Hpack is a format for Haskell packages. It is a modern alternative to the Cabal package format and follows different design principles.

The guiding design principles for Hpack are:

• Don't require the user to state the obvious, make sensible assumptions by default
• Give the user 100% control when needed
• Don't require the user to repeat things, facilitate DRYness

Hpack packages are described in a file named package.yaml. Both cabal2nix and stack support package.yaml natively. For other build tools the hpack executable can be used to generate a .cabal file from package.yaml.

There is reference documentation below, but introductory documentation is still lacking. For the time being, take a look at the slides from my talk about Hpack at the Singapore Haskell meetup: http://typeful.net/talks/hpack

• Given this package.yaml running hpack will generate hpack.cabal
• Given this package.yaml running hpack will generate getopt-generics.cabal
• Given this package.yaml running hpack will generate sensei.cabal
• Given this package.yaml running hpack will generate base-orphans.cabal

Hpack does not require you to specify a cabal-version manually. When generating a .cabal file, Hpack sets the cabal-version automatically based on the features that are used.

If you want to override this behavior you can use verbatim to set cabal-version manually, e.g.:

verbatim:
  cabal-version: 2.2

Hpack allows the inclusion of common fields from a file on GitHub or a local file.

To use this feature a user must specify a GitHub repository, Git reference and a path to a file within that repository; alternatively, a path to the local file must be given.

Example:

defaults:
  github: sol/hpack-template
  ref: 2017
  path: defaults.yaml

This will include all common fields from https://github.com/sol/hpack-template/blob/2017/defaults.yaml into the package specification.

Exactly one of github and local must be given in a defaults section.

Hpack supports shorthand syntax for specifying github and ref as a string:

defaults: sol/hpack-template@2017

This is equivalent to:

defaults:
  github: sol/hpack-template
  ref: 2017

Note: Hpack caches downloaded files under ~/.hpack/defaults/<owner>/<repo>/<path>. Once downloaded, a file is reused from the cache. If the content on GitHub changes the file is not updated. For this reason it is recommended to only use tags as Git references.

• If a defaults file has changed on GitHub and you want to use the latest version, then you have to delete that file from the cache manually.

• If you want to prevent Hpack from accessing the network to download a defaults file, then you can achieve this by adding that file to the cache manually.

These fields can be specified top-level or on a per section basis; top-level values are merged with per section values.

build-tools: A set of Haskell executables that are needed to build this component

Each element consists of a name and an optional version constraint.

The name can be specified in two ways:

1. Qualified: <package>:<executable>
2. Unqualified: <executable>

A qualified name refers to an executable named <executable> from a package named <package>.

An unqualified name either refers to an executables in the same package, or if no such executable exists it is desugared to <executable>:<executable>.

build-tools can be specified as a list or a mapping.

Examples:

build-tools:
  - alex
  - happy:happy
  - hspec-discover == 2.*

build-tools:
  alex: 3.2.*
  happy:happy: 1.19.*
  hspec-discover: 2.*

When generating a .cabal file each element of build-tools is either added to build-tools or build-tool-depends.

If the name refers to one of alex, c2hs, cpphs, greencard, haddock, happy, hsc2hs or hscolour then the element is added to build-tools, otherwise it is added to build-tool-depends.

This is done to allow compatibility with a wider range of Cabal versions.

Note: Unlike Cabal, Hpack does not accept system executables as build-tools. Use system-build-tools if you need this.

Dependencies can be specified as either a list or an object. These are equivalent:

  dependencies:
    - base >= 4.10.1.0
    - containers >= 5.10

  dependencies:
    base: ">= 4.10.1.0"
    containers: ">= 5.10"

The individual dependencies can also be specified as an object:

  dependencies:
    - name: base
      version: ">= 4.10.1.0"
    - name: containers

You can use objects at both levels, or have a mix of valid ways to specify the individual dependencies:

  dependencies:
    base:
      version: ">= 4.10.1.0"
    # If you don't give a version, it defaults to 'any version'.
    containers: {}
    transformers: ">= 0.5.5.0 && < 5.6"

Individual dependencies as objects are only supported from version 0.31.0.

When a dependency is specified as an object, you can use the mixin field to control what modules from the dependency your program will see and how its signatures are filled in:

  dependencies:
    # This gives you a shorter name to import from, and hides the other modules.
    - name: containers
      mixin:
        - (Data.Map.Lazy as Map)
    # This hides the System.IO.Unsafe module, and leaves the other modules unchanged.
    - name: base
      mixin:
        - hiding (System.IO.Unsafe)
    # This exposes only the listed modules - you won't be able to import the others!
    - name: lens
      mixin:
        - (Control.Lens, Data.Set.Lens, Data.Map.Lens as MapL)
    # This will rename the module, and expose the others.
    - name: transformers
      mixin:
        - hiding (Control.Monad.Trans.State.Lazy)
        - (Control.Monad.Trans.State.Lazy as State)

For more information, see the Cabal documentation.

Hint: you can hide the Prelude module from base, and then rename an alternative prelude to Prelude so that it doesn't need to be imported!

mixin was added in version 0.31.0.

Conditionals with no else branch:

• Must have a condition field
• May have any number of other fields

For example,

when:
  - condition: os(darwin)
    extra-lib-dirs: lib/darwin

becomes

if os(darwin)
  extra-lib-dirs:
    lib/darwin

Conditionals with an else branch:

• Must have a condition field
• Must have a then field, itself an object containing any number of other fields
• Must have a else field, itself an object containing any number of other fields
• All other top-level fields are ignored

For example,

when:
  - condition: flag(fast)
    then:
      ghc-options: -O2
    else:
      ghc-options: -O0

becomes

if flag(fast)
  ghc-options: -O2
else
  ghc-options: -O0

At place where you can specify a list of files you can also use glob patterns. Glob patters and ordinary file names can be freely mixed, e.g.:

extra-source-files:
  - static/*.js
  - static/site.css

Glob patterns are expanded according to the following rules:

• ? and * are expanded according to POSIX (they match arbitrary characters, except for directory separators)
• ** is expanded in a zsh-like fashion (matching across directory separators)
• ?, * and ** do not match a . at the beginning of a file/directory

(since hpack-0.24.0)

In cases where Hpack does not (yet!) support what you want to do, you can use the verbatim field to pass things to Cabal verbatim. It is recognized top-level, in sections, and in conditionals.

verbatim accepts an object or a string (or a list of objects and strings).

Disclaimer: The content of verbatim fields are merged into the generated .cabal file as a final step, after Hpack is done with most of its work. Before that final step Hpack does not look at any verbatim fields. Consequently, the content of a verbatim field does not affect any other fields that are populated by Hpack. As an example, if you use verbatim to override hs-source-dirs, the overridden information will not be used when Hpack infers exposed-modules or other-modules.

When an object is used:

• field values can be strings, numbers, booleans, or null
• existing .cabal fields can be overridden
• existing .cabal fields can be removed by overriding with null
• additional .cabal fields can be added

Example:

tests:
  spec:
    main: Spec.hs
    source-dirs: test
    verbatim:
      type: detailed-0.9     # change type from exitcode-stdio-1.0
      default-language: null # remove default-language

When a string is used:

• it will be added verbatim, indented to match the indentation of the surrounding context.
• all existing .cabal fields are left untouched

Example:

verbatim: |
  build-tool-depends:
      hspec-discover:hspec-discover == 2.*

You can combine the use of objects and strings to gain more fine-grained control, e.g. you can remove an existing field with an object and then include it with a string so that you have 100% control over the layout.

verbatim:
  - build-depends: null
  - |
    -- let's use Cabal 5.0 dependency syntax
    build-depends:
      hspec: [2-3[

It is possible to use YAML anchors (&), aliases (*) and merge keys (<<) to define fields and reference them later.

executables:
  my-exe-1: &my-exe
    main: my-exe-1.hs
    dependencies: [base, my-lib]
    ghc-options: [-threaded]
  my-exe-2:
    <<: *my-exe
    main: my-exe-2.hs

Fields that start with an underscore are ignored by hpack, so they can be used to declare aliases:

_exe-ghc-options: &exe-ghc-options
  - -threaded
  - -rtsopts

executables:
  my-exe-1:
    ghc-options: *exe-ghc-options

It is also possible to use the !include directive:

# ...

tests:
  hlint: !include "../common/hlint.yaml"

hlint.yaml:

source-dirs: test
main: hlint.hs
dependencies: [base, hlint]

This can also be used to provide entire libraries of snippets:

_common/lib: !include "../common/lib.yaml"

name: example1
version: '0.1.0.0'
synopsis: Example
<<: *legal

<<: *defaults

library:
  source-dirs: src

tests:
  hlint: *test_hlint

lib.yaml:

- &legal
  maintainer: Some One <someone@example.com>
  copyright: (c) 2017 Some One
  license: BSD3

- &defaults
  dependencies:
    - base
    - containers
  ghc-options:
    - -Wall
    - -Werror

- &test_hlint
  source-dirs: test
  main: hlint.hs
  dependencies: [hlint]

To run hpack automatically on modifications to package.yaml add the following to your ~/.vimrc:

autocmd BufWritePost package.yaml call Hpack()

function Hpack()
  let err = system('hpack ' . expand('%'))
  if v:shell_error
    echo err
  endif
endfunction

Stack has built-in support for Hpack. If you are using Stack you can use package.yaml instead of a .cabal file. No additional steps are required.

You can get binaries for use on Travis CI with:

curl -sSL https://github.com/sol/hpack/raw/master/get-hpack.sh | bash

(both Linux and OS X are supported)