scan() as transforming operator

Scan.swift

(1...4)
    .publisher
    .scan([]) { (values, value) -> [String] in
        values + [String(value)]
}
.sink { print($0) }

// Output:
//    ["1"]
//    ["1", "2"]
//    ["1", "2", "3"]
//    ["1", "2", "3", "4"]