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```console
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$ ormolu --mode inplace $(find . -name '*.hs')
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```
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## Magic comments
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Ormolu understands two magic comments:
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```haskell
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{- ORMOLU_DISABLE -}
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```
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and
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```haskell
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{- ORMOLU_ENABLE -}
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```
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This allows us to disable formatting selectively for code between these
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markers or disable it for the entire file. To achieve the latter, just put
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`{- ORMOLU_DISABLE -}` at the very top. Note that the source code should
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still be parseable even without the “excluded” part. Because of that the
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magic comments cannot be placed arbitrary, but should rather enclose
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independent top-level definitions.
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## Current limitations
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* CPP support is experimental. CPP is virtually impossible to handle
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correctly, so we process them as a sort of unchangeable snippets. This
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works only in simple cases when CPP conditionals surround top-level
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declarations. See the [CPP][design-cpp] section in the design notes for
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a discussion of the dangers.
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* Input modules should be parsable by Haddock, which is a bit stricter
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criterion than just being valid Haskell modules.
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* Various minor idempotence issues, most of them are related to comments.
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## Running on Hackage
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It's possible to try Ormolu on arbitrary packages from Hackage. For that
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execute (from the root of the cloned repo):
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```console
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$ nix-build -A hackage.<package>
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```
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Then inspect `result/log.txt` for possible problems. The derivation will
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also contain formatted `.hs` files for inspection and original inputs with
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`.hs-original` extension (those are with CPP dropped, exactly what is fed
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into Ormolu).
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## Editor integration
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We know of the following editor integrations:
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* [Emacs][emacs-package]
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* [VS Code][vs-code-plugin]
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* vim: [neoformat][neoformat], [vim-ormolu][vim-ormolu]
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## Arch Linux
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To install Ormolu on Arch Linux, one can use [the package on AUR][aur]:
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```console
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yay -S ormolu
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```
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## Contributing
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See [CONTRIBUTING.md][contributing].
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## License
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See [LICENSE.md][license].
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Copyright © 2018–present Tweag I/O
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[design-cpp]: https://github.com/tweag/ormolu/blob/master/DESIGN.md#cpp
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[contributing]: https://github.com/tweag/ormolu/blob/master/CONTRIBUTING.md
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[license]: https://github.com/tweag/ormolu/blob/master/LICENSE.md
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[haskell-src-exts]: https://hackage.haskell.org/package/haskell-src-exts
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[emacs-package]: https://github.com/vyorkin/ormolu.el
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[vs-code-plugin]: https://marketplace.visualstudio.com/items?itemName=sjurmillidahl.ormolu-vscode
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[vim-ormolu]: https://github.com/sdiehl/vim-ormolu
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[neoformat]: https://github.com/sbdchd/neoformat
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[aur]: https://aur.archlinux.org/packages/ormolu
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