Better list comprehension about m2 HOT 7 OPEN

We already have f \ L and L / f as shortcuts for apply(L, f). That seems pretty compact!

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Compactness is not the main problem, mostly proximity to math notation and readability. directSum(L / a -> (...)) seems worse to me.

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To be more specific, here's why I don't think f \ L and L / f are any better:

i1 : max({1,2,3} / i -> 10*i)
stdio:1:13:(3): error: expected parenthesized argument list or symbol

Maybe this is solvable independently, but even then I think L / f and f \ L are good for piping and other purposes, not set comprehension.

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-> has really low precedence, so maybe adding a new operator with even lower precedence would work. But I'm not sure what that would be. The natural choices I think would be : or | to look like set-builder notation, but both already exist in the language with higher precedence.

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Personally, I generally prefer the L/f, f/L notation to list comprehensions in e.g. python, not just due to compactness, but also readability, although of course, this kind of like/dislike is a personal judgement. I suggest we try to improve usability of these notions to fix the kind of issues @mahrud found.

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Maybe the examples I gave weren't good ones, but I don't consider L/f and f\L to be list comprehension at all. For instance, here's an example from python:

fruits = ["apple", "banana", "cherry", "kiwi", "mango"]
newlist = [x for x in fruits if "a" in x]

We can't do this with L/f or even apply alone, we need select. This is what I think of as list comprehension: (ignore that this one is infinite)

Haskell's is pretty nice:

s = [ 2*x | x <- [0..], x^2 > 3 ]

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In light of #1978, it would be great if the following were equivalent:

{ 2^x for x in {1..5,3,2:2} if odd x } -- syntactic sugar akin to { 2^x | x \in {1,2,3,4,5,3,2,2}, odd x }
for x in splice(1..5,3,2:2) list if odd x then x^2 else continue

The benefit is that if we wanted an array, we could simply switch to:

[2^x for x in {1..5,3,2:2} if odd x]

As a first step, at least this should work:

for x in splice(1..5,3,2:2) if odd x then x^2

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