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ormolu/README.md

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# Ormolu
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[![CircleCI](https://circleci.com/gh/tweag/ormolu/tree/master.svg?style=svg&circle-token=cfd37a39265561eb44e608f97cf953cb2a394c03)](https://circleci.com/gh/tweag/ormolu/tree/master)
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Ormolu is a formatter for Haskell source code. The projects was created with
the following features in mind:
* Using GHC's own parser to avoid parsing problems caused by
[`haskell-src-exts`][haskell-src-exts].
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* Let some whitespace be programmable. The layout of the input
influence 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.
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* Writing code in such a way so it's easy to modify and maintain. Roughly,
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it means that the project follows the path of [`hindent`][hindent] and is
very much about printing AST in a particular way.
* 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.
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* Idempotency: 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
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The easiest way to build the project is with Nix:
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```console
$ nix-build
```
Or with `cabal-install` from the Nix shell:
```console
$ nix-shell --run "cabal new-build"
```
Alternatively, `stack` could be used with a `stack.yaml` file as follows.
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```console
$ cat stack.yaml
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resolver: lts-13.19
packages:
- '.'
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$ stack build
```
## Usage
The following will print the formatted output to the standard output.
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```console
$ ormolu Module.hs
```
Add `--mode inplace` to replace the contents of the input file with the
formatted output.
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```console
$ ormolu --mode inplace Module.hs
```
A configuration file can specify the language extensions to use by default.
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```console
$ cat ormolu.yaml
ghc-opts:
- -XLambdaCase
unsafe: False
debug: False
$ ormolu -c ormolu.yaml Module.hs
```
## Contributing
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See [CONTRIBUTING.md](./CONTRIBUTING.md).
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## License
See [LICENSE.md](./LICENSE.md).
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Copyright © 20182019 Tweag I/O
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[haskell-src-exts]: https://hackage.haskell.org/package/haskell-src-exts
[hindent]: https://hackage.haskell.org/package/hindent