# Gild
[](https://github.com/tfausak/cabal-gild/actions/workflows/workflow.yaml)
[](https://hackage.haskell.org/package/cabal-gild)
[](https://www.stackage.org/package/cabal-gild)
Gild is an opinionated command line utility that formats Haskell package
descriptions, which are also known as `*.cabal` files. Gild can also be used to
automatically discover `exposed-modules`; see [the pragmas section][] for more
about that.
[the pragmas section]: #pragmas
To run Gild in CI, consider using [cabal-gild-setup-action][].
[cabal-gild-setup-action]: https://github.com/tfausak/cabal-gild-setup-action
Gild started as a fork of [phadej/cabal-fmt][], but is now totally separate.
For a brief summary of the differences between Gild and `cabal-fmt`, read [the
announcement post][].
[phadej/cabal-fmt]: https://github.com/phadej/cabal-fmt
[the announcement post]: https://taylor.fausak.me/2024/02/17/gild/
## Summary
Given a package description like this:
``` cabal
CABAL-VERSION : 3.0
name : example
version : 0.0.0.0
library
build-depends: mtl>=2.3, base
ghc-options:-Wall
if impl(ghc>=9.8)
ghc-options: -Wmissing-role-annotations
```
Gild will produce output like this:
``` cabal
cabal-version: 3.0
name: example
version: 0.0.0.0
library
build-depends:
base,
mtl >=2.3,
ghc-options: -Wall
if impl(ghc >= 9.8)
ghc-options: -Wmissing-role-annotations
```
See [the installation section][] for how to get Gild and [the usage section][]
for how to use it.
[the installation section]: #installation
[the usage section]: #usage
## Goals
- There should be no configuration options. As long as the output format is
reasonable, the specifics should not matter too much. This means the amount
of indentation cannot be changed, for example.
- The output should be diff friendly. This means things generally go on their
own line, trailing commas are used when possible, and elements are sorted
where it makes sense.
- The output should be semantically the same as the input. This means no
normalization or canonicalization. For example, separate `build-depends`
fields are not merged together.
- It should be possible to format other files that use the same format as
package descriptions. This means `cabal.project` files can be formatted as
well.
- The focus should be mostly on formatting. There should be no effort made to
validate input or provide any checks or lints. However some quality of life
features, like automatic module discovery, are desireable.
- The command line utility should be fast enough to run on every save. It
should not need network access.
## Installation
Go to [the latest release page][] and download the binary for your platform.
[the latest release page]: https://github.com/tfausak/cabal-gild/releases/latest
### From Source
In general you should prefer downloading the appropriate binary for you
platform. However it is possible to build Gild from source. It supports Linux,
macOS, and Windows along with the three most recent versions of GHC. Any other
configurations are unsupported.
With Cabal:
``` sh
$ cabal install cabal-gild
```
With Stack:
``` sh
$ stack install cabal-gild
```
## Usage
Gild is a command line utility named `cabal-gild`. By default it reads from
standard input (STDIN) and writes to standard output (STDOUT). Its behavior can
be modified with command line options, which are described below.
### Options
Run `cabal-gild --help` to see the options that Gild supports. They are:
- `--help`: Prints the help message to STDOUT then exits successfully.
- `--version`: Prints the version number to STDOUT then exits successfully.
- `--input=FILE`: Uses `FILE` as the input. If this is `-` (which is the
default), then the input will be read from STDIN.
- `--mode=MODE`: Sets the mode to `MODE`, which must be either `format` (the
default) or `check`. When the mode is `format`, Gild will output the
formatted package description. When the mode is `check`, Gild will exit
successfully if the input is already formatted, otherwise it will exit
unsuccessfully.
- `--output=FILE`: Uses `FILE` as the output. If this is `-` (which is the
default), then the output will be written to STDOUT. To modify a file in
place, use the same file as both input and output. For example:
``` sh
$ cabal-gild --input p.cabal --output p.cabal
```
This option is ignored when the mode is `check`.
- `--stdin=FILE`: When reading input from STDIN, use `FILE` as the effective
input file. This is useful when a file's contents are already available, like
in an editor. For example:
``` sh
$ cabal-gild --stdin p.cabal < p.cabal
```
This option is ignored when the input is not `-`.
### Pragmas
Gild supports special comments in package descriptions that act as pragmas.
Each pragma starts with `-- cabal-gild:`. Pragmas must be the last comment
before a field.
- `-- cabal-gild: discover DIRECTORY [DIRECTORY ...]`: This pragma will
discover any Haskell files in any of the given directories and use those to
populate the list of modules or signatures. For example, given this input:
``` cabal
library
-- cabal-gild: discover source/library
exposed-modules: ...
```
Assuming there is a single Haskell file at `source/library/M.hs`, Gild will
produce this output:
``` cabal
library
-- cabal-gild: discover source/library
exposed-modules: M
```
This pragma only works with the `exposed-modules`, `other-modules`, and
`signatures` fields. It will be ignored on all other fields.
Any existing modules or signatures in the list will be ignored. The entire
field (including comments) will be replaced. This means adding, removing, and
renaming modules or signatures should be handled automatically.
This pragma searches for files with any of the following extensions: `*.chs`,
`*.cpphs`, `*.gc`, `*.hs`, `*.hsc`, `*.hsig`, `*.lhs`, `*.lhsig`, `*.ly`,
`*.x`, or `*.y`,