dhall-haskell/dhall/README.md

dhall

For installation or development instructions, see:

• dhall-haskell - README

Full documentation here:

• dhall instructions

Introduction

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

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

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.

Example

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 FromDhall 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.

Standard-compatibility table

Haskell package version	Supported standard version
1.20.*	5.0.0
1.19.*	4.0.0
1.18.*	3.0.0
1.17.*	2.0.0
1.16.*	1.0.0