comprehension syntax should respect the local variables defined in if clause

[yatra9]

It will be nice if comprehension syntax respects the local variables defined in if clause.

For example, filter_map of Ruby 2.7 or Rust will be implemented as follows

filter_map(f, a) = (y for x in a if (y=f(x)) !== nothing)

Python 3.8 will introduce the assignment expressions which makes this possible.
https://docs.python.org/3.8/whatsnew/3.8.html
https://www.python.org/dev/peps/pep-0572/#simplifying-list-comprehensions

[JeffBezanson]

This is not so natural to implement in julia, since the expression y is not lexically inside the block with y=f(x), either in the source code or in what we lower it to. But this can be written as

(y for x in a for y in (f(x),) if y !== nothing)

[tkf]

I think a possible lowering of

($map_body for x in a if begin
    y1 = ...
    y2 = ...
    ...
    yn = ...
    $predicate
end)

is:

itr1 = Base.Generator(a) do x
    y1 = ...
    y2 = ...
    ...
    yn = ...
    (x, y1, y2, ..., yn)
end
itr2 = Iterators.filter(itr1) do (x, y1, y2, ..., yn)
    $predicate
end
itr3 = Base.Generator(itr2) do (x, y1, y2, ..., yn)
    $map_body
end
itr3

But this breaks the current behavior:

julia> let y
           itr = (y for x in 1:2 if (y = 2x; true))
           collect(itr), y
       end
([2, 4], 4)

[non-Jedi]

On discourse, @dhanak proposed the following syntax as an alternative:

(y for x in a let y = f(x) if y !== nothing)

Personally, I feel as if comprehension syntax is complex enough already, but I thought it worthwhile to close the loop.

[StefanKarpinski]

I have an alternative proposal here of allow trailing let clauses in comprehensions, which would let you write

(y for x in a for y in (f(x),) if y !== nothing)

a little more cleanly as:

(y for x in a let y = f(x) if y !== nothing)

This seems pretty clear to me and I can't imagine what else this would mean.

[flenzen]

Or, adopting a syntax inspired by Haskell:

(y for x in a if y !== nothing where y = f(x))