When thinking of immutable state, I imagine a large

input->[processingbox]->output

box being broken into a chain of multiple smaller mini-boxes,

input0->[pbox0]->output0 -> [pbox1]->output1 ->[pbox2]->output3

with each individual box accomplishing a change in its input to its output, without keeping any internal mutable state.

Which reminds me of Linear-Time-Invariant systems. As long as each part of the system is linear and time-invariant, a large system can be composed of simpler parts and still be analyzed by deriving a composite transfer function. The composite system can be studied for its stability and loss/gain characteristics and adjustments made in the system design.

I suspect a similar payoff occurs with functional programming. But it is not so clearly stated or visible in the ability to analyze large programs. Erlang is essentially a version of event driven programming. The conciseness of expression is encouraging, but I hope to arrive at good examples of program composition.

Meanwhile the ‘if’ statement in erlang is a curveball. In addition to the odd syntax, and statements like true->false, one finds one cannot place a log statement anywhere inside a set of statements to see what’s going on. Punctuation rules. The trick is to place the log a comma before the last expression before the semicolon.

if
   Val >= Left andalso Val =< Right ->
   io:format("==> ok~p<~p<~p\n\n",[Left,Val,Right]),
   true;

   true ->
   io:format("==>not ok ~p<~p<~p\n\n",[Left,Val,Right]),
   false
end.

The semicolons indicate different phrases which are pattern-matching options. The commas are continued statements within the same phrase. The last statement in the phrase is the return value. The last phrase is typically a catch-all, in this case returning a value of false. The case statement is more commonly used than the if. The -> denotes  ‘lhs condition is mapped to rhs’ .  

The ‘for’ statement does not exist at all and below are alternatives.

foreach(Fun, List)->ok, %% just applies a function Fun to each element of a list
foldl(Fun, Accumulator, List)-> Acc1,  %% fold list left to right into an accumulator
foldl(Fun, Accumulator, List)-> Acc1 %% fold list right to left into an accumulator
map(Fun, List1) -> List2  %% maps a list to a new list

The = sign in erlang is used for (a) immutable assignment of a term to a value and (b) for comparing if a previously assigned term or expression of terms is the same as another term or value – this is called pattern matching. One cannot reassign a term to a new value, so the math notion of equality always holds. The = used in C/Java could be called mutable assignment, every variable is hiding within it mutable state. A term in erlang does have an internal state of bound or unbound. On a successful pattern match any unbound variable becomes bound.

1> [H|T] = [1,2,3].

2> io:write(H) .

To generate a list of numbers, like range(n) in python, there is lists:seq(1,n). io:fwrite(“~p”, [lists:seq(1,20)]).

The philosophy behind some of these decisions is discussed here https://news.ycombinator.com/item?id=13499377. An objective is to make it impossible to write runaway loops – to make the runtime system always be able to recover and reallocate its cycles. A related aspect is to let-it-fail and not write a lot of exception handling, defensive code. 

The erlang work was done while modifying RabbitMQ (written in erlang) to support features for certificate based authentication for MQTT while consulting at GE Predix.