What should one do, when implementing a Generator, if the underlying data
source can experience errors? Generator.next isn’t allowed to throw, only to
return nil at the end of the sequence.
The only idea I can think of is to add an `error` property to my Generator
implementation, and request that the caller check it after the iteration ends,
and/or a `checkForError()` method that can throw an error if there is one:
var q: Query = db.query(…)
for (row in q) { … }
q.checkForError()
As the example implies, this problem comes up when creating Swift bindings for
database iterators/cursors. Generator is the obvious protocol to implement,
especially since without it you don’t get the idiomatic for/in loop, but the
`next` method does have to hit the database, so it has the possibility of I/O
errors.
Another possibility is to do all the failable database work up front in the
`query` method that creates the Generator, but this means all the results have
to be cached in memory, which isn’t scaleable. (Keep in mind I work for a
database company <http://couchbase.com/> whose customers often have
multi-terabyte data sets.)
I know there’s been a proposal (by Brent Royal-Gordon) to allow property
accessors and subscripts to throw, which I strongly endorse, but I think we
also need to allow Generators to throw. Fodder for the swift-evolution list…
—Jens_______________________________________________
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