How to launch an external procedure in OpenEdge Progress-4GL - include

Question

While learning about OpenEdge Progress-4GL, I stumbled upon running external procedures, and I just read following line of code, describing how to do this:
RUN p-exprc2.p.
For a person with programming experience in C/C++, Java and Delphi, this makes absolutely no sense: in those languages there is a bunch of procedures (functions), present in external files, which need to be imported, something like:
filename "file_with_external_functions.<extension>"
===================================================
int f1 (...){
return ...;
}
int f2 (...){
return ...;
}
filename "general_file_using_the_mentioned_functions.<extension>"
=================================================================
#import file_with_external_functions.<extension>;
...
int calculate_f1_result = f1(...);
int calculate_f2_result = f2(...);
So, in other words: external procedures (functions) mean that you make a list of procedures (functions), you put all of them and in case needed, you import that file and launch the procedure (function) when you need it.
In Progress 4GL, it seems you are launching the entire file!
Although this makes no sense at all in C/C++, Java, Delphi, I believe this means that Progress procedure files (extension "*.p") only should contain one procedure, and the name of the file is then the name of that procedure.
Is that correct and in that case, what's the sense of the PERSISTENT keyword?
Thanks in advance
Dominique

Answer 1

There are a lot of options to the RUN statement: https://documentation.progress.com/output/ua/OpenEdge_latest/index.html#page/dvref%2Frun-statement.html%23
But, in the simple case, if you just:
RUN name.p.
You are invoking a procedure. It might be internal, "super", "persistent" or external. It could also be an OS DLL.
The interpreter will first search for an internal procedure with that name. Thus:
procedure test.p.
message "yuck".
end.
run test.p.
Will run the internal procedure "test.p". A "local" internal procedure is defined inside the same compilation unit as the RUN statement. (Naming an internal procedure with ".p" is an abomination, don't do it. I'm just showing it to clarify how RUN resolves names.)
If a local internal procedure is not found then the 4gl interpreter will look for a SESSION SUPER procedure with that name. These are instantiated by first running a PERSISTENT procedure.
If no matching internal procedure or SUPER procedure is found the 4gl will search the PROPATH looking for a matching procedure (it will first look for a compiled version ending with .r) and, if found, will RUN that.
There are more complex ways to run procedures using handles and the IN keyword. You can also pass parameters and "compile on the fly" arguments. The documentation above gets into all of that. My answer is just covering a simple RUN name.p.

Answer 2

Progress was originally implemented as a procedural language which did it's thing by running programs. That's what you're seeing with the "run" statement.
If one was to implement this in OO, it'd look something like this:
NEW ProgramName(Constructor,Parameter,List).
Progress added support for OO development which does things in a way you seem more familiar with.