Callbacks are a signal that your operation is done, and the data is ready... usually they have some kind of state (i.e. success, error, the data read from disk

Callbacks are a signal that your operation is done, and the data is ready... usually they have some kind of state (i.e. success, error, the data read from disk/api, etc). They are usually used within one program itself, and not across systems. Something like the Firebase real time database, or webhooks, are for callback achross systems, but they always are used to pass data/state, and not functions itself. Again, SQL and maybe Regex (which is not a full programming language anyway), as the most successful alternatives to normal Object Oriented/procedural programing. Most functional languages have not become successful. I learned Prolog in the 90s, and I could see how it might be useful in some situations, but it is hard to do anything practical with it. Some functional concepts, like map/reduce, have become popular in mainstream languages, as they are useful in certain scenarios, but pure functional languages have not.