@swift-4.1.3
import Foundation protocol Operand: Numeric {} typealias Operator<Value: Operand> = (Value, Value) -> Value indirect enum Node<Value: Operand> { case v(Value) case op(Node<Value>, Operator<Value>, Node<Value>) func evaluate() -> Value { switch self { case .v(let value): return value case .op(let operand1, let `operator`, let operand2): return `operator`(operand1.evaluate(), operand2.evaluate()) } } } extension Int: Operand {} extension Node: ExpressibleByIntegerLiteral where Value == Int { typealias IntegerLiteralType = Int init(integerLiteral value: Value) { self = .v(value) } } let root: Node<Int> = .op(1, +, .op(2, *, 3)) print(root.evaluate())
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