Can you define what you mean by "flattening" and "arbitrary depth"? My understanding of these things does not lead me to see unresolved problems in the original post. (Also, I do not believe that the NestedLoops code you provided is equivalent to seq_prod.)

In the OP's example while loop, the iterator returns scalar values. Thus I don't see how the seq_prod of such iterators could "flatten" the results: there are no intermediate array results to flatten.

On the arbitrary depth issue, if you mean that iter_prod works only for a fixed number of iterators, that is not the case. You can pass it any number of iterators, and (as long as they are independent) it will yield an iterator over their Cartesian product. If you mean something else, say that the Cartesian product is not what is desired, we can derive a suitable combinator to replace seq_prod2 and use it (with reduce) to build the desired output iterator. For example, we could define a combinator that takes the product of two iterators when the second is generated dynamically from the output of the first.

Cheers,
Tom