[erlang-questions] Erlang and mutable state

[istarex]

Hi all,

I had a question about Erlang and mutable state.  I understand that
Erlang avoids the concurrency nightmare of other languages by avoiding
mutable state shared between processes.  But why Erlang doesn't allow
mutable state isolated within a process (such that no other process
could directly access or change that state)?  I'm wondering what in
Erlang's programming model prevents this.  If anybody could point me
to a good link or paper that describes the reason, I'd really
appreciate it.

-Istarex