"Gadts" with a base protocol

not-really-gadts-protocol.swift

protocol Expr {
	associatedtype A
	
	func eval() -> A
}
struct Const<T>: Expr {
	typealias A = T
	let value: A
	func eval() -> A {
		return value
	}
}

struct Add<E1: Expr, E2: Expr>: Expr where E1.A == Int, E2.A == Int {
	typealias A = Int
	let lhs: E1
	let rhs: E2
	
	func eval() -> Int {
		return lhs.eval() + rhs.eval()
	}
}

struct Equal<E1: Expr, E2: Expr>: Expr where E1.A == Bool, E2.A == Bool {
	typealias A = Bool
	let lhs: E1
	let rhs: E2
	
	func eval() -> Bool {
		return lhs.eval() == rhs.eval()
	}
}

let x = Add(lhs: Const(value: 1), rhs: Const(value: 2))
print(type(of: x))