[Haskell-cafe] Higher order functor package?

[Erik Hesselink]

I think the keyword you're looking for might be 'indexed', although that
also seems to be used for something different (two indices for pre and post
conditions). Your functor seems to be in 'index-core' [0], and probably
other places (it seems there was something in category-extras but it's
unclear where it went).

Regards,

Erik

[0]
http://hackage.haskell.org/package/index-core-1.0.4/docs/Control-IMonad-Core.html

On 7 July 2017 at 06:48, Clinton Mead <clintonmead at gmail.com> wrote:

> Consider the illustrative code below:
>
>
> {-# LANGUAGE GADTs #-}
> {-# LANGUAGE DataKinds #-}
> {-# LANGUAGE KindSignatures #-}
> {-# LANGUAGE RankNTypes #-}
> {-# LANGUAGE PolyKinds #-}
> {-# LANGUAGE StandaloneDeriving #-}
> {-# LANGUAGE UndecidableInstances #-}
>
> data Param = Param1 | Param2
>
> data T (p :: Param) where
>   TInt :: Int -> T Param1
>   TInteger :: Integer -> T Param1
>   TBool :: Bool -> T Param2
>
> data U (p :: Param) where
>   UDouble :: Double -> U Param1
>   UString :: String -> U Param2
>
> data F (t :: Param -> *) where
>   F :: t Param1 -> t Param2 -> F t
>
> f :: T a -> U a
> f (TInt x) = UDouble (fromIntegral x)
> f (TInteger x) = UDouble (fromIntegral x)
> f (TBool x) = UString (show x)
>
> class MyFunctor f where
>   myFmap :: (forall a. t a -> u a) -> f t -> f u
>
> instance MyFunctor F where
>   myFmap f (F x1 x2) = F (f x1) (f x2)
>
> deriving instance Show (U a)
> deriving instance (Show (t Param1), Show (t Param2)) => Show (F t)
>
> main = print $ myFmap f (F (TInt 42) (TBool False))
>
>
> Basically this is a sort of "higher order" functor, but I can't seem to
> fit it into an ordinary functor.
>
> But it seems like I'm reinventing the wheel, as my code is suspiciously
> like `Functor` but only slightly different.
>
> Has this sort of class already been created and if so what package is it
> in?
>
>
>
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