zip2 and eachline don't play nice because next() is called twice

[swadey]

Suppose you have test.txt which contains:

1
2
3
4
5

Running this code:

f1   = open("test.txt")
letters = ['a', 'b', 'c', 'd', 'e']
for (number, letter) in zip(eachline(f1), letters)
  println(strip(number), " ", letter)
end
close(f1)

Results in this:

[splunk:~/projects/julia]⇒ julia ziptest.jl
1 a
3 b
5 c

This seems wrong and it's caused by the fact that the next() method of EachLine changes state but it's called twice in Zip2 causing one state to be skipped per iteration:

next(z::Zip2, st) = ((next(z.a,st[1])[1], next(z.b,st[2])[1]),                                                                                                                        
                     (next(z.a,st[1])[2], next(z.b,st[2])[2])) 

Since next() can, in general have side effects, the I'm assuming the iterator protocol requires that next() only be called once per iteration, so I've submitted a corresponding PR #7370 (with the corresponding test). If that's not true, then EachLine should be fixed.

[IainNZ]

See also #6284 which I think is related in spirit

[swadey]

right. It's the same problem but in a different part of iterators.jl. I guess it should be corrected everywhere or next() should be side-effect free everywhere.

[JeffBezanson]

fixed by #7370