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How to integrate Dhall

Gabriel Gonzalez edited this page Aug 20, 2019 · 8 revisions

You can integrate the Dhall configuration language into your project in one of the following three ways:

  • Language support for reading Dhall configuration files (this is the only way to get complete support for Dhall, including invoking Dhall functions from your application)
  • An external dhall-to-${FORMAT} executable that transforms Dhall into another configuration format
  • The dhall text command combined with a pure Dhall function that renders your desired file format

Language support

The following languages can read in Dhall configuration files directly using a package that natively implements Dhall:

  • Clojure - Via the dhall-clj package
  • Haskell - Via the dhall package
  • Ruby - Via the dhall-ruby package

The following languages can read in Dhall configuration files by building on top of one of the above three implementations:

  • Eta - Via the Haskell implementation
  • Java - The above Eta implementation also provides Java bindings
  • Nix - Via the Haskell implementation

The following language bindings are currently in progress:

  • Go - Via the dhall-golang package
  • Rust - Via the dhall-rust package
  • PureScript - Via the dhall-purescript package

The following integrations built on top of another implementation are still in progress:

  • C - Using the Haskell package as a shared library

External executable

You can convert Dhall to one of the following configuration formats if your language does not natively support Dhall.

  • YAML - Via the dhall-to-yaml executable

  • JSON - Via the dhall-to-json executable

  • XML - Via the dhall-to-xml executable

  • Bash - Via the dhall-to-bash executable

    Technically not a "configuration format", but many programs are configured via Bash scripts

  • Kubernetes - Via the dhall-to-yaml executable

    dhall-kubernetes is a repository containing Kubernetes-related Dhall types and functions. However, you still need to use dhall-to-yaml to generate the YAML configuration that Kubernetes expects since Kubernetes is not Dhall-aware

  • Go - Via the dhall-to-json executable

    This implementation invokes a dhall-to-json subprocess to convert a Dhall configuration file to JSON and then read the JSON configuration into Go

Render within Dhall

"Render within Dhall" is a special case of "External executable" where the external executable is the dhall text command (i.e. the text subcommand of the dhall executable). The difference is that the logic to generate a given configuration file format is implemented within Dhall and then the expression is interpreted and displayed using the dhall text command.

This can just mean templating an existing format, like markdown:

-- ./template.dhall

let Prelude = https://prelude.dhall-lang.org/package.dhall

let toItem = \(item : Text) -> "* ${item}"

let toList = Prelude.Text.concatMapSep "\n" Text toItem

let fruits = [ "apple", "orange", "banana" ]

in  ''
    # About me

    These are my favorite fruits:

    ${toList fruits}
    ''
$ dhall text --file ./template.dhall
# About me

These are my favorite fruits:

* apple
* orange
* banana

... or you can provide a more structured integration implemented in "pure Dhall". For example, the Prelude natively supports rendering well-formed JSON using the same dhall text command:

-- ./json.dhall

let JSON = https://raw.githubusercontent.com/dhall-lang/dhall-lang/master/Prelude/JSON/package.dhall

in  JSON.render (JSON.array [ JSON.number 1.0, JSON.bool True ])
$ dhall text --file './json.dhall' 
[ 1.0, true ]

"Pure Dhall" integrations include:

  • JSON - Via the Dhall Prelude

  • XML - Via the Dhall Prelude

  • NetHack - Via the dhall-nethack repository

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