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

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

Table of contents

• Getting Started
• Example Configuration
• Features
• Documentation
• Standard Library
• Development Status
• Overview
  • Interpreter
  • Language Bindings
    • Haskell
    • Nix
  • Compilers
    • JSON and YAML
    • Bash
    • Text
• Design Philosophy
• Name

Getting started

The easiest way to get started experimenting with Dhall is to install the dhall-to-json and/or dhall-to-yaml executables, which enable you to generate JSON and YAML, respectively, on the command line. Platform- and runtime-specific installation instructions can be found in the Dhall wiki.

Example Configuration

$ cat ./makeUser.dhall

-- This is a single-line comment

{- This is a
   block comment
-}

-- This file stores an anonymous function (analogous to a "template")

   -- ↓↓↓↓ The function's input is an argument named `user`
    \(user : Text)
          -- ↑↑↑↑ ... that has type `Text`

-- The remainder of this file is the function's output
-- (a.k.a. "the body of the function")
-> 

 -- ↓↓↓ Use `let` to define intermediate variables
    let homeDirectory = "/home/${user}"

                           -- ↓↓↓↓↓↓↓↓↓↓↓↓↓↓↓↓ String interpolation
    let privateKeyFile = "${homeDirectory}/id_rsa"

    let publicKeyFile = "${privateKeyFile}.pub"

        -- Records begin with `{`, end with `}`, and separate fields with `,`
    in  { homeDirectory  = homeDirectory
        , privateKeyFile = privateKeyFile
        , publicKeyFile  = publicKeyFile
        } : ./User.dhall
       -- ↑ The `:` symbol begins a type annotation (optional in this case)
       --
       -- We can store expressions (even types) in files (like `./User.dhall`)

$ cat ./User.dhall

-- This file stores a Dhall type (analogous to a "schema")

-- This is the type of a record which has three fields:
--
-- * The first field is named `homeDirectory` and has type `Text`
-- * The second field is named `privateKeyFile` and has type `Text`
-- * The third field is named `publicKeyFile` and has type `Text`
--
-- The order of fields does not matter
{ homeDirectory  : Text
, privateKeyFile : Text
, publicKeyFile  : Text
}

$ cat ./configuration.dhall

-- This is our top-level configuration file

-- We can import any Dhall expression, even a function, from another file
let makeUser = ./makeUser.dhall

-- We can import Dhall expressions from URLs, too
let generate =
        https://prelude.dhall-lang.org/List/generate
            -- ... and optionally protect them with integrity checks
            sha256:3d7c09834af3d0657f070e6bb42c0c8e7e9f9e9649f47de52aed8a7c095daa80

        -- We can provide a fallback mirror if the first import fails
      ? https://raw.githubusercontent.com/dhall-lang/Prelude/302881a17491f3c72238975a6c3e7aab603b9a96/List/generate

        -- We can fall back to anything, like a local file
      ? /usr/local/share/dhall/Prelude/List/generate

-- We can also define functions inline within the same file
let makeBuildUser =
      \(index : Natural) -> makeUser "build${Natural/show index}"

-- We can import types, too
let User = ./User.dhall

      -- Lists begin with `[`, end with `]` and separate elements with `,`
in    [ -- We can inline the configuration for any given user
        --
        -- This is useful for users with a non-standard configuration
        { homeDirectory  = "/home/jenkins"
        , privateKeyFile = "/etc/jenkins/id_rsa"
        , publicKeyFile  = "/etc/jenkins/id_rsa.pub"
        }

        -- We can use our `makeUser` function to stamp out users that follow
        -- a standard pattern
      , makeUser "john"

      , makeUser "mary"

      , makeUser "alice"

      ]

 -- ↓ This is the list concatenation operator
    # ( -- Let's add the current $USER to the list, too

                 -- ↓↓↓↓↓↓↓↓ We can import from environment variables, too
        [ makeUser (env:USER as Text) ]
                          -- ↑↑↑↑↑↑↑ Adding "as Text" imports raw Text instead
                          --         of a Dhall expression

        -- What if the `USER` environment variable is not set?
        --
        -- No problem; fall back to appending an empty list
      ? ([] : List User)
      )

      -- Let's also generate 7 users using makeBuildUser
    # generate 7 User makeBuildUser

$ # Now convert to JSON
$ dhall-to-json --pretty <<< './configuration.dhall'

[
  {
    "homeDirectory": "/home/jenkins",
    "privateKeyFile": "/etc/jenkins/id_rsa",
    "publicKeyFile": "/etc/jenkins/id_rsa.pub"
  },
  {
    "homeDirectory": "/home/john",
    "privateKeyFile": "/home/john/id_rsa",
    "publicKeyFile": "/home/john/id_rsa.pub"
  },
  {
    "homeDirectory": "/home/mary",
    "privateKeyFile": "/home/mary/id_rsa",
    "publicKeyFile": "/home/mary/id_rsa.pub"
  },
  {
    "homeDirectory": "/home/alice",
    "privateKeyFile": "/home/alice/id_rsa",
    "publicKeyFile": "/home/alice/id_rsa.pub"
  },
  {
    "homeDirectory": "/home/gabriel",
    "privateKeyFile": "/home/gabriel/id_rsa",
    "publicKeyFile": "/home/gabriel/id_rsa.pub"
  },
  {
    "homeDirectory": "/home/build0",
    "privateKeyFile": "/home/build0/id_rsa",
    "publicKeyFile": "/home/build0/id_rsa.pub"
  },
  {
    "homeDirectory": "/home/build1",
    "privateKeyFile": "/home/build1/id_rsa",
    "publicKeyFile": "/home/build1/id_rsa.pub"
  },
  {
    "homeDirectory": "/home/build2",
    "privateKeyFile": "/home/build2/id_rsa",
    "publicKeyFile": "/home/build2/id_rsa.pub"
  },
  {
    "homeDirectory": "/home/build3",
    "privateKeyFile": "/home/build3/id_rsa",
    "publicKeyFile": "/home/build3/id_rsa.pub"
  },
  {
    "homeDirectory": "/home/build4",
    "privateKeyFile": "/home/build4/id_rsa",
    "publicKeyFile": "/home/build4/id_rsa.pub"
  },
  {
    "homeDirectory": "/home/build5",
    "privateKeyFile": "/home/build5/id_rsa",
    "publicKeyFile": "/home/build5/id_rsa.pub"
  },
  {
    "homeDirectory": "/home/build6",
    "privateKeyFile": "/home/build6/id_rsa",
    "publicKeyFile": "/home/build6/id_rsa.pub"
  }
]

$ # ... or YAML
$ dhall-to-yaml <<< './configuration.dhall'

- privateKeyFile: /etc/jenkins/id_rsa
  publicKeyFile: /etc/jenkins/id_rsa.pub
  homeDirectory: /home/jenkins
- privateKeyFile: /home/john/id_rsa
  publicKeyFile: /home/john/id_rsa.pub
  homeDirectory: /home/john
- privateKeyFile: /home/mary/id_rsa
  publicKeyFile: /home/mary/id_rsa.pub
  homeDirectory: /home/mary
- privateKeyFile: /home/alice/id_rsa
  publicKeyFile: /home/alice/id_rsa.pub
  homeDirectory: /home/alice
- privateKeyFile: /home/gabriel/id_rsa
  publicKeyFile: /home/gabriel/id_rsa.pub
  homeDirectory: /home/gabriel
- privateKeyFile: /home/build0/id_rsa
  publicKeyFile: /home/build0/id_rsa.pub
  homeDirectory: /home/build0
- privateKeyFile: /home/build1/id_rsa
  publicKeyFile: /home/build1/id_rsa.pub
  homeDirectory: /home/build1
- privateKeyFile: /home/build2/id_rsa
  publicKeyFile: /home/build2/id_rsa.pub
  homeDirectory: /home/build2
- privateKeyFile: /home/build3/id_rsa
  publicKeyFile: /home/build3/id_rsa.pub
  homeDirectory: /home/build3
- privateKeyFile: /home/build4/id_rsa
  publicKeyFile: /home/build4/id_rsa.pub
  homeDirectory: /home/build4
- privateKeyFile: /home/build5/id_rsa
  publicKeyFile: /home/build5/id_rsa.pub
  homeDirectory: /home/build5
- privateKeyFile: /home/build6/id_rsa
  publicKeyFile: /home/build6/id_rsa.pub
  homeDirectory: /home/build6

To learn more about core language features, read:

• Core language features

For an even longer hands-on tutorial, read:

• Getting started: Generate JSON or YAML

Features:

• Total - Evaluation always terminates and never hangs or infinitely loops
• Safe - Evaluation never crashes or throws exceptions
• Distributed - Expressions can reference other expressions by URL or path
• Strongly normalizing - All expressions (even functions) have a normal form
• Statically typed - Configurations can be validated ahead-of-time
• Strongly typed - No coercions, casts or subtyping
• Built-in data types - Includes lists, anonymous records and anonymous unions

Documentation

The Dhall language originated as a Haskell-specific configuration file format and is expanding to support more languages and file formats. Consequently, the Haskell package for Dhall still hosts the official tutorial and language manual:

• Dhall tutorial and manual

... which will eventually become a language-agnostic tutorial

You can also read about the original motivation behind the language here:

• Dhall - A non-Turing-complete configuration language

Finally, we have a cheatsheet for a very condensed overview and quick lookup:

• Dhall Cheatsheet

Standard Library

Dhall's Standard Library is called Prelude. It implements various utilities to work with the builtin types. Where to find it:

• Official link - https://prelude.dhall-lang.org
• Github repo
• Nix: both dhall and dhall.prelude derivations are built, install the Prelude with e.g. nix-env -iA nixpkgs.dhall.prelude

Development status

The current version and versioning policy is detailed in the Versioning document, and you can see the latest changes in the Changelog.

There is an effort under way to formalize the language semantics for Dhall, to help with porting it to other languages.
If you would like to assist with either standardizing the language or creating new bindings just open a new issue or contribute to existing ones in the issue tracker.

Overview

You can use Dhall in one of three ways:

• Interpreter: You can install a language-independent command-line program that can import, type-check and evaluate Dhall expressions
• Language binding: You can use a language-specific library to load Dhall configuration files directly into your programs
• Compilers: You can compile Dhall expressions to several common configuration file formats using command-line utilities

I recommend the following progression for adopting Dhall:

• Install the interpreter to learn more about the language
• If you like what you see, then:
  • If your preferred programming language supports Dhall:
    • Use the language binding to configure your programs using Dhall
  • If your language does not yet support Dhall
    • Compile Dhall to a supported configuration format (such as JSON)

The following sections tour each of these use cases in more detail

Interpreter

You can install a Dhall interpreter that can type-check and evaluate Dhall expressions from the command line. You can use this interpreter to:

• Learn how the language works:

  $ dhall <<< 'True && False'
  Bool
  
  False

  The first line of output shows the inferred type of an expression:

  $ dhall <<< 'List/length'
  ∀(a : Type) → List a → Natural
  
  List/length

  The second line of output shows the fully evaluated normal form of an expression:

  $ dhall <<< 'λ(x : Text) → let y = True in if y != False then x else "?"'
  ∀(x : Text) → Text
  
  λ(x : Text) → x

• Validate a configuration file ahead of time against a schema:

  $ cat config
  { foo = List/length Natural [2, 3, 5], bar = True && False }

  $ cat schema
  { foo : Natural, bar : Bool }

  Dhall lets you import expressions and types by their path:

  $ dhall <<< './config : ./schema'
  { bar : Bool, foo : Natural }
  
  { bar = False, foo = 3 }

  Schema validation is the same thing as a type annotation

• Detect type errors:

  $ dhall <<< 'λ(x : Integer) → x && True'
  
  Use "dhall --explain" for detailed errors
  
  Error: ❰&&❱ only works on ❰Bool❱s
  
  x && True
  
  (stdin):1:18

  You can ask the type checker to go into more detail using the --explain flag:

  $ dhall --explain <<< 'λ(x : Integer) → x && True'
  
  
  x : Integer
  
  Error: ❰&&❱ only works on ❰Bool❱s
  
  Explanation: The ❰&&❱ operator expects two arguments that have type ❰Bool❱
                                                                                  
  For example, this is a valid use of ❰&&❱:                           
                                                                                  
                                                                                  
      ┌───────────────┐                                                           
      │ True && False │                                               
      └───────────────┘                                                           
                                                                                  
                                                                                  
  You provided this argument:                                                     
                                                                                  
  ↳ x                                                                
                                                                                  
  ... which does not have type ❰Bool❱ but instead has type:                       
                                                                                  
  ↳ Integer                                                                
  
  ────────────────────────────────────────────────────────────────────────────────
  
  x && True
  
  (stdin):1:18

• Resolve remote expressions:

  dhall <<< 'https://raw.githubusercontent.com/dhall-lang/Prelude/35deff0d41f2bf86c42089c6ca16665537f54d75/List/replicate'
  
  ∀(n : Natural) → ∀(a : Type) → ∀(x : a) → List a
  
  λ(n : Natural) → λ(a : Type) → λ(x : a) → List/build a (λ(list : Type) → λ(cons : a → list → list) → Natural/fold n list (cons x))

  You can import arbitrary expressions by URL. In fact, the Dhall Prelude is hosted on GitHub to be imported in this way:

  • Dhall Prelude

• Reduce an expression to normal form:

  $ cat ./example
  let replicate = https://raw.githubusercontent.com/dhall-lang/Prelude/35deff0d41f2bf86c42089c6ca16665537f54d75/List/replicate
  
  let exclaim = λ(t : Text) → t ++ "!"
  
  in  λ(x : Text) → replicate 3 Text (exclaim x)

  You can reduce functions to normal form, even when they haven't been applied to all of their arguments

  $ dhall <<< './example'
  ∀(x : Text) → List Text
  
  λ(x : Text) → [x ++ "!", x ++ "!", x ++ "!"] : List Text

  The normal form is equivalent to the original program except stripped of all imports and indirection

Learn more:

• GitHub repository
• Language guide
• Blog post

Language Bindings

You can use Dhall to configure programs written in other languages. This is the most common use case for Dhall: a type-safe and non-Turing-complete configuration language

Dhall currently supports two complete language bindings:

• Haskell
• Nix

... and four language bindings in progress:

• Clojure
• Java - via Eta
• PureScript
• Python

The following two bindings are not maintained but you may find them useful as a starting point:

• Scala
• Rust

Haskell

You can load Dhall expressions into Haskell using the input function:

>>> input auto "True && False' :: IO Bool
False

The compiler validates the expression against the expected type:

>>> input auto "1" :: IO Bool
*** Exception: 
Error: Expression doesn't match annotation

1 : Bool

(input):1:1

You can even marshall some Dhall functions into Haskell:

>>> twice <- input auto "λ(x : Integer) → [x, x]" :: IO (Integer -> [Integer])
>>> twice 5
[5,5]

You can also decode Dhall expressions into Haskell types that derive Generic:

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

import Dhall

data Example = Example { foo :: Natural, 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 = [3.0, 4.0, 5.0]
}

... and prints the configuration:

$ ./example
Example {foo = 1, bar = [3.0,4.0,5.0]}

Learn more:

• GitHub repository
• Tutorial

Nix

nixpkgs provides a dhallToNix utility which you can use to translate a Dhall expression to the corresponding Nix expression. This lets you carve out a typed and total subset of Nix

For example, this Dhall expression:

let
  pkgs = import <nixpkgs> { };

in
  pkgs.dhallToNix ''
    { foo = λ(x : Bool) → [x, x]
    , bar = Natural/even 2
    , baz = https://raw.githubusercontent.com/dhall-lang/Prelude/35deff0d41f2bf86c42089c6ca16665537f54d75/Text/concat
    }
  ''

... translates to this Nix expression

{ foo = x : [ x x ];
  bar = true;
  baz = xs:
    (t: xs: t: cons:
      builtins.foldl' (f: y: ys: f (cons y ys)) (ys: ys) xs
    ) {} xs {} (x: y: x + y) "";
}

Learn more:

• dhallToNix function
• GitHub repository
• Tutorial
• Blog post

Compilers

You can compile Dhall expressions to other configuration formats, such as:

• JSON and YAML
• Bash declarations
• Text (i.e. Dhall as a template engine)

These compilers can only translate a subset of Dhall to these other formats. For example, you cannot translate Dhall functions to JSON

JSON and YAML

The dhall-to-json executable lets you compile Dhall expressions to JSON:

$ dhall-to-json <<< '{ foo = 1, bar = True, baz = [1, 2, 3] }'
{"foo":1,"baz":[1,2,3],"bar":true}

YAML is already a superset of JSON, but the dhall-to-yaml executable lets you compile Dhall expressions to idiomatic YAML syntax, too:

dhall-to-yaml <<< '{ foo = 1, bar = True, baz = [1, 2, 3] }'
foo: 1
baz:
- 1
- 2
- 3
bar: true

Some configuration file formats are supersets of JSON such as the HashiCorp configuration language (Used to configure terraform) so you can compile Dhall expressions to these configuration file formats, too.

Learn more:

• GitHub repository
• Tutorial
• Blog post

Bash

You can also compile Dhall expressions to either Bash expressions:

$ dhall-to-bash <<< 'List/length Bool [True, False]'
2

... or Bash declarations:

$ dhall-to-bash --declare FOO <<< '{ bar = 1, baz = "ABC" }'
declare -r -A FOO=([bar]=1 [baz]=ABC)
$ eval $(dhall-to-bash --declare FOO <<< '{ bar = 1, baz = "ABC" }')
$ echo ${FOO[bar]}
1
$ echo ${FOO[baz]}
ABC

Learn more:

• GitHub repository
• Tutorial
• Blog post

Text

You can also use the dhall-to-text executable as a template engine

For example, given this template:

$ cat function
    λ(record : { name        : Text
               , value       : Double
               , taxed_value : Double
               , in_ca       : Bool
               }
     ) → ''
Hello ${record.name}
You have just won ${Double/show record.value} dollars!
${ if record.in_ca
   then "Well, ${Double/show record.taxed_value} dollars, after taxes"
   else ""
 }
''

... and this payload:

$ cat value
{ name        = "Chris"
, value       = 10000.0
, taxed_value = 6000.0
, in_ca       = True
}

... you can apply the template to the payload and render the result using the dhall-to-text executable:

$ dhall-to-text <<< './function ./value'
Hello Chris
You have just won 10000.0 dollars!
Well, 6000.0 dollars, after taxes

Learn more:

• GitHub repository and tutorial
• Blog post

Design philosophy

Programming languages are all about design tradeoffs and the Dhall language uses the following guiding principles (in order of descending priority) that help navigate those tradeoffs:

• Polish

  The language should delight users. Error messages should be fantastic, execution should be snappy, documentation should be excellent, and everything should "just work".

• Simplicity

  When in doubt, cut it out. Every configuration language needs bindings to multiple programming languages, and the more complex the configuration language the more difficult to create new bindings. Let the host language that you bind to compensate for any missing features from Dhall.

• Beginner-friendliness

  Dhall needs to be a language that anybody can learn in a day and debug with little to no assistance from others. Otherwise people can't recommend Dhall to their team with confidence.

• Robustness

  A configuration language needs to be rock solid. The last thing a person wants to debug is their configuration file. The language should never hang or crash. Ever.

• Consistency

  There should only be one way to do something. Users should be able to instantly discern whether or not something is possible within the Dhall language or not.

The Dhall configuration language is also designed to negate many of the common objections to programmable configuration files, such as:

"Config files shouldn't be Turing complete"

Dhall is not Turing-complete. Evaluation always terminates, no exceptions

"Configuration languages become unreadable due to abstraction and indirection"

Every Dhall configuration file can be reduced to a normal form which eliminates all abstraction and indirection

"Users will go crazy with syntax and user-defined constructs"

Dhall is a very minimal programming language. For example: you cannot even compare strings for equality. The language also forbids many other common operations in order to force users to keep things simple

The biggest issue with using Dhall as a configuration language is that there are currently only Haskell bindings. If you would like to contribute bindings to another language then go for it, otherwise I will do my best to contribute them as time permits.

Name

The language is named after a Dustman from the game Planescape: Torment who belongs to a faction obsessed with death (termination). The logo represents his quill and inkwell

The name rhymes with "tall"/"call"/"hall" (i.e. "dɔl" for a US speaker or "dɔːl" for a UK speaker using the International Phonetic Alphabet).