Why does Flow.reduce return a list of tuples

Questions / Help
1

I’m currently looking at Flow and I can’t understand why every example that reduces the events into a map with a reduce stage return a list of tuples if we pipe the flow into a Enum.map(&(&1)), Enum.sort() or Enum.to_list().

If we look at the example given on the Flow.reduce/3 docs, this is exactly what happens:

    iex> flow = Flow.from_enumerable(["the quick brown fox"]) |> Flow.flat_map(fn word ->
    ...>    String.graphemes(word)
    ...> end)
    iex> flow = flow |> Flow.partition |> Flow.reduce(fn -> %{} end, fn grapheme, map ->
    ...>   Map.update(map, grapheme, 1, & &1 + 1)
    ...> end)
    iex> Enum.sort(flow)
    [{" ", 3}, {"b", 1}, {"c", 1}, {"e", 1}, {"f", 1},
     {"h", 1}, {"i", 1}, {"k", 1}, {"n", 1}, {"o", 2},
     {"q", 1}, {"r", 1}, {"t", 1}, {"u", 1}, {"w", 1},
     {"x", 1}]

My interpretation is that each reduce stage is definitely returning the map resulting from the Map.update/4 operation, but each individual map is being converted into a list on an intermediate step (since %{1 => 2, 3 => 4} |> Enum.map(& &1) == [{1, 2}, {3, 4}]). Why is this? Is there a way for me to get the “raw” reduce result from each stage?

Thanks in advance!

2

The reason probably is that under the hood, the different Flow calls call Enum.map or one of the other Enum-operations. These work using the Enumerable protocol, which will always convert the enumerable, no matter what it was, to a list as part of its inner workings.

For many functions that Enum provides this can be overridden by supplying a collectable as optional second (or third, i.e. for functions that support it always the last), into which the results will then be collected.

Asides from this, Enum.sort will always return a list, as many other containers (notably including maps) are not ordered.

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