notice the single quotes ' instead of double "

Generally when interacting directly with Erlang libs, they expect char lists

Yes, they are.

The spec of :gen_tcp.connect/3:

connect(Address, Port, Options) → {ok, Socket} | {error, Reason}

Types

• Address = inet:socket_address() | inet:hostname()
• [other types omitted since they are unimportant here]

inet:hostname() (which is :inet.hostname in elixir syntax) is defined as follows:

hostname() = atom() | string()

Erlangs string() type is equal to elixirs charlist type.

charlists can be created using the charlist literal, which is an single-quoted “string”. Also one can use the c and C sigils to create them, when one wants to be a little bit more explicit.

And as @PatNowak correctly tells, you can convert anything implementing List.Chars by passing it to Kernel.to_charlist/1

You should mark this thread as solved so others can see the answer right away and see it as solved in the thread list.