When thinking of immutable state, I imagine a large
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 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, turns out one cannot place a log statement anywhere inside it to see what’s going on. Punctuation rules. The trick is to place the log a comma before the last expression before the semicolon.
Val >= Left andalso Val =< Right ->
io:format("==>not ok ~p<~p<~p\n\n",[Left,Val,Right]),
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. Note the case statement is far more commonly used than the if.
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
Note that to generate a list of numbers, like range(n) in python, there is lists:seq(1,n).