2018-11-25 16:34:28 +03:00
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# Ormolu
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2018-11-25 17:09:03 +03:00
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[](https://circleci.com/gh/tweag/ormolu/tree/master)
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2019-05-10 17:36:51 +03:00
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Ormolu is a formatter for Haskell source code. The projects was created with
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the following features 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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* 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
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very much about printing AST in a particular way.
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* Implementing one “true” formatting style which admits no configuration.
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2019-05-10 19:53:16 +03:00
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* Let the layout of the input influence the layout choices in the output.
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This means that the choices between single-line/multi-line layouts in each
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particular situation are made by the user, not by an algorithm. This makes
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the implementation simpler and leaves some control to the user while still
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guaranteeing that the formatted code is stylistically consistent.
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* Idempotency: 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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* The project is commercially backed by a company, that is, Tweag. This
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guarantees that it'll be actively maintained and bugs will be fixed.
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## Contribution
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Issues (bugs, feature requests or otherwise feedback) may be reported in
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[the GitHub issue tracker for this
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project](https://github.com/tweag/ormolu/issues). Pull requests are also
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welcome.
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### What to hack on?
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Right now there are two options for people who want to contribute:
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* Implementing rendering of AST. This is the main focus right now because we
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want to have a MVP which can render all syntactical constructions found in
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Haskell source code. Once that is achieved we'll polish the tool
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iteratively.
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* [Fixing bugs][bugs]. If this seems more interesting than implementing
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rendering of AST, you are welcome to do this as well.
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### Implementing rendering of AST
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The `Ormolu.Printer.Combinators` module provides a DSL for rendering of GHC
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AST. You'll probably only need this one module for writing new rendering
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functions. The module documents how to use the printing combinators it
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provides. Consult the Haddocks to learn more about them.
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2019-05-10 17:36:51 +03:00
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Create new modules corresponding to the things you want to render under
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`Ormolu.Printer.Meat`. For example, there are `Ormolu.Printer.Meat.Type` and
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`Ormolu.Printer.Meat.Declaration.Data`.
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2019-05-10 19:53:16 +03:00
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Concrete rendering functions get their names by appending `p_` to the name
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of the type which describes a particular part of AST. For example, the
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function for rendering types is correspondingly:
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```haskell
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p_hsType :: HsType GhcPs -> R ()
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```
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In general rendering functions like this take 1 or more arguments and
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produce `R ()` which is a rendering action.
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### Testing
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Testing has been taken good care of and now it amounts to just adding
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examples under `data/examples`. Each example is a pair of files:
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`<example-name>.hs` for input and `<example-name>-out.hs` for corresponding
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expected output.
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Testing is performed as following:
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* Given snippet of source code is parsed and pretty-printed.
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* The result of printing is parsed back again and the AST is compared to the
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AST obtained from the original file. They should match.
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* The output of printer is checked against the expected output.
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* Idempotency property is verified: formatting already formatted code
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results in exactly the same output.
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Examples can be organized in sub-directories, see the existing ones for
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inspiration.
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2018-11-25 16:34:28 +03:00
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## License
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2019-03-21 17:43:58 +03:00
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Copyright © 2018–2019 Tweag I/O
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2019-05-10 17:36:51 +03:00
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[haskell-src-exts]: https://hackage.haskell.org/package/haskell-src-exts
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[hindent]: https://hackage.haskell.org/package/hindent
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[bugs]: https://github.com/tweag/ormolu/issues?q=is%3Aissue+is%3Aopen+label%3Abug
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