Enum.join

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

join(enumerable, joiner \\ "")

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Specs

join(t(), String.t()) :: String.t()

Joins the given enumerable into a string using joiner as a separator.

If joiner is not passed at all, it defaults to an empty string.

All elements in the enumerable must be convertible to a string, otherwise an error is raised.

Examples

iex> Enum.join([1, 2, 3])
"123"

iex> Enum.join([1, 2, 3], " = ")
"1 = 2 = 3"