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{-# LANGUAGE RankNTypes, DeriveGeneric, TypeApplications, DataKinds, ExistentialQuantification, TemplateHaskell #-}
{-# OPTIONS_GHC -dsuppress-idinfo #-}
module GenericLens (main) where
import GHC.Generics
import Data.Generics.Product
import Test.Inspection
data Record = MkRecord { fieldA :: Int
, fieldB :: Bool
} deriving Generic
type Lens' s a = forall f. Functor f => (a -> f a) -> s -> f s
fieldALensManual :: Lens' Record Int
fieldALensManual f (MkRecord a b) = (\a -> MkRecord a b) <$> f a
-- Coyoneda optimization
data Coyoneda f b = forall a. Coyoneda (a -> b) (f a)
instance Functor (Coyoneda f) where
fmap f (Coyoneda g fa) = Coyoneda (f . g) fa
inj :: Functor f => Coyoneda f a -> f a
inj (Coyoneda f a) = fmap f a
proj :: Functor f => f a -> Coyoneda f a
proj fa = Coyoneda id fa
ravel :: Functor f => ((a -> Coyoneda f b) -> (s -> Coyoneda f t))
-> (a -> f b) -> (s -> f t)
ravel coy f s = inj $ coy (\a -> proj (f a)) s
-- the examples
fieldALensGeneric :: Lens' Record Int
fieldALensGeneric = field @"fieldA"
fieldALensGenericYoneda :: Lens' Record Int
fieldALensGenericYoneda = ravel (field @"fieldA")
main :: IO ()
main = return ()
-- the check
inspect $ 'fieldALensManual === 'fieldALensGeneric
inspect $ 'fieldALensManual === 'fieldALensGenericYoneda
|
