A Swift playground for the beginning of the book, The Little Typer

LittleTyper.swift

import Cocoa
import Foundation

protocol AtomType {}
struct Atom: AtomType {}

func == <Atom1, Atom2> (lhs: Atom1, rhs: Atom2) -> Bool where Atom1: AtomType, Atom2: AtomType {
    return type(of: lhs) == type(of: rhs) // maybe
}

func == (lhs: (Atom, Atom), rhs: (Atom, Atom)) -> Bool {
    return true // ??
}

func cons<CarType, CdrType>(_ car: CarType, _ cdr: CdrType) -> (CarType, CdrType) {
    return (car, cdr)
}

func car<CarType, CdrType>(_ pair: (CarType, CdrType)) -> CarType {
    return pair.0
}

func cdr<CarType, CdrType>(_ pair: (CarType, CdrType)) -> CdrType {
    return pair.1
}

let ratatouille = Atom()
let garlic = Atom()
let meal = cons(ratatouille, garlic)

let tomato = Atom()
let egg = Atom()
let omelette = cons(tomato, egg)

print(ratatouille == garlic)
print(meal == meal)
print(ratatouille == car(meal))
print(garlic == cdr(meal))
print(tomato == garlic)