Chain ⛓

public indirect enum Chain {
    case end
    case complete(E.Function?)
    case link(E.Function, Chain)
    case background(E.Function, Chain)
    case multi([Chain])
}

let output = Chain.link(
    .out { "First" },
    .link(
        .in {
            print("Value: \($0)")
        },
        .multi(
            [
                .multi(
                    [
                        .end,
                        .end,
                        .end
                    ]
                ),
                .link(
                    .out { "Link" },
                    .link(
                        .out { "Last" },
                        .complete(
                            .inout { value in
                                guard case .string(let value) = value else {
                                    XCTFail()
                                    return .void
                                }
                                
                                return  .string("\(value) !!!")
                            }
                        )
                    )
                )
            ]
        )
    )
)
.run(name: "ChainTests-testOutput")

(lldb) po output
▿ Variable
  ▿ array : 2 elements
    ▿ 0 : Variable
      - string : "First"
    ▿ 1 : Variable
      ▿ array : 2 elements
        - 0 : E.Variable.void
        ▿ 1 : Variable
          ▿ array : 2 elements
            ▿ 0 : Variable
              ▿ array : 3 elements
                ▿ 0 : Variable
                  ▿ array : 1 element
                    - 0 : E.Variable.void
                ▿ 1 : Variable
                  ▿ array : 1 element
                    - 0 : E.Variable.void
                ▿ 2 : Variable
                  ▿ array : 1 element
                    - 0 : E.Variable.void
            ▿ 1 : Variable
              ▿ array : 2 elements
                ▿ 0 : Variable
                  - string : "Link"
                ▿ 1 : Variable
                  ▿ array : 2 elements
                    ▿ 0 : Variable
                      - string : "Last"
                    ▿ 1 : Variable
                      ▿ array : 1 element
                        ▿ 0 : Variable
                          - string : "Last !!!"

(lldb) po output.flatten
▿ Variable
  ▿ array : 8 elements
    ▿ 0 : Variable
      - string : "First"
    - 1 : E.Variable.void
    - 2 : E.Variable.void
    - 3 : E.Variable.void
    - 4 : E.Variable.void
    ▿ 5 : Variable
      - string : "Link"
    ▿ 6 : Variable
      - string : "Last"
    ▿ 7 : Variable
      - string : "Last !!!"