Group within a list

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21

If performance is something important then there’s also Enum functions, which get closer to manual recursion as well:

defmodule Mod do
  def run(list) do
    Enum.chunk_while(list, :start, &chunk_fun/2, &after_fun/1)
  end

  defp chunk_fun([key, value], :start), do: {:cont, {key, [value]}}
  defp chunk_fun([key, value], {key, rest}), do: {:cont, {key, [value | rest]}}
  defp chunk_fun([key, value], acc), do: {:cont, emit_acc(acc), {key, [value]}}

  defp after_fun({_, _} = acc), do: {:cont, emit_acc(acc), acc}
  defp after_fun(acc), do: {:cont, acc}

  defp emit_acc({key, list}), do: [key, format(list)]

  defp format([el]), do: "#{el}"
  defp format([last | rest]), do: "#{rest |> Enum.reverse() |> Enum.join(", ")} and #{last}"
end

Name                   ips        average  deviation         median         99th %
hissssst          714.47 K        1.40 μs  ±2591.76%        1.01 μs        1.71 μs
chunk_while       577.99 K        1.73 μs  ±1484.17%        1.35 μs        2.47 μs

Comparison: 
hissssst          714.47 K
chunk_while       577.99 K - 1.24x slower +0.33 μs
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