You introduce references (make_ref) very quickly, but it may be useful to note that in some cases they can be very useful. In particular I'm missing a note that it's cheap to pattern match on references and that they are very useful as keys, even in a distributed system (the name of a node is part of the reference value, so refs are not likely to collide even when many different nodes generate them).

One big improvement would be to add a note to "2.4.10 Other built-in types" that references are very useful as unique keys. "unique piece of information" doesn't quite convey the purpose of references that well.

Thanks for this comment! I think the best way to go about it would be to add an example of tagging a message with a reference, and then matching on a corresponding response.