Expr.swift

//: Playground - noun: a place where people can play

import Cocoa

// Operations
protocol Stringable {
    func stringify() -> String
}

protocol Evaluatable {
    func evaluate() -> Double
}

// Types
protocol Expr {}

struct Constant: Expr {
    let value: Double
}

struct BinaryPlus<LH: Expr, RH: Expr>: Expr {
    let lh: LH, rh: RH
}

// Add implementations
extension Constant: Evaluatable {
    func evaluate() -> Double {
        return value
    }
}

extension Constant: Stringable {
    func stringify() -> String {
        return String(value)
    }
}

extension BinaryPlus: Stringable where LH: Stringable, RH: Stringable {
    func stringify() -> String {
        return "\(lh.stringify()) + \(rh.stringify())"
    }
}

extension BinaryPlus: Evaluatable where LH: Evaluatable, RH: Evaluatable {
    func evaluate() -> Double {
        return lh.evaluate() + rh.evaluate()
    }
}