[dhall/en] Add "Learn Dhall in Y minutes"

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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/)