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356 lines
12 KiB
Markdown
356 lines
12 KiB
Markdown
# Ormolu
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[](http://opensource.org/licenses/BSD-3-Clause)
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[](https://hackage.haskell.org/package/ormolu)
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[](http://stackage.org/nightly/package/ormolu)
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[](http://stackage.org/lts/package/ormolu)
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[](https://github.com/tweag/ormolu/actions/workflows/ci.yml)
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* [Installation](#installation)
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* [Building from source](#building-from-source)
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* [Usage](#usage)
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* [Ormolu Live](#ormolu-live)
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* [Editor integration](#editor-integration)
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* [Haskell Language Server](#haskell-language-server)
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* [GitHub actions](#github-actions)
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* [Language extensions, dependencies, and fixities](#language-extensions-dependencies-and-fixities)
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* [Magic comments](#magic-comments)
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* [Regions](#regions)
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* [Exit codes](#exit-codes)
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* [Using as a library](#using-as-a-library)
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* [Troubleshooting](#troubleshooting)
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* [Operators are being formatted weirdly!](#operators-are-being-formatted-weirdly)
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* [Limitations](#limitations)
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* [Running on Hackage](#running-on-hackage)
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* [Forks and modifications](#forks-and-modifications)
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* [Contributing](#contributing)
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* [License](#license)
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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 certain situations are made by the user,
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not by an algorithm. This makes the implementation simpler and leaves some
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control to the user while still guaranteeing that the formatted code is
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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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* The formatting style aims to result in minimal diffs.
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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 so that the formatter can be used in large
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projects.
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Try it out in your browser at <https://ormolu-live.tweag.io>!
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See [Ormolu Live](#ormolu-live) for more info.
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## Installation
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The [release page][releases] has binaries for Linux, macOS and Windows.
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You can also install using `cabal` or `stack`:
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```console
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$ cabal install ormolu
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$ stack install ormolu
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```
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Ormolu is also included in several package repositories. E.g., on Arch Linux,
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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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## Building from source
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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
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```
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Make sure to accept the offered Nix caches (in particular the IOG cache),
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otherwise building may take a very long time.
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Alternatively, `stack` could be used as follows:
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```console
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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 flakes, this snippet may come in handy:
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```nix
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{
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inputs.ormolu.url = "github:tweag/ormolu";
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outputs = { ormolu, ... }: {
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# use ormolu.packages.${system}.default here
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};
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}
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```
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## Usage
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The following will print the formatted output to the standard output.
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```console
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$ 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 $(find . -name '*.hs')
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```
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Or find all files in a project with `git ls-files`:
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```console
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$ ormolu --mode inplace $(git ls-files '*.hs')
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```
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To check if files are are already formatted (useful on CI):
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```console
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$ ormolu --mode check $(find . -name '*.hs')
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```
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#### :zap: Beware git's `core.autocrlf` on Windows :zap:
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Ormolu's output always uses LF line endings. In particular,
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`ormolu --mode check` will fail if its input is correctly formatted
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*except* that it has CRLF line endings. This situation can happen on Windows
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when checking out a git repository without having set [`core.autocrlf`](
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https://www.git-scm.com/docs/git-config#Documentation/git-config.txt-coreautocrlf)
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to `false`.
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### Ormolu Live
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On every new commit to `master`, [Ormolu Live](./ormolu-live) is deployed to
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https://ormolu-live.tweag.io. Older versions are available at
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https://COMMITHASH--ormolu-live.netlify.app.
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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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### Haskell Language Server
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[Haskell Language Server](https://haskell-language-server.readthedocs.io)
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has built-in support for using Ormolu as a formatter.
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### GitHub actions
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[`run-ormolu`][run-ormolu] is the recommended way to ensure that a project
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is formatted with Ormolu.
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### Language extensions, dependencies, and fixities
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Ormolu automatically locates the Cabal file that corresponds to a given
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source code file. Cabal files are used to extract both default extensions
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and dependencies. Default extensions directly affect behavior of the GHC
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parser, while dependencies are used to figure out fixities of operators that
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appear in the source code. Fixities can also be overridden via an `.ormolu`
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file which should be located at a higher level in the file system hierarchy
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than the source file that is being formatted. When the input comes from
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stdin, one can pass `--stdin-input-file` which will give Ormolu the location
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that should be used as the starting point for searching for `.cabal` and
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`.ormolu` files.
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Here is an example of `.ormolu` file:
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```haskell
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infixr 9 .
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infixr 5 ++
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infixl 4 <$
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infixl 1 >>, >>=
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infixr 1 =<<
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infixr 0 $, $!
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infixl 4 <*>, <*, *>, <**>
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```
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It uses exactly the same syntax as usual Haskell fixity declarations to make
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it easier for Haskellers to edit and maintain.
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As of Ormolu 0.7.0.0, `.ormolu` files can also contain instructions about
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module re-exports that Ormolu should be aware of. This might be desirable
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because at the moment Ormolu cannot know about all possible module
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re-exports in the ecosystem and only few of them are actually important when
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it comes to fixity deduction. In 99% of cases the user won't have to do
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anything, especially since most common re-exports are already programmed
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into Ormolu. (You are welcome to open PRs to make Ormolu aware of more
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re-exports by default.) However, when the fixity of an operator is not
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inferred correctly, making Ormolu aware of a re-export may come in handy.
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Here is an example:
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```haskell
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module Control.Lens exports Control.Lens.At
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module Control.Lens exports "lens" Control.Lens.Lens
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```
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Module re-export declarations can be mixed freely with fixity overrides, as
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long as each declaration is on its own line. As of Ormolu 0.7.1.0 explicit
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package names are allowed in re-export declarations (see the example above).
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Finally, all of the above-mentioned parameters can be controlled from the
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command line:
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* Language extensions can be specified with the `-o` or `--ghc-opt` flag.
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* Dependencies can be specified with the `-p` or `--package` flag.
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* Fixities can be specified with the `-f` or `--fixity` flag.
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* Re-exports can be specified with the `-r` or `--reexport` flag.
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Searching for `.cabal` and `.ormolu` files can be disabled by passing
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`--no-cabal` and `--no-dot-ormolu` respectively.
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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 for Ormolu to work the
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fragments where Ormolu is enabled must be parseable on their own. Because of
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that the magic comments cannot be placed arbitrarily, but rather must
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enclose independent top-level definitions.
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### Regions
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One can ask Ormolu to format a region of input and leave the rest
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unformatted. This is accomplished by passing the `--start-line` and
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`--end-line` command line options. `--start-line` defaults to the beginning
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of the file, while `--end-line` defaults to the end.
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Note that the selected region needs to be parseable Haskell code on its own.
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### Exit codes
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Exit code | Meaning
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----------|-----------------------------------------------
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0 | Success
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1 | General problem
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2 | CPP used (deprecated)
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3 | Parsing of original input failed
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4 | Parsing of formatted code failed
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5 | AST of original and formatted code differs
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6 | Formatting is not idempotent
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7 | Unrecognized GHC options
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8 | Cabal file parsing failed
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9 | Missing input file path when using stdin input and accounting for .cabal files
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10 | Parse error while parsing fixity overrides
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100 | In checking mode: unformatted files
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101 | Inplace mode does not work with stdin
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102 | Other issue (with multiple input files)
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### Using as a library
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The `ormolu` package can also be depended upon from other Haskell programs.
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For these purposes only the top `Ormolu` module should be considered stable.
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It follows [PVP](https://pvp.haskell.org/) starting from the version
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0.5.3.0. Rely on other modules at your own risk.
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## Troubleshooting
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### Operators are being formatted weirdly!
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This can happen when Ormolu doesn't know or can't determine the fixity of an
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operator.
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* If this is a custom operator, see the instructions in the [Language
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extensions, dependencies, and
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fixities](#language-extensions-dependencies-and-fixities) section to
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specify the correct fixities in a `.ormolu` file.
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* If this is a third-party operator (e.g. from `base` or some other package
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from Hackage), Ormolu probably doesn't recognize that the operator is the
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same as the third-party one.
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Some reasons this might be the case:
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* You might have a custom Prelude that re-exports things from Prelude
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* You might have `-XNoImplicitPrelude` turned on
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If any of these are true, make sure to specify the reexports correctly in
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a `.ormolu` file.
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You can see how Ormolu decides the fixity of operators if you use `--debug`.
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## 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 a
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discussion of the dangers.
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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 .#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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## Forks and modifications
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We know of the following actively maintained forks:
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* [Fourmolu][fourmolu], which uses 4-space indentation and allows arbitrary
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configuration.
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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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[aur]: https://aur.archlinux.org/packages/ormolu
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[design-cpp]: https://github.com/tweag/ormolu/blob/master/DESIGN.md#cpp
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[emacs-package]: https://github.com/vyorkin/ormolu.el
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[haskell-src-exts]: https://hackage.haskell.org/package/haskell-src-exts
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[neoformat]: https://github.com/sbdchd/neoformat
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[releases]: https://github.com/tweag/ormolu/releases
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[run-ormolu]: https://github.com/haskell-actions/run-ormolu
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[vim-ormolu]: https://github.com/sdiehl/vim-ormolu
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[vs-code-plugin]: https://marketplace.visualstudio.com/items?itemName=sjurmillidahl.ormolu-vscode
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[fourmolu]: https://github.com/fourmolu/fourmolu
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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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