> Another solution is to put concatenation into your string type

Aah, the Erlang way of handling strings.

On Beam (Erlang's VM), that goes as deep as IO. It's perfectly fine to pass a (possibly nested) list of strings (whether charlists or binaries) to an IO function, and the system just knows how to deal with that.

An iolist isn't a string, you can't pass it to the uppercase function for instance. It's really meant for I/O as the name implies. Regular string concatenation is optimized to avoid copying when possible: https://www.erlang.org/doc/system/binaryhandling.html#constr...