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+---
+language: Dhall
+filename: learndhall.dhall
+contributors:
+ - ["Gabriel Gonzalez", "http://www.haskellforall.com/"]
+---
+
+Dhall is a programmable configuration language that provides a non-repetitive
+alternative to YAML.
+
+You can think of Dhall as: JSON + functions + types + imports
+
+Note that while Dhall is programmable, Dhall is not Turing-complete. Many
+of Dhall's features take advantage of this restriction to provider stronger
+safety guarantees and more powerful tooling.
+
+```haskell
+-- Single-line comment
+
+{- Multi-line comment
+
+ Unicode is fine 🙂
+
+ This file is a valid Dhall expression that evaluates to a large record
+ collecting the results of each step.
+
+ You can view the results by interpreting the file:
+
+ $ dhall --file learndhall.dhall
+
+ {- Comments can be nested -}
+-}
+
+let greeting = "Hello, world!"
+
+let fruits = "🍋🍓🍍🍉🍌"
+
+let interpolation = "Enjoy some delicious fruit: ${fruits}"
+
+let multilineText {- Inline comments work, too -} =
+ ''
+ Leading whitespace is stripped from multi-line text literals.
+
+ That means you can freely indent or dedent a text literal without
+ changing the result.
+
+ Relative indentation within the literal is still preserved.
+
+ Other than that, the text literal is preserved verbatim, similar to a
+ "literal" YAML multiline string.
+ ''
+
+let bool = True
+
+-- Type annotations on bindings are optional, but helpful, so we'll use them
+let annotation : Bool = True
+
+let renderedBool : Text = if bool then "True" else "False"
+
+-- Natural numbers are non-negative and are unsigned
+let naturalNumber : Natural = 42
+
+-- Integers may be negative, but require an explicit sign, even if positive
+let positiveInteger : Integer = +1
+
+let negativeInteger : Integer = -12
+
+let pi : Double = 3.14159265359
+
+{- You can use a wider character range for identifiers (such as quotation
+ marks and whitespace) if you quote them using backticks
+-}
+let `Avogadro's Number` : Double = 6.0221409e+23
+
+let origin : { x : Double, y : Double } = { x = 0.0, y = 0.0 }
+
+let somePrimes : List Natural = [ 2, 3, 5, 7, 11 ]
+
+{- A schema is the same thing as a type
+
+ Types begin with an uppercase letter by convention, but this convention is
+ not enforced
+-}
+let Profile : Type
+ = { person :
+ { name : Text
+ , age : Natural
+ }
+ , address :
+ { country : Text
+ , state : Text
+ , city : Text
+ }
+ }
+
+let john : Profile =
+ { person =
+ { name = "John Doe"
+ , age = 67
+ }
+ , address =
+ { country = "United States"
+ , state = "Pennsylvania"
+ , city = "Philadelphia"
+ }
+ }
+
+let philadelphia : Text = john.address.city
+
+{- Enum alternatives also begin with an uppercase letter by convention. This
+ convention is not enforced
+-}
+let DNA : Type = < Adenine | Cytosine | Guanine | Thymine >
+
+let dnaSequence : List DNA = [ DNA.Thymine, DNA.Guanine, DNA.Guanine ]
+
+let compactDNASequence : List DNA =
+ let a = DNA.Adenine
+ let c = DNA.Cytosine
+ let g = DNA.Guanine
+ let t = DNA.Thymine
+ in [ c, t, t, a, t, c, g, g, c ]
+
+-- You can transform enums by providing a record with one field per alternative
+let theLetterG : Text =
+ merge
+ { Adenine = "A"
+ , Cytosine = "C"
+ , Guanine = "G"
+ , Thymine = "T"
+ }
+ DNA.Guanine
+
+let presentOptionalValue : Optional Natural = Some 1
+
+let absentOptionalValue : Optional Natural = None Natural
+
+let points : List { x : Double, y : Double } =
+ [ { x = 1.1, y = -4.2 }
+ , { x = 4.4, y = -3.0 }
+ , { x = 8.2, y = -5.5 }
+ ]
+
+{- `Natural -> List Natural` is the type of a function whose input type is a
+ `Natural` and whose output type is a `List Natural`
+
+ All functions in Dhall are anonymous functions (a.k.a. "lambdas"),
+ which you can optionally give a name
+
+ For example, the following function is equivalent to this Python code:
+
+ lambda n : [ n, n + 1 ]
+
+ ... and this JavaScript code:
+
+ function (n) { return [ n, n + 1 ]; }
+-}
+let exampleFunction : Natural -> List Natural =
+ \(n : Natural) -> [ n, n + 1 ]
+
+-- Dhall also supports Unicode syntax, but this tutorial will stick to ASCII
+let unicodeFunction : Natural → List Natural =
+ λ(n : Natural) → [ n, n + 1 ]
+
+-- You don't need to parenthesize function arguments
+let exampleFunctionApplication : List Natural =
+ exampleFunction 2
+
+let functionOfMultipleArguments : Natural -> Natural -> List Natural =
+ \(x : Natural) -> \(y : Natural) -> [ x, y ]
+
+let functionAppliedToMultipleArguments : List Natural =
+ functionOfMultipleArguments 2 3
+
+{- Same as `exampleFunction` except we gave the function's input type a
+ name: "n"
+-}
+let namedArgumentType : forall (n : Natural) -> List Natural =
+ \(n : Natural) -> [ n, n + 1 ]
+
+{- If you name a function's input type, you can use that name later within the
+ same type
+
+ This lets you write a function that works for more than one type of input
+ (a.k.a. a "polymorphic" function)
+-}
+let duplicate : forall (a : Type) -> a -> List a =
+ \(a : Type) -> \(x : a) -> [ x, x ]
+
+let duplicatedNumber : List Natural =
+ duplicate Natural 2
+
+let duplicatedBool : List Bool =
+ duplicate Bool False
+
+{- The language also has some built-in polymorphic functions, such as:
+
+ List/head : forall (a : Type) -> List a -> Optional a
+-}
+let firstPrime : Optional Natural = List/head Natural somePrimes
+
+let functionOfARecord : { x : Natural, y : Natural } -> List Natural =
+ \(args : { x : Natural, y : Natural }) -> [ args.x, args.y ]
+
+let functionAppliedToARecord : List Natural =
+ functionOfARecord { x = 2, y = 5 }
+
+{- All type conversions are explicit
+
+ `Natural/show` is a built-in function of the following type:
+
+ Natural/show : Natural -> Text
+
+ ... that converts `Natural` numbers to their `Text` representation
+-}
+let typeConversion : Natural -> Text =
+ \(age : Natural) -> "I am ${Natural/show age} years old!"
+
+-- A template is the same thing as a function whose output type is `Text`
+let mitLicense : { year : Natural, copyrightHolder : Text } -> Text =
+ \(args : { year : Natural, copyrightHolder : Text }) ->
+''
+Copyright ${Natural/show args.year} ${args.copyrightHolder}
+
+Permission is hereby granted, free of charge, to any person obtaining a copy of
+this software and associated documentation files (the "Software"), to deal in
+the Software without restriction, including without limitation the rights to
+use, copy, modify, merge, publish, distribute, sublicense, and/or sell copies
+of the Software, and to permit persons to whom the Software is furnished to do
+so, subject to the following conditions:
+
+The above copyright notice and this permission notice shall be included in all
+copies or substantial portions of the Software.
+
+THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR
+IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY,
+FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT. IN NO EVENT SHALL THE
+AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER
+LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING FROM,
+OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN THE
+SOFTWARE.
+''
+
+-- Template instantiation is the same thing as function application
+let templatedLicense : Text =
+ mitLicense { year = 2019, copyrightHolder = "Jane Smith" }
+
+{- You can import expressions by URL
+
+ Also, like Bash, you can import code from your local filesystem (not shown)
+
+ Security-conscious users can pin remotely-imported expressions by adding a
+ semantic integrity check. The interpreter rejects any attempt to tamper with
+ an expression pinned in this way. However, behavior-preserving refactors
+ of imported content will not perturb the hash.
+
+ Imported expressions pinned in this way are also locally cached in a
+ content-addressable store (typically underneath `~/.cache/dhall`)
+-}
+let Natural/sum : List Natural -> Natural =
+ https://prelude.dhall-lang.org/Natural/sum
+ sha256:33f7f4c3aff62e5ecf4848f964363133452d420dcde045784518fb59fa970037
+
+let twentyEight : Natural = Natural/sum somePrimes
+
+-- A package is the same thing as a (possibly nested) record that you can import
+let Prelude = https://prelude.dhall-lang.org/package.dhall
+
+let false : Bool = Prelude.Bool.not True
+
+-- You can import the raw contents of a file by adding `as Text` to an import
+let sourceCode : Text = https://prelude.dhall-lang.org/Bool/not as Text
+
+-- You can import environment variables, too:
+let presentWorkingDirectory = env:PWD as Text
+
+-- You can provide a fallback expression if an import fails
+let home : Optional Text = Some env:HOME ? None Text
+
+-- Fallback expressions can contain alternative imports of their own
+let possiblyCustomPrelude =
+ env:DHALL_PRELUDE
+ ? https://prelude.dhall-lang.org/package.dhall
+
+{- Tie everything together by auto-generating configurations for 10 build users
+ using the `generate` function:
+
+ Prelude.List.generate
+ : Natural -> forall (a : Type) -> (Natural -> a) -> List a
+-}
+let buildUsers =
+ let makeUser = \(user : Text) ->
+ let home = "/home/${user}"
+ let privateKey = "${home}/id_ed25519"
+ let publicKey = "${privateKey}.pub"
+ in { home = home
+ , privateKey = privateKey
+ , publicKey = publicKey
+ }
+
+ let buildUser =
+ \(index : Natural) -> makeUser "build${Natural/show index}"
+
+ let Config =
+ { home : Text
+ , privateKey : Text
+ , publicKey : Text
+ }
+
+ in Prelude.List.generate 10 Config buildUser
+
+-- Present all of the results in a final record
+in { greeting = greeting
+ , fruits = fruits
+ , interpolation = interpolation
+ , multilineText = multilineText
+ , bool = bool
+ , annotation = annotation
+ , renderedBool = renderedBool
+ , naturalNumber = naturalNumber
+ , positiveInteger = positiveInteger
+ , negativeInteger = negativeInteger
+ , pi = pi
+ , `Avogadro's Number` = `Avogadro's Number`
+ , origin = origin
+ , somePrimes = somePrimes
+ , john = john
+ , philadelphia = philadelphia
+ , dnaSequence = dnaSequence
+ , compactDNASequence = compactDNASequence
+ , theLetterG = theLetterG
+ , presentOptionalValue = presentOptionalValue
+ , absentOptionalValue = absentOptionalValue
+ , points = points
+ , exampleFunction = exampleFunction
+ , unicodeFunction = unicodeFunction
+ , exampleFunctionApplication = exampleFunctionApplication
+ , functionOfMultipleArguments = functionOfMultipleArguments
+ , functionAppliedToMultipleArguments = functionAppliedToMultipleArguments
+ , namedArgumentType = namedArgumentType
+ , duplicate = duplicate
+ , duplicatedNumber = duplicatedNumber
+ , duplicatedBool = duplicatedBool
+ , firstPrime = firstPrime
+ , functionOfARecord = functionOfARecord
+ , functionAppliedToARecord = functionAppliedToARecord
+ , typeConversion = typeConversion
+ , mitLicense = mitLicense
+ , templatedLicense = templatedLicense
+ , twentyEight = twentyEight
+ , false = false
+ , sourceCode = sourceCode
+ , presentWorkingDirectory = presentWorkingDirectory
+ , home = home
+ , buildUsers = buildUsers
+ }
+```
+
+To learn more, visit the official website, which also lets you try the
+language live in your browser:
+
+* [https://dhall-lang.org](http://dhall-lang.org/)