What exactly is "the whole point of treating functions as data"?
Of course you can declare everything is data. You don't need higher order functions for that. But real data is simple and introspectable. Computation is not.
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.
nitpick: in Javascript you can actually call functionName.toString() and get the implementation. AFAIK angular did this trick to implement their dependency injection mechanism.
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.)
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:
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?