or, to steal ashley yakeley's marvellous quip, "every sufficiently well-documented lisp program contains an ML program in its comments" (:

So there's some common ground between map, filter, flatmap, etc and these transducers allow you to compose those common-ground representations? I mean, you can define `map` as a callable chain of `(key-by #(true)) -> (map-apply fn) -> (filter-trues)` and `filter` as `(key-by predicate) -> (map-apply identity) -> (filter-trues)` where key-by, map-apply and filter-trues are some low-level primitives? Is that correct?

More precisely:

  -- Left reduce
  type Reducer a r = r -> a -> r

  -- Here's where then rank-2 type is needed
  type Transducer a b = forall r . Reducer a r -> Reducer b r