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{-# LANGUAGE CPP #-}
#if __GLASGOW_HASKELL__ >= 702
{-# LANGUAGE Safe #-}
#endif
#if __GLASGOW_HASKELL__ >= 710
{-# LANGUAGE AutoDeriveTypeable #-}
#endif
-----------------------------------------------------------------------------
-- |
-- Module : Control.Applicative.Lift
-- Copyright : (c) Ross Paterson 2010
-- License : BSD-style (see the file LICENSE)
--
-- Maintainer : R.Paterson@city.ac.uk
-- Stability : experimental
-- Portability : portable
--
-- Adding a new kind of pure computation to an applicative functor.
-----------------------------------------------------------------------------
module Control.Applicative.Lift (
-- * Lifting an applicative
Lift(..),
unLift,
mapLift,
-- * Collecting errors
Errors,
runErrors,
failure
) where
import Data.Functor.Classes
import Control.Applicative
import Data.Foldable (Foldable(foldMap))
import Data.Functor.Constant
import Data.Monoid (Monoid(..))
import Data.Traversable (Traversable(traverse))
-- | Applicative functor formed by adding pure computations to a given
-- applicative functor.
data Lift f a = Pure a | Other (f a)
instance (Eq1 f) => Eq1 (Lift f) where
liftEq eq (Pure x1) (Pure x2) = eq x1 x2
liftEq _ (Pure _) (Other _) = False
liftEq _ (Other _) (Pure _) = False
liftEq eq (Other y1) (Other y2) = liftEq eq y1 y2
{-# INLINE liftEq #-}
instance (Ord1 f) => Ord1 (Lift f) where
liftCompare comp (Pure x1) (Pure x2) = comp x1 x2
liftCompare _ (Pure _) (Other _) = LT
liftCompare _ (Other _) (Pure _) = GT
liftCompare comp (Other y1) (Other y2) = liftCompare comp y1 y2
{-# INLINE liftCompare #-}
instance (Read1 f) => Read1 (Lift f) where
liftReadsPrec rp rl = readsData $
readsUnaryWith rp "Pure" Pure `mappend`
readsUnaryWith (liftReadsPrec rp rl) "Other" Other
instance (Show1 f) => Show1 (Lift f) where
liftShowsPrec sp _ d (Pure x) = showsUnaryWith sp "Pure" d x
liftShowsPrec sp sl d (Other y) =
showsUnaryWith (liftShowsPrec sp sl) "Other" d y
instance (Eq1 f, Eq a) => Eq (Lift f a) where (==) = eq1
instance (Ord1 f, Ord a) => Ord (Lift f a) where compare = compare1
instance (Read1 f, Read a) => Read (Lift f a) where readsPrec = readsPrec1
instance (Show1 f, Show a) => Show (Lift f a) where showsPrec = showsPrec1
instance (Functor f) => Functor (Lift f) where
fmap f (Pure x) = Pure (f x)
fmap f (Other y) = Other (fmap f y)
{-# INLINE fmap #-}
instance (Foldable f) => Foldable (Lift f) where
foldMap f (Pure x) = f x
foldMap f (Other y) = foldMap f y
{-# INLINE foldMap #-}
instance (Traversable f) => Traversable (Lift f) where
traverse f (Pure x) = Pure <$> f x
traverse f (Other y) = Other <$> traverse f y
{-# INLINE traverse #-}
-- | A combination is 'Pure' only if both parts are.
instance (Applicative f) => Applicative (Lift f) where
pure = Pure
{-# INLINE pure #-}
Pure f <*> Pure x = Pure (f x)
Pure f <*> Other y = Other (f <$> y)
Other f <*> Pure x = Other (($ x) <$> f)
Other f <*> Other y = Other (f <*> y)
{-# INLINE (<*>) #-}
-- | A combination is 'Pure' only either part is.
instance (Alternative f) => Alternative (Lift f) where
empty = Other empty
{-# INLINE empty #-}
Pure x <|> _ = Pure x
Other _ <|> Pure y = Pure y
Other x <|> Other y = Other (x <|> y)
{-# INLINE (<|>) #-}
-- | Projection to the other functor.
unLift :: (Applicative f) => Lift f a -> f a
unLift (Pure x) = pure x
unLift (Other e) = e
{-# INLINE unLift #-}
-- | Apply a transformation to the other computation.
mapLift :: (f a -> g a) -> Lift f a -> Lift g a
mapLift _ (Pure x) = Pure x
mapLift f (Other e) = Other (f e)
{-# INLINE mapLift #-}
-- | An applicative functor that collects a monoid (e.g. lists) of errors.
-- A sequence of computations fails if any of its components do, but
-- unlike monads made with 'ExceptT' from "Control.Monad.Trans.Except",
-- these computations continue after an error, collecting all the errors.
--
-- * @'pure' f '<*>' 'pure' x = 'pure' (f x)@
--
-- * @'pure' f '<*>' 'failure' e = 'failure' e@
--
-- * @'failure' e '<*>' 'pure' x = 'failure' e@
--
-- * @'failure' e1 '<*>' 'failure' e2 = 'failure' (e1 '<>' e2)@
--
type Errors e = Lift (Constant e)
-- | Extractor for computations with accumulating errors.
--
-- * @'runErrors' ('pure' x) = 'Right' x@
--
-- * @'runErrors' ('failure' e) = 'Left' e@
--
runErrors :: Errors e a -> Either e a
runErrors (Other (Constant e)) = Left e
runErrors (Pure x) = Right x
{-# INLINE runErrors #-}
-- | Report an error.
failure :: e -> Errors e a
failure e = Other (Constant e)
{-# INLINE failure #-}
|
