"for z in zip()" doesn't terminate

[stevengj]

for z in zip()
    @show z
end

is an infinite loop, printing z = () over and over.

Arguably, it should execute once with z = (), or maybe throw an error, or maybe execute zero times (like Python), but in any case the current behavior seems bad.

[tkf]

I'd say that the behavior of zip is correct. Observe that zip is a monoid in the sense that zip(a, zip(b, c)) and zip(zip(a, b), c) are equivalent modulo how the elements are nested in the resulting element tuples (it's possible to write define an equivalence relation ==′ by ignoring the difference between (x, (y, z)) and ((x, y), z)). Furthermore, like other monoids, it defines the vararg method satisfying zip(args...) ==′ reduce(zip, args) when args is nonempty. From this observation, zip() should be the identity element; i.e., zip(zip(), a) ==′ zip(a). An empty stream of ()s satisfies this. Indeed,

julia> for (_, x) in zip(zip(), 1:3)  # this is like `0 + 3`
           @show x
       end
x = 1
x = 2
x = 3

From this point of view, the definition foreach(f) = (f(); nothing) is wrong. It would have been OK if the vararg extension of foreach used product and not zip.

Ref: #35293

[Moelf]

@tkf ehh... why would 99.9% of the users care if zip is a monoid? and did we promise it was a monoid?

in fact there's a single (place) mention of "monoid" in the entire Julia base:

julia/doc/src/manual/strings.md

Lines 526 to 528 in df5b081

	More precisely, the set of all finite-length strings *S* together with the string concatenation operator
	`*` forms a [free monoid](https://en.wikipedia.org/wiki/Free_monoid) (*S*, `*`). The identity element
	of this set is the empty string, `""`. Whenever a free monoid is not commutative, the operation is

I'm not saying what you're saying is wrong / irrelevant, just asking from a typical user perspective. If you have a user of Julia asking (e.g. a grad student from not CS) "why does empty zip have infinite length", would you give them the above speech?

[rafaqz]

Maybe a simpler grad-student ready explanation is "zip() is like a functional while true".

[Moelf]

and what is zip(a,b)?.... this doesn't make sense with the "zip things together" metaphor, zip(something, something) is finite length but zip nothing is infinite length?

[jariji]

zip(xss...) stops when any of its inputs stops. If xss is empty, then none of its inputs ever stops, so the overall expression doesn't terminate.

Adding inputs can only make it stop sooner.

This is the same kind of reasoning that explains

julia> all(!isempty, Vector[])
true

For the opposite behavior, see zip_longest, where adding inputs can only make it stop later.

[LilithHafner]

If this is a common error, perhaps we can make zip() throw in 2.0, otherwise, I don't think there is much to be done about this.

[Moelf]

If xss is empty, then none of its inputs ever stops,

by this logic for x in () or for x in 1:1 should infinite loop

[Moelf]

zip() throw in 2.0,

I'm 99% sure nobody actually relies on this producing infinite loop, we can fix this as a minor change in 1.x

[jariji]

for x in () iterates length(()) times and for x in 1:1 iterates length(1:1) times. I don't see why either would be infinite?

julia> collect(reduce(zip, [1:3, 4:6]; init=zip()))
3-element Vector{Tuple{Tuple{Tuple{}, Int64}, Int64}}:
 (((), 1), 4)
 (((), 2), 5)
 (((), 3), 6)

This code seems fine to me, I wouldn't try to ban it.

[LilithHafner]

I'm 99% sure nobody actually relies on this producing infinite loop, we can fix this as a minor change in 1.x

Feel free to make a PR; we can check with PkgEval.

[Moelf]

@jariji

zip(xss...) stops when any of its inputs stops

because I would describe iterating 1:1 or () as "inputs stops"

btw the current behavior comes from #29238 with no discussion on this specific point