Re: [Haskell-cafe] Higher order functor package?

7 Jul 2017

      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...

On 7 July 2017 at 06:48, Clinton Mead  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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Re: [Haskell-cafe] Higher order functor package?

Erik Hesselink