dhall-haskell/dhall

dhall 1.19.1

Dhall is a programmable configuration language that is not Turing-complete

You can think of Dhall as: JSON + functions + types + imports

You will probably want to read the language-agnostic README here:

• dhall-lang README

This repository (and this README) focuses on the Haskell implementation of Dhall

Motivation

"Why not configure my program using JSON or YAML?"

JSON or YAML are suitable for small configuration files, but larger configuration files with complex schemas require programming language features to reduce repetition. Otherwise, the repetitive configuration files become error-prone and difficult to maintain/migrate.

This post explains in more detail the motivation behind programmable configuration files:

• Programmable configuration files

"Why not configure my program using Haskell code?"

You probably don't want to rebuild your program every time you make a configuration change. Recompilation is slow and requires the GHC toolchain to be installed anywhere you want to make configuration changes.

Quick start

Given this Haskell program saved to example.hs:

-- example.hs

{-# LANGUAGE DeriveGeneric     #-}
{-# LANGUAGE OverloadedStrings #-}

import Dhall

data Example = Example { foo :: Integer, bar :: Vector Double }
    deriving (Generic, Show)

instance Interpret Example

main :: IO ()
main = do
    x <- input auto "./config"
    print (x :: Example)

... which reads in this configuration file:

$ cat ./config
{ foo = 1
, bar = ./bar
}

... which in turn references this other file:

$ cat ./bar
[3.0, 4.0, 5.0]

... you can interpret the Haskell program like this:

$ nix-shell ../nix/test-dhall.nix
[nix-shell]$ runghc example.hs
Example {foo = 1, bar = [3.0,4.0,5.0]}

You can also interpret Dhall programs directly using the installed command-line compiler:

$ dhall
List/head Double ./bar
<Ctrl-D>
Optional Double

Some 3.0

... and you can reference remote expressions or functions by their URL, too:

$ dhall
let null = https://raw.githubusercontent.com/dhall-lang/Prelude/35deff0d41f2bf86c42089c6ca16665537f54d75/List/null
in  null Double ./bar
<Ctrl-D>
Bool

False

Now go read the Dhall tutorial to learn more

Building this project

Nix + Cabal is the recommended workflow for project development since continuous integration uses Nix to build and test the project. Other development tools and workflows are also supported on a best-effort basis.

You can build the project using only Nix by running this command from the root of the repository:

$ nix-build

More commonly, you will want to incrementally build the project using cabal. You can either do so inside of a nix-shell:

$ nix-shell
[nix-shell]$ cabal configure
[nix-shell]$ cabal build
[nix-shell]$ cabal test

... or you can add nix: True to your ~/.cabal/config file and then you can run the same cabal commands without an explicit nix-shell:

$ cabal configure
$ cabal build
$ cabal test