hpack-dhall

Haskell package descriptions in Dhall.

This package named hpack-dhall as described in package.dhall.

{ name =
    "hpack-dhall"
...
, library =
    { exposed-modules = "Hpack.Dhall" }
, executables =
    { dhall-hpack-cabal = ...
    , dhall-hpack-json = ...
    , dhall-hpack-yaml = ...
    , dhall-hpack-dhall = ...
    }
}

This .cabal creating executable can be run over its own package description;

> stack install --stack-yaml=stack-8.4.4.yaml
...
Copied executables to /.../hpack-dhall/__bin:
- dhall-hpack-cabal
- dhall-hpack-dhall
- dhall-hpack-json
- dhall-hpack-yaml

> __bin/dhall-hpack-cabal package.dhall
hpack-dhall.cabal is up-to-date

> __bin/dhall-hpack-cabal --force package.dhall
generated hpack-dhall.cabal

Using one of the golden tests for example, there are executables to show the dhall with the imports made as well as json and yaml equivalents;

> __bin/dhall-hpack-dhall test/golden/hpack-dhall-cabal/empty-package.dhall
{ name = "empty-package" }

> __bin/dhall-hpack-json test/golden/hpack-dhall-cabal/empty-package.dhall
{
    "name": "empty-package"
}

> __bin/dhall-hpack-yaml test/golden/hpack-dhall-cabal/empty-package.dhall
name: empty-package

By going from hpack package fields to cabal package properties, we are not required to state what can be inferred or defaulted, easing the burden of completing a package description by hand. For example other-modules can be inferred by taking the set difference between modules on disk and the set of exposed-modules.

By using an hpack-like Dhall dialect here rather than the YAML of hpack we’re able to;

• Add types to the fields.
• Safely import from other *.dhall files.
• Use functions.

Imports and Functions

With this safer and more capable alternative input format for hpack, we’re able to simply describe the package and by using imports and functions we can do more such as configuring linting;

> cat default-extensions.dhall
{ default-extensions =
    [ "DataKinds"
    , "DeriveFunctor"
    ...
    , "TupleSections"
    , "UndecidableInstances"
    ]
}

> cat hlint.dhall
    let Prelude/List/map =
          https://raw.githubusercontent.com/dhall-lang/Prelude/35deff0d41f2bf86c42089c6ca16665537f54d75/List/map

in  let defs = ./default-extensions.dhall

in  let f = λ(s : Text) → "-X" ++ s

in  { arguments = Prelude/List/map Text Text f defs.default-extensions }

> dhall-to-yaml < ./hlint.dhall > ./.hlint.yaml

> cat .hlint.yaml
arguments:
- -XDataKinds
- -XDeriveFunctor
...
- -XTupleSections
- -XUndecidableInstances

We can pull those same default-extensions into a package description;

> cat package.dhall
    let defs = ./defaults.dhall

in    defs
    ⫽ ./default-extensions.dhall
    ⫽ { name =
          "flight-units"
    ...
      , github =
          "blockscope/flare-timing/units"
    ...
      , dependencies =
            defs.dependencies
          # [ "numbers"
            , "fixed"
            , "bifunctors"
            , "text"
            , "formatting"
            , "uom-plugin"
            , "siggy-chardust"
            ]
    ...
      }

Formatting

We can consistently format package.dhall and other *.dhall imports using dhall;

> stack install dhall --stack-yaml=stack-dhall.yaml
> __bin/dhall format --inplace package.dhall

Restrictions

Using hpack’s conditionals in a list in package.dhall can cause an error because lists in Dhall must have elements of the same type;

From stack’s package.yaml;

executables:
  stack:
    main: Main.hs
    source-dirs: src/main
    generated-other-modules:
    - Build_stack
    - Paths_stack
    ghc-options:
    - -threaded
    - -rtsopts
    dependencies:
    - stack
    when:
    - condition: flag(static)
      ld-options:
      - -static
      - -pthread
    - condition: ! '!(flag(disable-git-info))'
      cpp-options: -DUSE_GIT_INFO
      dependencies:
      - githash
      - optparse-simple
    - condition: flag(hide-dependency-versions)
      cpp-options: -DHIDE_DEP_VERSIONS
    - condition: flag(supported-build)
      cpp-options: -DSUPPORTED_BUILD

This can be represented in package.dhall as;

, executables =
    { stack =
        { main =
            "Main.hs"
        , source-dirs =
            [ "src/main" ]
        , generated-other-modules =
            [ "Build_stack", "Paths_stack" ]
        , ghc-options =
            [ "-threaded", "-rtsopts" ]
        , dependencies =
            [ "stack" ]
        , when =
            [ { condition =
                  "flag(static)"
              , cpp-options =
                  [] : List Text
              , dependencies =
                  [] : List Text
              , ld-options =
                  [ "-static", "-pthread" ]
              }
            , { condition =
                  "!(flag(disable-git-info))"
              , cpp-options =
                  [ "-DUSE_GIT_INFO" ]
              , dependencies =
                  [ "githash", "optparse-simple" ]
              , ld-options =
                  [] : List Text
              }
            , { condition =
                  "flag(hide-dependency-versions)"
              , cpp-options =
                  [ "-DHIDE_DEP_VERSIONS" ]
              , dependencies =
                  [] : List Text
              , ld-options =
                  [] : List Text
              }
            , { condition =
                  "flag(supported-build)"
              , cpp-options =
                  [ "-DSUPPORTED_BUILD" ]
              , dependencies =
                  [] : List Text
              , ld-options =
                  [] : List Text
              }
            ]
        }
    }

Continuous Integration

With haskell-ci tooling installed, generate the .travis.yml setup with;

> haskell-ci --output=.travis.yml --config=cabal.haskell-ci hpack-dhall.cabal
*INFO* Generating Travis-CI config for testing for GHC versions: 8.2.2 8.4.3 8.4.4 8.6.3