r' or 'Lens').
--
-- This is usually applied in the 'Control.Monad.Reader.Reader'
-- 'Control.Monad.Monad' @(->) s@.

-- @
-- 'previews' = 'views' '.' 'pre'
-- @
--
-- @
-- 'previews' :: 'Getter' s a     -> (a -> r) -> s -> 'Maybe' r
-- 'previews' :: 'Fold' s a       -> (a -> r) -> s -> 'Maybe' r
-- 'previews' :: 'Lens'' s a      -> (a -> r) -> s -> 'Maybe' r
-- 'previews' :: 'Iso'' s a       -> (a -> r) -> s -> 'Maybe' r
-- 'previews' :: 'Traversal'' s a -> (a -> r) -> s -> 'Maybe' r
-- @
--
-- However, it may be useful to think of its full generality when working with
-- a 'Monad' transformer stack:
--
-- @
-- 'previews' :: 'MonadReader' s m => 'Getter' s a     -> (a -> r) -> m ('Maybe' r)
-- 'previews' :: 'MonadReader' s m => 'Fold' s a       -> (a -> r) -> m ('Maybe' r)
-- 'previews' :: 'MonadReader' s m => 'Lens'' s a      -> (a -> r) -> m ('Maybe' r)
-- 'previews' :: 'MonadReader' s m => 'Iso'' s a       -> (a -> r) -> m ('Maybe' r)
-- 'previews' :: 'MonadReader' s m => 'Traversal'' s a -> (a -> r) -> m ('Maybe' r)
-- @
previews :: MonadReader s m => Getting (First r) s a -> (a -> r) -> m (Maybe r)
previews l f = asks (getFirst . foldMapOf l (First #. Just . f))
{-# INLINE previews #-}

-- | Retrieve a function of the first index and value targeted by an 'IndexedFold' or
-- 'In
previews :: MonadReader s m => Getting (First r) s a -> (a -> r) -> m (Maybe r)
previews l f = asks (getFirst . foldMapOf l (First #. Just . f))
previews l f = asks (getFirst . foldMapOf l (First #. Just . f))