Representing Algebraic Data Type?

ADT.swift

struct Decl {
    struct Option {
        enum Segment {
            case one(String, [String])
            indirect case map(Segment, Segment)
        }
        typealias Field = (name: String?, type: Segment)
        let name: String
        let fields: [Field]
    }
    let name: String
    let variables: [String]
    let options: [Option]
}

enum Type {
    enum Primitive {
        case int
        case bool
    }
    case primitive(Primitive)
    case declared(Decl)
    case variable(String)
    indirect case map(Type, Type)
}

typealias Environment = [String: Type]

var global: Environment = ["Int": .primitive(.int), "Bool": .primitive(.bool)]

Decl(
    name: "Suit",
    variables: [],
    options: [
        Decl.Option(name: "spade", fields: []),
        Decl.Option(name: "heart", fields: []),
        Decl.Option(name: "diamond", fields: []),
        Decl.Option(name: "club", fields: [])])

Decl(
    name: "Optional",
    variables: ["T"],
    options: [
        Decl.Option(name: "none", fields: []),
        Decl.Option(name: "some", fields: [
            Decl.Option.Field(name: nil, type: .one("T", []))])])