Alternate version of Linq101: Query Reuse

query_reuse_alternate.exs

# This is a hacky way of doing the same as this:
# https://github.com/omnibs/elixir-linq-examples#linq101-query-reuse
numbers = [5, 4, 1, 3, 9, 8, 6, 7, 2, 0]

# We use agents to hold mutable state.
{:ok, pid} = Agent.start_link(fn -> numbers end)

# We turn our agent into a stream to defer linking
# the Stream.filter to the value of the array.
# Otherwise Agent.update below would have no effect.
numbers_stream = Stream.resource(
    fn -> Agent.get(pid, & &1) end,
    fn 
      [head | tail] -> {[head], tail}
      _ -> {:halt, nil}
    end,
    fn _ -> end)

low_numbers = numbers_stream
  |> Stream.filter(& &1 <= 3)

IO.puts "First run numbers <= 3:"
for n <- low_numbers, do: IO.puts n

Agent.update(pid, fn x -> x |> Enum.map(& -&1) end)

IO.puts "Second run numbers <= 3:"
for n <- low_numbers, do: IO.puts n