Added some macro iterators

[carlobaldassi]

As per the discussion in 1bbeeee.

This adds macro versions of

• zip
• enumerate
• take/takestrict
• drop
• chain

which work on for loops. They all follow the same general syntax: whenever one would write

for VARS in ITERATOR(ARGUMENTS)
  BODY
end

the corresponding macro version is:

@ITERATOR for VARS in ARGUMENTS
  BODY
end

[timholy]

Really beautiful. I made a trivial variable-naming comment simply to prove I had looked at it 😄. I say merge at will.

[kmsquire]

I really like the idea here. I'd like to offer a thought on the interface.

Having the iterator name out front is necessary (because it's a macro, after all), but somewhat harder to read, in my opinion. What about something like

@zip for (x,y) in zip(a,b)
   @show x,y
end

Or, just using one macro for all iterators

@iterate for (x,y) in zip(a,b)
   @show x,y
end

In the first, the zip is obviously extraneous, but it's easier to see what's happening (and easier to switch back to the non-macro version).

The second would be harder to implement, and may not be worth it, but the syntax is easy (and also makes it easier to switch back to the non-macro version).

Thoughts?

[kmsquire]

Okay, @iterate doesn't work as a prefix, because there's already an iterator by that name. (Although I would prefer that one be named something like applyiter.)

[StefanKarpinski]

How about @itr? It's sort of an annotation that the thing that's coming is a for loop over an iterator that should be transformed.

[kmsquire]

I like @itr.

[carlobaldassi]

Using @itr to dispatch to the correct macro seems a good idea, I'll try that.

[carlobaldassi]

@itr macro implemented.

[kmsquire]

LGTM

[carlobaldassi]

Ok I'll squash, clean up a bit and merge.

[timholy]

Yay! Really nice to have this in Iterators. Will presumably get much more attention that way.