[eru]

Yes, even pure functions are not introspectable. All you can really do with a function is call it on a value and get a return value. (For the sake of simplicity, I am ignoring side effects here.)

Let me try to rephrase with an example. Eg you might have an API for a file that allows you to open a file, manipulate it, and then close it. That's a very procedural interface.

As an alternative, think of an interface like the following:

    withOpenFile(filename, manipulator)

that opens a the file, calls the manipulator function on the contents, and automatically closes it.

Or compare map, filter and reduce vs manually iterating over a collection of items.

I did really like your grandfather comment (https://news.ycombinator.com/item?id=11099224). I hope I've cleared up my point?

[anonyfox]

nitpick: in Javascript you can actually call functionName.toString() and get the implementation. AFAIK angular did this trick to implement their dependency injection mechanism.

[eru]

Yeah, but that's evil. And even with the source code, the only decidable properties of Turing complete systems are trivial. (Eg you can't even tell in general whether a given function will eventually return or loop forever from the source.)

[jstimpfle]

That doesn't show the actual values in the closure. Which are subject to change anyway.

[jstimpfle]

Thanks :-)