A.Enum.map_join

You're seeing just the function map_join, go back to A.Enum module for more information.
Link to this function

map_join(enumerable, joiner \\ "", mapper)

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Specs

map_join(t(val), String.t(), (val -> String.Chars.t())) :: String.t()
when val: value()

Maps and joins the given enumerable in one pass.

Mirrors Enum.map_join/3 with higher performance for Aja structures.