public static ChannelReader<T> Select<T, S>(this ChannelReader<S> reader,
                                                 Func<S, long, T> map,
                                                 ChannelParallelOptions options)

using Amphisbaena;
using Amphisbaena.Linq;

...

int[] data = new int[] {1, 2, 3};

string report = string.Join(Environment.NewLine, data
  .ToChannelReader()
  .Select((item, index) => $"A[{index}] = {item};")
  .ToEnumerable());