Comment by nivertech

Of course in Elixir[1] reduce is more efficient (at least memory-wise) than map/flat_map, but in case of scripting it usually doesn't matter (unless this JSON file is huge, but it's fully loaded into memory and parsed there anyway).

I would rewrite the example from the post like this (NOTE: I didn't run it, so it might be slightly incorrect):

    #/usr/bin/env elixir

    "file.json"
    |> File.read!()
    |> Jason.decode!()
    |> Enum.flat_map(& &1["children"])
    |> Enum.map(&{&1["id"], &1})
    |> Enum.into(%{})

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1. although map is much easier to parallelize/vectorize (e.g. in Array PLs)