Loops are a lot of fun in Common Lisp, what with LOOP (https://gigamonkeys.com/book/loop-for-black-belts.html), ITERATE (https://common-lisp.net/project/iterate/doc/Don_0027t-Loop-I...), and FOR (https://shinmera.github.io/for/). But their fundamental problem of trying to extract a "bird's eye view" of what you're actually trying to do remains here.

Maybe a new iteration construct specifically supporting parallelization would help here. Some of these things might already work quite well, for example it should not be difficult to discern from (let me use ITERATE's example)

    (iterate (for el in list)
             (finding el minimizing (length el)))

that you're computing argmin using a pure function, which should be easy to do in parallel, whereas the way you'd typically write this in C would be probably difficult for the compiler to confirm as such (it would have to look for pattern such as "if (f(current)<f(best)) { best=current; }" or something like that. (BTW have you seen another loop construct supporting argmin/argmax in another language? I haven't so far.) Whether this could be done somewhat generally and extensibly is something I have to ponder on.