How is a type-erased generic wrapper implemented?

Question

I need to implement a type-erasing wrapper for my own structure, very similar to SequenceOf, GeneratorOf, etc. So I started by trying to just re-im

Answer 1

Try:

struct MySequenceOf : SequenceType {
    private let _generate:() -> MyGeneratorOf

    init(_ makeUnderlyingGenerator: () -> G) {
        _generate = { MyGeneratorOf(makeUnderlyingGenerator()) }
    }

    init(_ base: S) {
        _generate = { MyGeneratorOf(base.generate()) }
    }

    func generate() -> MyGeneratorOf {
        return _generate()
    }
}

struct MyGeneratorOf : GeneratorType, SequenceType {

    private let _next:() -> T?

    init(_ nextElement: () -> T?) {
        _next = nextElement
    }

    init(var _ base: G) {
        _next = { base.next() }
    }

    mutating func next() -> T? {
        return _next()
    }

    func generate() -> MyGeneratorOf {
        return self
    }
}

The basic strategy of implementing ProtocolOf is, like this:

protocol ProtocolType {
    typealias Value
    func methodA() -> Value
    func methodB(arg:Value) -> Bool
}

struct ProtocolOf:ProtocolType {
    private let _methodA: () -> T
    private let _methodB: (T) -> Bool

    init(_ base:B) {
        _methodA = { base.methodA() }
        _methodB = { base.methodB($0) }
    }

    func methodA() -> T { return _methodA() }
    func methodB(arg:T) -> Bool { return _methodB(arg) }
}

---

Added to answering @MartinR in comment.

Is there a special reason that _generate is a closure and not the generator itself?

First of all, I think, It's a matter of specification or semantics.

Needless to say, the difference is "when to create the generator".

Consider this code:

class Foo:SequenceType {
    var vals:[Int] = [1,2,3]
    func generate() -> Array.Generator {
        return vals.generate()
    }
}

let foo = Foo()
let seq = MySequenceOf(foo)
foo.vals = [4,5,6]
let result = Array(seq)

The problem is: result should be [1,2,3] or [4,5,6]? My MySequenceOf and built-in SequenceOf results the latter. I just matched the behaviors with built-in one.