Notes from my stream on 2017/01/16

dhall.swift

// Dhall, a terminating configuration language
// Syntax: https://github.com/Gabriel439/Haskell-Dhall-Library/blob/master/src/Dhall/Core.hs

// This gist's history:
// 1. Contents at stream end
// 2. Clean up free vs. bound variables

enum Expr {
  case constant(Int)
  case variable(Bound)
  indirect case `let`(Free, equalTo: Expr, in: Expr)
}

final class Free {
  func ref() -> Bound {
    return Bound(to: self)
  }

  // Debugging
  var name: String
  init(_ name: String) {
    self.name = name
  }
}
struct Bound {
  let to: Free

  // Debugging
  var name: String {
    return to.name
  }
}

extension Expr {
  var debugDescription: String {
    switch self {
    case let .constant(c):
      return "\(c)"
    case let .variable(v):
      return v.name
    case let .`let`(v, e, b):
      return "let \(v.name) = \(e.debugDescription) in \(b.debugDescription)"
    }
  }
}

// let x = 5 in let y = x in y
// evaluates to: 5

func makeY(_ x: Free) -> Expr {
  let y = Free("y")
  return Expr.`let`(y, equalTo: .variable(x.ref()), in: .variable(y.ref()))
}
let x = Free("x")
let e = Expr.`let`(x, equalTo: .constant(5), in: makeY(x))
e.debugDescription
//switch e.normal {
//case let .variable(v):
//  v.value
//default:
//  break
//}