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.

Just small note that from Elixir 1.5 you should use String.to_charlist and previous version - to_char_list is deprecated.