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{-# LANGUAGE AllowAmbiguousTypes #-}
{-# LANGUAGE DataKinds #-}
{-# LANGUAGE DeriveFunctor #-}
{-# LANGUAGE FlexibleContexts #-}
{-# LANGUAGE FlexibleInstances #-}
{-# LANGUAGE FunctionalDependencies #-}
{-# LANGUAGE PolyKinds #-}
{-# LANGUAGE RankNTypes #-}
{-# LANGUAGE ScopedTypeVariables #-}
{-# LANGUAGE TypeFamilies #-}
{-# LANGUAGE TypeApplications #-}
{-# LANGUAGE TypeOperators #-}
{-# LANGUAGE UndecidableInstances #-}
module T11068b (Lens', GField(..)) where
import Data.Kind
import Data.Type.Bool
import Data.Type.Equality
import GHC.Generics
import GHC.TypeLits
-- Code taken from the optics / generic-lens-lite library.
----------------------------------------
-- Profunctors
data Context a b t = Context (b -> t) a
deriving Functor
class Profunctor p where
dimap :: (a -> b) -> (c -> d) -> p b c -> p a d
lmap :: (a -> b) -> p b c -> p a c
rmap :: (c -> d) -> p b c -> p b d
class Profunctor p => Strong p where
first' :: p a b -> p (a, c) (b, c)
second' :: p a b -> p (c, a) (c, b)
linear :: LensVL s t a b -> p a b -> p s t
linear f = dimap
((\(Context bt a) -> (a, bt)) . f (Context id))
(\(b, bt) -> bt b)
. first'
{-# INLINE linear #-}
data Store a b s t = Store (s -> a) (s -> b -> t)
instance Profunctor (Store a b) where
dimap f g (Store get set) = Store (get . f) (\s -> g . set (f s))
lmap f (Store get set) = Store (get . f) (\s -> set (f s))
rmap g (Store get set) = Store get (\s -> g . set s)
instance Strong (Store a b) where
first' (Store get set) = Store (get . fst) (\(s, c) b -> (set s b, c))
second' (Store get set) = Store (get . snd) (\(c, s) b -> (c, set s b))
----------------------------------------
-- Lens
type LensVL s t a b = forall f. Functor f => (a -> f b) -> s -> f t
type LensVL' s a = LensVL s s a a
newtype Lens s t a b = Lens (forall p. Strong p => p a b -> p s t)
type Lens' s a = Lens s s a a
lens :: (s -> a) -> (s -> b -> t) -> Lens s t a b
lens get set = Lens $ dimap (\s -> (get s, s))
(\(b, s) -> set s b)
. first'
lensVL :: LensVL s t a b -> Lens s t a b
lensVL l = Lens (linear l)
withLens :: Lens s t a b -> ((s -> a) -> (s -> b -> t) -> r) -> r
withLens (Lens l) k = case l $ Store id (\_ -> id) of
Store get set -> k get set
----------------------------------------
-- Field
class GField (name :: Symbol) s a | name s -> a where
gfield :: Lens' s a
instance
( Generic s
, path ~ GetPathTree name (Rep s)
, GFieldSum name s path (Rep s) a
) => GField name s a where
gfield = withLens
(lensVL (\f s -> to <$> gfieldSum @name @s @path f (from s)))
(\get set -> lensVL $ \f s -> set s <$> f (get s))
{-# INLINE gfield #-}
data Void0
-- | Hidden instance.
instance a ~ Void0 => GField name Void0 a where
gfield = lensVL id
class GFieldSum (name :: Symbol) s (path :: PathTree) (g :: Type -> Type) a
| name g -> a where
gfieldSum :: LensVL' (g x) a
instance
( GFieldSum name s path V1 a
, TypeError ('Text "Type " ':<>: Quoted ('ShowType s) ':<>:
'Text " has no data constructors")
) => GFieldSum name s path V1 a where
gfieldSum = error "unreachable"
instance
( GFieldSum name s path g a
) => GFieldSum name s path (M1 D m g) a where
gfieldSum f (M1 x) = M1 <$> gfieldSum @name @s @path f x
instance
( GFieldSum name s path1 g1 a
, GFieldSum name s path2 g2 a
) => GFieldSum name s ('PathTree path1 path2) (g1 :+: g2) a where
gfieldSum f (L1 x) = L1 <$> gfieldSum @name @s @path1 f x
gfieldSum f (R1 y) = R1 <$> gfieldSum @name @s @path2 f y
{-# INLINE gfieldSum #-}
instance
( path ~ FromMaybe
(TypeError
('Text "Type " ':<>: Quoted ('ShowType s) ':<>:
'Text " doesn't have a field named " ':<>: Quoted ('Text name)))
mpath
, GFieldProd name s path g a
) => GFieldSum name s ('PathLeaf mpath) (M1 C m g) a where
gfieldSum f (M1 x) = M1 <$> gfieldProd @name @s @path f x
class GFieldProd (name :: Symbol) s (path :: [Path]) g a | name g -> a where
gfieldProd :: LensVL' (g x) a
instance
( GFieldProd name s path g1 a
) => GFieldProd name s ('PathLeft : path) (g1 :*: g2) a where
gfieldProd f (x :*: y) = (:*: y) <$> gfieldProd @name @s @path f x
instance
( GFieldProd name s path g2 a
) => GFieldProd name s ('PathRight : path) (g1 :*: g2) a where
gfieldProd f (x :*: y) = (x :*:) <$> gfieldProd @name @s @path f y
instance
( a ~ b -- for better error message if types don't match
) => GFieldProd name s '[] (M1 S ('MetaSel ('Just name) su ss ds) (Rec0 b)) a where
gfieldProd f (M1 (K1 x)) = M1 . K1 <$> f x
----------------------------------------
-- Helpers
type family Quoted (s :: ErrorMessage) :: ErrorMessage where
Quoted s = 'Text "‘" ':<>: s ':<>: 'Text "’"
data PathTree
= PathTree PathTree PathTree
| PathLeaf (Maybe [Path])
| NoPath
data Path = PathLeft | PathRight
-- | Compute paths to a field for a generic representation of a data type.
type family GetPathTree (name :: Symbol) g :: PathTree where
GetPathTree name (M1 D _ g) = GetPathTree name g
GetPathTree name V1 = 'NoPath
GetPathTree name (g1 :+: g2) = 'PathTree (GetPathTree name g1)
(GetPathTree name g2)
GetPathTree name (M1 C _ g) = 'PathLeaf (GetPath name g '[])
-- | Compute path to a constructor in a sum or a field in a product.
type family GetPath (name :: Symbol) g (acc :: [Path]) :: Maybe [Path] where
GetPath name (M1 D _ g) acc = GetPath name g acc
-- Find path to a constructor in a sum type
GetPath name (M1 C ('MetaCons name _ _) _) acc = 'Just (Reverse acc '[])
GetPath name (g1 :+: g2) acc = Alt (GetPath name g1 ('PathLeft : acc))
(GetPath name g2 ('PathRight : acc))
-- Find path to a field in a product type
GetPath name (M1 S ('MetaSel ('Just name) _ _ _) _) acc = 'Just (Reverse acc '[])
GetPath name (g1 :*: g2) acc = Alt (GetPath name g1 ('PathLeft : acc))
(GetPath name g2 ('PathRight : acc))
GetPath _ _ _ = 'Nothing
-- | Reverse a type-level list.
type family Reverse (xs :: [k]) (acc :: [k]) :: [k] where
Reverse '[] acc = acc
Reverse (x : xs) acc = Reverse xs (x : acc)
type family FromMaybe (def :: a) (m :: Maybe a) :: a where
FromMaybe _ ('Just a) = a
FromMaybe def 'Nothing = def
-- | Type-level mplus for 'Maybe'.
type family Alt (m1 :: Maybe a) (m2 :: Maybe a) :: Maybe a where
Alt ('Just a) _ = 'Just a
Alt _ b = b
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