Erlang beginnings: moving a function from an escript into OTP - erlang

Question

There is a simple implementation of the factorial function in an 'escript' in the Erlang docs. The factorial function is given as:
fac(0) -> 1;
fac(N) -> N * fac(N-1).
That's all fine, I can get this to work, no problem.
I would however like to know how I can implement this same, simple factorial function in an 'OTP way' using rebar3?
Just to be clear, my questions are:
Where does the code go?
How would I call it from the shell?
Could I also run it from the command line like I do via the escript example?
FYI, I have gotten started with rebar3. Here is where I am at:
rebar3 new app factorial
creates a few files but specifically the code is in 3 files in a src directory. I can see that a supervisor is being used, seems fine.
I can interact with this project from the shell:
$ rebar3 shell
1> application:which_applications().
[{factorial,"An OTP application","0.1.0"},
{inets,"INETS CXC 138 49","7.0.3"},
{ssl,"Erlang/OTP SSL application","9.1.1"},
{public_key,"Public key infrastructure","1.6.4"},
{asn1,"The Erlang ASN1 compiler version 5.0.8","5.0.8"},
{crypto,"CRYPTO","4.4"},
{stdlib,"ERTS CXC 138 10","3.7"},
{kernel,"ERTS CXC 138 10","6.2"}]
2> application:stop(factorial).
=INFO REPORT==== 21-Jan-2019::12:42:07.484244 ===
application: factorial
exited: stopped
type: temporary
ok
3> application:start(factorial).
ok

Answer 1

Where does the code go?
To 'call code in the OTP way', you can put it behind a gen_server.
For this simple factorial function, I added a new file factorial.erl within the src directory which is pretty much a standard gen_server skeleton with my factorial function as one of the callbacks:
% factorial.erl
-module(factorial).
-behaviour(gen_server).
-export([start_link/0, stop/0, calc/1]).
<boilerplate gen_server stuff here, like init, etc.>
calc(N) ->
{ok, Result} = gen_server:call(?SERVER, {calc, N}),
{ok, Result}.
handle_call({calc, N}, _From, State) ->
Factorial = factorial(N),
Reply = {ok, Factorial},
{reply, Reply, State};
factorial(0) ->
1;
factorial(N) ->
N * factorial(N-1).
Since my rebar3 new app factorial created a supervisor, I modified the supervisor's init so that it calls my factorial module:
% factorial_sup.erl
<skeleton supervisor stuff here>
init([]) ->
Server = {factorial, {factorial, start_link, []},
permanent, 2000, worker, [factorial]},
Children = [Server],
RestartStrategy = {one_for_one, 0, 1},
{ok, {RestartStrategy, Children}}.
How do I call it from the shell?
$ rebar3 shell
<Enter>
1> factorial:calc(5).
{ok,120}
Since this is running under a supervisor, we can still stop and restart it:
2> application:stop(factorial).
=INFO REPORT==== 22-Jan-2019::13:31:29.243520 ===
application: factorial
exited: stopped
type: temporary
ok
3> factorial:calc(5).
** exception exit: {noproc,{gen_server,call,[factorial,{calc,5}]}}
in function gen_server:call/2 (gen_server.erl, line 215)
in call from factorial:calc/1 (/Users/robert/git/factorial/src/factorial.erl, line 32)
4> application:start(factorial).
ok
5> factorial:calc(5).
{ok,120}
How do I create an executable?
Work in progress :-).