Erlang does have some mutability (the process dictionary, and the mailbox), but the fact that the language is largely immutable is leveraged by the GC: if I remember correctly the private heap GC is a generational semi-space copying GC, but because the language is immutable it doesn’t need to have write barriers, to perform a minor collection only a scan of the stack（and process dict) is necessary to know the valid roots of the young heap.