IOasProtocol.swift

protocol IO {
    associatedtype CollectionType: Collection
    init(_ source: CollectionType)
    func go() -> AnyCollection<CollectionType.Element>
}

struct Print<T> : IO where T: Collection {
    typealias CollectionType = T
    let source: CollectionType
    init(_ source: T) {
        self.source = source
    }
    func go() -> AnyCollection<T.Element>? {
        if let first = source.first {
            print(first)
            return AnyCollection(source).dropFirst()
        }
        return nil
    }
}

Print().apply("abc")

struct AndThen<T: IO> : IO where T: Collection {
    typealias CollectionType = T.CollectionType
    let left: T
    let right: T
    init(_ left: T, _ right: T) {
        self.left = left
        self.right = right
    }
    func go() -> AnyCollection<T.CollectionType.Element> {
        <#code#>
    }
}