For the downvoters, I think giving examples of what's NOT a function would start an interesting conversation, especially if you don't know how it could possibly be interesting!
I mean trivially you could say it's a function from (entire machine state) to (entire machine state), but typically we ignore the trivial solution because it's not interesting.
Even non-functional relations can be turned into functions (domain has to change). Like a circle, which is not a function of the x-axis, can be parameterized by an angle theta (... `0 <= theta < 2pi`)
The answer is pretty much, yes, everything can be a function. e.g. A KV Map can be a function "K -> Maybe V"
P.S. If this style of thinking appeals to you, go read Algebra Driven Design! https://leanpub.com/algebra-driven-design
it's a bit like saying "everything is a process", or "everything is a part of the same singular process that has been playing out since observable history". there's some interesting uses you can get out of that framing, but it's not generally applicable in the way that something like "all programs are unable to tell when a process will halt" is.
but if you really want to harvest the downvotes, I haven't found a better lure than "everything, including the foundations of mathematics, is just a story we are telling each other/ourselves." I know it's true and I still hate it, myself. really feels like it's not true. but, obviously, that's just the english major's version of "everything is a function".
An fexpr is a function.
> An fexpr is a function.
Try to implement a short circuiting logical `and` operator as a function :-)