My worry is that it would be impossible to debug the internal expression inside the for without some sort of inline text debugger though, wouldn't it? Even with the <exp>?<T>:<F> that TS currently has, debuging nested clauses is a big deal, and folks are steered strongly to IF statement steps.
That being said, alot of the need for a FOR in a expression is because of non trivial ability to do math/copy data on arrays that requires writing very simple (but annoying) code fragments to handle it or a subsequence dedicated to the looping.
What if TS just provided a series of common utility subsequences for these sorts of "I need to take this array and implement X expression against every element" type thing? Immediate problem I can see is what is 'common' in this usecase....
