Base.isdone for Stream

[baumgold]

Implement Base.isdone for Arrow.Stream

https://github.com/JuliaLang/julia/blob/v1.8.5/base/essentials.jl#L882-L895

This enables calling Base.isempty(::Arrow.Stream) without affecting the state of the Arrow.Stream object.

[quinnj]

Hmmmm, I'm not sure I'm following the changes or reasoning here. Can you explain how isempty changes the current Arrow.Stream state? I'm not super familiar w/ Base.isdone or how it's used for other types or why; if you could share an example, that would be great. Another question that comes to mind is why we shouldn't define isempty(::Arrow.Stream) ourselves if it's causing problems?

[baumgold]

@quinnj - Iterating on Arrow.Stream modifies the inputindex field, making it stateful iteration rather than stateless. The default implementation for Base.isempty uses iteration to check if there are further elements remaining:

https://github.com/JuliaLang/julia/blob/v1.8.5/base/essentials.jl#L785-L789

Unfortunately this doesn't work for stateful iterators since calling Base.isdone modifies the iterator's state. A solution to this is to define Base.isdone:

https://github.com/JuliaLang/julia/blob/v1.8.5/base/essentials.jl#L882-L895

Note the doc-string explains this quite clearly:

This function provides a fast-path hint for iterator completion. This is useful for mutable iterators that want to avoid having elements consumed, if they are not going to be exposed to the user (e.g. to check for done-ness in isempty or zip). Mutable iterators that want to opt into this feature should define an isdone method that returns true/false depending on whether the iterator is done or not. Stateless iterators need not implement this function. If the result is missing, callers may go ahead and compute iterate(x, state...) === nothing to compute a definite answer.