Generic Show

show.swift

enum Sum<A,B> {
    case left(A)
    case right(B)
}

struct Product<A,B> {
    let a: A
    let b: B
}

struct Iso<A,B> {
    let to: (A) -> B
    let from: (B) -> A
}

struct Unit { }

extension Unit: Show {
    var text: String {
        return ""
    }
}


protocol Show {
    var text: String { get }
}

extension Sum: Show where A: Show, B: Show {
    var text: String {
        switch self {
        case .left(let a): return ".left(\(a.text)"
        case .right(let b): return ".right(\(b.text)"
        }
    }
}

extension Product: Show where A: Show, B: Show {
    var text: String {
        return "(\(self.a), \(self.b))"
    }
}

protocol Generic {
    associatedtype Rep
    var to: Rep { get }
}

struct Rec<A> where A: Generic {
    let value: A
}

extension Rec: Show where A.Rep: Show {
    var text: String {
        return value.to.text
    }
}

extension Array: Generic {
    typealias Rep = Sum<Unit, Product<Element, Rec<Array<Element>>>>
    
    var to: Sum<Unit, Product<Element, Rec<Array<Element>>>> {
        if self.isEmpty { return .left(Unit()) }
        return .right(Product(a: self[0], b: Rec(value: Array(self.suffix(from: 1)))))
    }
}

extension Int: Show {
    var text: String {
        return "\(self)"
    }
}

let arr = [1,2,3]
arr.to.text