as I understand it...
a callback will be a method of the "base program" checking on the
"asynchronous" program to see if it is done, and to receive the results...
It isn't limited to "asynchronous" languages, or to multi-threaded
languages. And indeed, the concept, if it's the one I think you're
talking about, is used fairly extensively in things like Scott Klement's
This is just my understanding of the concept I *think* you're referring
to, but . . .
Imagine the following situation:
You're calling an API of some sort, that does some standardized process,
and produces results of some sort, and it defaults to, say, pouring
those results into a stream file in the IFS. But instead of that you
want to process those results directly in your own code, as they arrive.
So you pass, as one of the parameters to the API call, a procedure in
your own code, which the API then calls, whenever it has results for
your program to process.