Data.Generic.Rep.Functor.purs

-- | Failed experiment in generically deriving a `Functor` instance.
-- | Requires PureScript HEAD (what will be 0.12), and still doesn't compile :/
module Data.Generic.Rep.Functor
  ( class GenericMap
  , genericMap'
  ) where

import Prelude

import Data.Generic.Rep (class Generic, Argument(..), Constructor(..), Field(..), NoArguments(..), Product(..), Rec(..), Sum(..), from, to)

class GenericMap a b fa fb where
  genericMap' :: (a -> b) -> fa -> fb

instance genericMapNoArguments :: GenericMap a b NoArguments NoArguments where
  genericMap' f NoArguments = NoArguments

instance genericMapProduct :: (GenericMap a b fa fb, GenericMap a b ga gb) => GenericMap a b (Product fa ga) (Product fb gb) where
  genericMap' f (Product x y) = Product (genericMap' f x) (genericMap' f y)

instance genericMapSum ::
  ( GenericMap a b fa fb
  , GenericMap a b ga gb
  )
  => GenericMap a b (Sum fa ga) (Sum fb gb) where
  genericMap' f (Inl x) = Inl (genericMap' f x)
  genericMap' f (Inr y) = Inr (genericMap' f y)

instance genericMapConstructor :: GenericMap a b fa fb => GenericMap a b (Constructor name fa) (Constructor name fb) where
  genericMap' f (Constructor x) = Constructor (genericMap' f x)

instance genericMapArgumentI :: GenericMap a b (Argument a) (Argument b) where
  genericMap' f (Argument x) = Argument (f x)
else instance genericMapArgumentK :: GenericMap a b (Argument x) (Argument x) where
  genericMap' f (Argument x) = Argument x

instance genericMapRec :: GenericMap a b fa fb => GenericMap a b (Rec fa) (Rec fb) where
  genericMap' f (Rec x) = Rec (genericMap' f x)

instance genericMapFieldI :: GenericMap a b (Field name a) (Field name b) where
  genericMap' f (Field x) = Field (f x)
else instance genericMapFieldK :: GenericMap a b (Field name x) (Field name x) where
  genericMap' f (Field x) = Field x

genericMap :: forall f a b fa fb. Generic (f a) fa => Generic (f b) fb => GenericMap a b fa fb => (a -> b) -> f a -> f b
genericMap f = to <<< genericMap' f <<< from

data D a = C1 | C2 a Int

derive instance genericD :: Generic (D a) _
instance functorD :: Functor D where map = genericMap

{-
No type class instance was found for
|| 
||     Data.Generic.Rep.Functor.GenericMap a0
||                                         b1
||                                         (Argument Int)
||                                         (Argument Int)

The type checker can't commit to `genericMapArgumentK`, because that would
  require `a` and `b` being _apart_ from Int. That's obviously not the case,
  since we could substitute Int for `a`.

Swapping the order of instances for Argument doesn't help - then the type
  checked fails to commit to `genericMapArgumentI`, since it would require `a`
  being apart from `b`.

-}