Andrey Popp writes:
[(y, y) for y in (f(x) for x in some_iterable) if y < 2]
means producing a list by iterating over the generator, which iterates over the some_iterable, it is correct in algorithmic way, but not in semantic.
That's only true if you think of "iterable" as a generalized (ie, possibly infinite) sequence. However, you can also think of this, not as "iterating over a sequence of values created by iterating over another sequence of values," but rather "iterating 'application' over a sequence of filters." Then the "for" clause functions as a "where". The first "for" in Ben's expression is interpreted as "where y = f(x)", and the second "for" is interpreted as "where x = next(some_iterable)".
This is not fully general; it only works at all in a comprehension context, and I'm not entirely sure it works perfectly here. But it troubles me that we already have a way to say "where" at the upper levels of nesting, and you want to introduce a new "where" that is only useful at upper levels of nesting.
Another way to put this is that because these are iterables (streams) rather than sequences (ordered (finite) sets), it doesn't make sense to talk about "double interation." There are multiple levels of iterable here, but in the end there's only one iterative process, and all the levels share the same "iteration".