Bikeshedding filtered generator syntax

[ararslan]

This started coming up in #16389 but it's a little off topic, so perhaps we should branch this discussion out into a separate issue.

@StefanKarpinski mentioned the nice, familiar syntax

x^2 if x % 3 == 0 for x = 1:100

which follows more closely what other languages do.

I had also proposed using where, e.g.

x^2 for x = 1:100 where x % 3 == 0  # or,
x^2 where x % 3 == 0 for x = 1:100

This, at least in my mind, makes the filtering aspect a little clearer than if, since if looks like it should be modifying the computation rather than the iterator. (Which perhaps is a little narrow-minded of me.)

Thoughts?

[ararslan]

I could also imagine where extending to other scenarios, e.g.

s = 0
for x = 1:100 where x % 3 == 0
    s += 1
end

This super contrived example would be equivalent to

s = 0
for x = 1:100
    if x % 3 == 0
        s += 1
    end
end

(or whatever single-line if comes about from #16389)

[JeffBezanson]

This can be moved to #550. We are also likely to use where for #11310.

[ararslan]

@JeffBezanson Okay, sounds good. Thanks!