Hello, I'm not sure if this is what you're looking for, but vinyl + lens state monad combinators let you write something like this: foo :: State (Rec Foo [B,C,F]) Bar foo = ... bar = State (Rec Foo [A,B,C,D,E,F] bar = do ... x <- zoom rsubset foo ... rlens SA .= 3 ... Unfortunately Vinyl has O(n) lookup (unless it gets optimized away by sufficiently glorious haskell compiler, I guess, but I have no idea whether it actually can happen). But I'm not sure if the speed impact is noticeable, compared to using monad transformer stacks, for example. http://hackage.haskell.org/package/vinyl http://hackage.haskell.org/package/vinyl-0.5.1/docs/Data-Vinyl-Lens.html http://hackage.haskell.org/package/lens https://www.fpcomplete.com/school/to-infinity-and-beyond/pick-of-the-week/a-... Best regards, Marcin Mrotek