Stream.into

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

into(enum, collectable, transform \\ fn x -> x end)

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Specs

into(Enumerable.t(), Collectable.t(), (term() -> term())) :: Enumerable.t()

Injects the stream values into the given collectable as a side-effect.

This function is often used with run/1 since any evaluation is delayed until the stream is executed. See run/1 for an example.