What is Expressiveness in a Computer Language

[Joachim Durchholz]

Matthias Blume schrieb:
> Erlang relies on a combination of purity, concurrency, and message
> passing, where messages can carry higher-order values.
> 
> Data structures are immutable, and each computational agent is a
> thread.  Most threads consist a loop that explicitly passes state
> around.  It dispatches on some input event, applies a state
> transformer (which is a pure function), produces some output event (if
> necessary), and goes back to the beginning of the loop (by
> tail-calling itself) with the new state.

Actually any Erlang process, when seen from the outside, is impure: it 
has observable state.
However, from what I hear, such state is kept to a minimum. I.e. the 
state involved is just the state that's mandated by the purpose of the 
process, not by computational bookkeeping - you won't send file 
descriptors in a message, but maybe information about the state of some 
hardware, or about a permanent log.

So to me, the approach of Erlang seems to amount to "make pure 
programming so easy and efficient that aren't tempted to introduce state 
that isn't already there".

Regards,
Jo