[GitHub] [arrow-julia] baumgold commented on pull request #428: Base.isdone for Stream

["baumgold (via GitHub)" <gi...@apache.org>]

baumgold commented on PR #428:
URL: https://github.com/apache/arrow-julia/pull/428#issuecomment-1520107608

   @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.


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