# brittany
haskell source code formatter

![Output sample](https://github.com/lspitzner/brittany/raw/master/brittany-sample.gif)

(see another demonstration in [Showcase.md](Showcase.md))

This project's goals roughly are to:

- Be idempotent (this also directly ensures that only valid haskell is
  produced);
- Support the full ghc-haskell syntax including syntactic extensions;
- Retain newlines and comments unmodified;
- Be clever about using the available horizontal space while not overflowing
  it if it cannot be avoided;
- Be clever about aligning things horizontally.
- Have linear complexity in the size of the input.

At this point, these goals are completed to different degrees.
Currently, only type-signatures and function/value bindings are processed;
other module elements (data-decls, classes, instances, imports/exports etc.) are not
transformed in any way; this extends to e.g. bindings inside class instance
definitions - they won't be touched (yet).
By using ghc-exactprint as the parser, brittany supports full ghc syntax including
extensions, but many of the less common syntactic elements (even of 2010 haskell) are
not handled.
There are cases where comments are not re-inserted properly (leading to an error or
to affected comments being placed at a different location than in the input).

The current algorithm is rather clever about horizontal space while still being
linear in the size of the input (although the constant factor is not small).

# Important notes

- Requires `ghc`>=8
- `-XCPP` is not officially supported (yet).
- ~~Some commandline flags mentioned in the help don't work yet (and won't even
  be parsed correctly.)~~ (fixed in 0.4.0.0)
- some config values can not be configured via commandline yet.
- ~~Creates a `brittany.yaml` config file _in the current directory_.~~
  (fixed in 0.5.0.0; now creates user config file in `~/.brittany`;
  still reads `brittany.yaml` in current dir if present.)
- ~~Currently some unhandled syntactical constructs don't raise errors~~
  (fixed in 0.6.0.0)
- There are cases where comments are not copied to the output (this will
  be detected and the user will get an error); there are other cases where
  comments are moved slightly; there are also cases where comments result in
  wonky newline insertion (although this should be a purely aesthetic issue.)

# Building

(This does not cover _installation_. TODO)

via `cabal new-build`

~~~~.sh
mkdir brittany-project
cd brittany-project/
git clone https://github.com/lspitzner/butcher.git
git clone https://github.com/lspitzner/data-tree-print.git
git clone https://github.com/lspitzner/ghc-exactprint.git
git clone https://github.com/lspitzner/brittany.git
cat > cabal.project <<EOF
packages: data-tree-print butcher ghc-exactprint brittany
EOF
# cabal new-configure -w $PATH_TO_GHC_8
cabal new-build brittany/
~~~~

or via `stack`

~~~~.sh
git clone https://github.com/lspitzner/brittany.git
cd brittany
stack build
~~~~


# Usage

- Currently one mode of operation: Transform a single module. By default read
  from `stdin` and written to `stdout`, but commandline arguments allow to
  read/write from/to files.
- For stdin/stdout usage it makes sense to enable certain syntactic extensions
  by default, i.e. to add something like this to your
  `~/.brittany/config.yaml` (execute `brittany` once to create default):

  ~~~~
  conf_forward:
    options_ghc:
    - -XLambdaCase
    - -XMultiWayIf
    - -XGADTs
    - -XPatternGuards
    - -XViewPatterns
    - -XRecursiveDo
    - -XTupleSections
    - -XExplicitForAll
    - -XImplicitParams
    - -XQuasiQuotes
    - -XTemplateHaskell
    - -XBangPatterns
  ~~~~

# Implementation

I have started adding documentation about the main data type `BriDoc`; see the
"docs/implementation" folder. Start with "bridoc-design.md".