Erlang process gives undef error when running code from two modules - erlang

Question

I have two files, processes and calc(that calculates the area of depending upon the shape). I am new at Erlang and right now, I just want to run processes file which create the process and invokes the area() from the calc file/ module.
The code is as follows:
calc.erl:
- module(calc).
- export([start/0, area/0]).
- import(io, [fwrite/1]).
start() ->
% Pid = spawn(fun() -> loop(Args here) end),
PID = spawn(calc, area, []),
io:fwrite("PID: ~w", [PID]).
% PID ! {self(), {rectangle, 10, 20}},
area() ->
receive
{From , {rectangle, X, Y}} ->
From ! {rectangle, X*Y};
{From, {square, X}} ->
io:fwrite("in the Square area!"),
From ! {square, X*X}
end,
area().
processes.erl:
-module(processes).
-export([execute/0]).
-import(io,[fwrite/1]).
-import(calc, [area/0]).
execute() ->
PID = spawn(processes, area, []),
Square_Area = PID ! {self(), {square, 10}},
receive
{rectangle, Area} ->
io:fwrite("Rectangle area: ~w", [Area]);
{square, Area} ->
io:fwrite("Square area: ~w", [Area]);
Other ->
io:fwrite("In Other!")
end,
io:fwrite("Yolo: ~w", [Square_Area]).
When I run the command processes:execute(). after compiling and running the processes.erl file, I get the following error:
=ERROR REPORT==== 4-Sep-2022::20:24:26.720042 ===
Error in process <0.87.0> with exit value:
{undef,[{processes,area,[],[]}]}
Is this because the second file is not being loaded or am I writing wrong commands? Any help will be appreciated!

Answer 1

Let's check the error: It says
{undef, [{processes, area, [], []}]}
That means that the function area with no arguments ([]) is not defined (undef) in the module processes. To be precise, what that error means is that such a function is not exported from that module.
Which is correct. The function is defined in the module calc, right?
So, if you change…
PID = spawn(processes, area, []).
…to…
PID = spawn(calc, area, []).
…you should be fine :)
ℹ Extra Tips
You don't need to import anything (like io:fwrite/1 or calc:area/0 in order to use them - particularly, since you're using them in a fully qualified manner). import in Erlang has a different meaning and, in practice, its usage is not recommended at all.
Using Camel_Case for variables is not wrong, but it's more idiomatic to use…
PascalCase for variables
snake_case for atoms (including function and module names)
SCREAMING_SNAKE_CASE for macros