Lenses are not always better:
fresh :: FDState s c -> (Int, FDState s c)
fresh = id &&& id
>>> first (^. nextId)
>>> (\(i, s) -> (i, s & alive %~ Set.insert i))
>>> second (nextId %~ (+ 1))
fresh :: FDState s c -> (Int, FDState s c)
fresh s@FDState {_nextId = i, _alive = as} =
(i, s {_nextId = i + 1, _alive = Set.insert i as})
That’s not really an argument against lenses so much as an argument against extreme point-free style.
fresh :: FDState s c -> (Int, FDState s c) fresh s = (i, inc_s & alive %~ Set.insert i) where (i, inc_s) = s & nextID <<+~ 1
Edit: Fixed some of the operators being replaced with HTML entities. Edit 2: Hmm, they keep showing up fine in Preview but then getting replaced again. I’m not sure what I’m supposed to do to make them work.