Selecting

Main.hs

import Control.Lens
import Data.Monoid
import Data.List.Extra
import Data.Foldable

select as fbs f s =
  flip as s $ \a -> a <$ (
    flip (fbs a) s $ \b ->
      snd <$> f (a,b)
  )

selecting as fbs = 
  (fmap.fmap $ fold) . groupSort . toListOf (select as fbs)

example :: [(Bool, Sum Int)]
example = ([True,False], [1..10])
  & selecting (_1 . traversed)
          ( \b ->
              _2 . traversed
              . filtered ((==b) . even)
              . to Sum
          )

{- result:
[
    ( False
    , Sum
        { getSum = 25 }
    )
,
    ( True
    , Sum
        { getSum = 30 }
    )
]
-}