    {-| State and Reader operations specialized for working with total lenses. -}

    {-# LANGUAGE TypeOperators  #-}

    module Data.Label.Monadic
    (
    -- * 'MonadState' lens operations.
      gets
    , puts
    , modify
    , modifyAndGet
    , (=:)
    , (=.)

    -- * 'MonadReader' lens operations.
    , asks
    , local
    )
    where

    import Control.Monad
    import Data.Label.Mono (Lens)

    import qualified Data.Label.Total     as Total
    import qualified Control.Monad.Reader as Reader
    import qualified Control.Monad.State  as State

    -- | Get a value out of the state, pointed to by the specified lens.

    gets :: State.MonadState f m => Lens (->) f o -> m o
    gets = State.gets . Total.get

    -- | Set a value somewhere in the state, pointed to by the specified lens.

    puts :: State.MonadState f m => Lens (->) f o -> o -> m ()
    puts l = State.modify . Total.set l

    -- | Modify a value with a function somewhere in the state, pointed to by the
    -- specified lens.

    modify :: State.MonadState f m => Lens (->) f o -> (o -> o) -> m ()
    modify l = State.modify . Total.modify l

    -- | Alias for `puts' that reads like an assignment.

    infixr 2 =:
    (=:) :: State.MonadState f m => Lens (->) f o -> o -> m ()
    (=:) = puts

    -- | Alias for `modify' that reads more or less like an assignment.

    infixr 2 =.
    (=.) :: State.MonadState f m => Lens (->) f o -> (o -> o) -> m ()
    (=.) = modify

    -- | Fetch a value pointed to by a lens out of a reader environment.

    asks :: Reader.MonadReader f m => (Lens (->) f o) -> m o
    asks = Reader.asks . Total.get

    -- | Execute a computation in a modified environment. The lens is used to
    -- point out the part to modify.

    local :: Reader.MonadReader f m => (Lens (->) f o) -> (o -> o) -> m a -> m a
    local l f = Reader.local (Total.modify l f)

    -- | Modify a value with a function somewhere in the state, pointed to by the
    -- specified lens. Additionally return a separate value based on the
    -- modification.

    modifyAndGet :: State.MonadState f m => (Lens (->) f o) -> (o -> (a, o)) -> m a
    modifyAndGet l f =
      do (b, a) <- f `liftM` gets l
         puts l a
         return b