EnvT use case

gistfile1.txt

import Data.Tree
import Control.Comonad
import Control.Comonad.Trans.Class
import Control.Comonad.Trans.Env 

replusify:: MonadPlus m => [a] -> m a
replusify = msum . map return

class Treeish l where
    children :: MonadPlus m => l -> m l
    descendants :: MonadPlus m => l -> m l

instance Treeish (Tree a) where
    children = replusify . subForest
    descendants = replusify . flatten . duplicate

instance Treeish (EnvT e Tree a) where
    children  = replusify . map rootLabel . subForest . lower . duplicate
    descendants = replusify . flatten . lower . duplicate