# An iterator that maps each item to an iterator and yield
# each item from it.
class FlatMapIter[A,B]
        super FunIter[A,B]
        var f: Fun1[A, Iterator[B]]
        protected var inner: nullable Iterator[B] = null

        redef init
        do
                try_compute_inner
        end

        redef fun item
        do
                return inner.as(not null).item
        end

        redef fun is_ok
        do
                return inner != null
        end

        redef fun next
        do
                inner.next
                if not inner.is_ok then
                        super
                        try_compute_inner
                end
        end

        # Tries to resolve the inner iterator.
        # Assigns null to `inner` if it fails.
        protected fun try_compute_inner
        do
                inner = null
                if not my_iter.is_ok then return
                var res = f.call(my_iter.item)
                if res.is_ok then inner = res
        end
end