clarify set API

[JeffBezanson]

I suggest we remove "cute" definitions such as

|(s::Set...) = union(s...)

and add a subset or issubset function to go with contains. This makes it possible for Arrays to fully implement the set API without confusion.

ref #1963

[StefanKarpinski]

I concur.

[johnmyleswhite]

This seems like a clear win. If we were going to have anything cute, we could use unicode characters for set theoretic operations.

[StefanKarpinski]

Oh, that would be nice. I like that generic functions force us to be so careful about not mixing different meanings wantonly into the same operator/function. It's so tempting to pun on these things because other languages have. I feel a bit remorseful about some of our uses of the pipe – function chaining and command pipelines. But at last for chaining pipelines, it seems like the syntax is worth it.

[johnmyleswhite]

I personally would prefer that | were replaced with something less ambiguous, like ==>, but agree that the pipes are probably worth keeping despite being internally inconsistent.

[StefanKarpinski]

In the comman case, we could get rid of them by pushing them inside of the backitcks. That's a big step though because it turns backitcks into a real language instead of just a way express arrays of strings with interpolated expressions (which is already pretty complicated).

[johnmyleswhite]

That case seems particularly worth allowing since the shell tradition encourages thinking about pipes.

[JeffBezanson]

We could use |> as a pipe operator.

[pao]

We could use |> as a pipe operator.

Elixir is using this notation, so it's not entirely crazy. (That might be why you brought it up.)

[StefanKarpinski]

Yep, I sent Joe Armstrong's post about Elixir to Jeff yesterday, so I suspect that might be the impetus. I would be ok with that, although I wouldn't mind making => a more general operator or using ==>.

[toivoh]

I still would like to see \/ and /\ operators for set union and intersection.

[StefanKarpinski]

I'd rather just add support for ⋃ and ⋂ as Unicode operators with appropriate precedence and everything.

[toivoh]

That could be ok too. I'll have to figure out how to type them.

[ViralBShah]

I will register my vote against unicode operators once more here, as I have done before.