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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)
Ormolu is a formatter for Haskell source code.
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].
* Writing code is such a way so it's easy to modify and maintain. Roughly,
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.
* Let layout of input source code control layout choices in output. This
means that the choices between single-line/multi-line layouts in each
particular situation are made by user, not by an algorithm. This makes the
implementation simpler and leaves just enough control to the user while
still guaranteeing that the formatted code is stylistically consistent.
* 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.
* The project is commercially backed by a company, that is, Tweag. This
guarantees that it'll be actively maintained and bugs will be fixed.
## Contribution
Issues (bugs, feature requests or otherwise feedback) may be reported in
[the GitHub issue tracker for this
project](https://github.com/tweag/ormolu/issues). Pull requests are also
welcome.
### What to hack on?
Right now there are two options for people who want to contribute:
* Implementing rendering of AST. This is the main focus right now because we
want to have a MVP which can render all syntactical constructions found in
Haskell source code. Once that is achieved we'll polish the tool
iteratively.
* [Fixing bugs][bugs]. If this seems more interesting than implementing
rendering of AST, you are welcome to do this as well.
### Implementing rendering of AST
The `Ormolu.Printer.Combinators` module provides a DSL for rendering of GHC
AST. You'll probably only need this one module for writing new rendering
functions. The module documents how to use the printing combinators it
provides, consult the Haddocks to learn more about that.
Create new modules corresponding to the things you want to render under
`Ormolu.Printer.Meat`. For example, there are `Ormolu.Printer.Meat.Type` and
`Ormolu.Printer.Meat.Declaration.Data`.
Concrete rendering functions get their names by appending `p_` to record
field name which contains the part of AST of interest. For example, types
are often found in fields named `hsType`, so the function for rendering
types is correspondingly:
```haskell
p_hsType :: HsType GhcPs -> R ()
```
In general rendering functions like this take 1 or more arguments and
produce `R ()` which is a rendering action.
### Testing
Testing has been taken good care of and now it amounts to just adding
examples under `data/examples`. Each example is a pair of files:
`<example-name>.hs` for input and `<example-name>-out.hs` for corresponding
expected output.
Testing is performed as following:
* Given snippet of source code is parsed and pretty-printed.
* The result of printing is parsed back again and the AST is compared to the
AST obtained from the original file. They should match.
* The output of printer is checked against the expected output.
* Idempotency property is verified: formatting already formatted code
results in exactly the same output.
Examples can be organized in sub-directories, see the existing ones for
inspiration.
## License
Copyright © 2018–2019 Tweag I/O
[haskell-src-exts]: https://hackage.haskell.org/package/haskell-src-exts
[hindent]: https://hackage.haskell.org/package/hindent
[bugs]: https://github.com/tweag/ormolu/issues?q=is%3Aissue+is%3Aopen+label%3Abug