Generalization of 'when' to work with any Prism

prism_when.hs

{-# LANGUAGE TemplateHaskell #-}
import Control.Lens.TH
import Control.Lens.Prism

whenP :: Applicative m => APrism s t a b -> s -> (a -> m ()) -> m ()
whenP p s act = case matching p s of
  Left _ -> pure ()
  Right a -> act a

whenP' :: Applicative m => APrism s t a b -> a -> m () -> m ()
whenP' p s act = whenP p s (\_ -> act)

data Tree = Leaf | Node Tree Tree

$(makePrisms ''Tree)

errOnNil :: Tree -> Either String ()
errOnNil tree = do
  whenP' _Leaf $ Left "error: got a Nil!"