AnyResponse

router.get("foo") { req -> AnyResponse in
    if /* something */ {
        return AnyResponse(42)
    } else {
        return AnyResponse("string")
    }
}

This can also be done using a ResponseEncodable enum.

enum IntOrString: ResponseEncodable {
    case int(Int)
    case string(String)

    func encode(for req: Request) throws -> EventLoopFuture<Response> {
        switch self {
        case .int(let i): return try i.encode(for: req)
        case .string(let s): return try s.encode(for: req)
        }
    }
}

router.get("foo") { req -> IntOrString in
    if /* something */ {
        return .int(42)
    } else {
        return .string("string")
    }
}