question: Can Done() be called before Interator is has finished?

[nemith]

I am implementing an iterator that reads from a channel and I want to make sure that the go routine is completed. Is there ever a case were Done() would be called before finishing the iterator (i.e Next() returns false)?

https://pkg.go.dev/go.starlark.net/starlark#Iterator

[alandonovan]

Yes, the client of an iterator is under no obligation to read to the end, so in a loop such as this:

iter := iterable.Iterator()
defer iter.Done()
var x Value
for iter.Next(&x) {
    ...body...
}

if the body contains a break statement, Done will be called without Next ever having returned false.

However, the client is obliged to call Done, so you could cancel your goroutine there.