[Haskell-cafe] ambiguous type variable question

[Zoran Bošnjak]

Hi Tom,
thanks a lot for your answers. It works as expected. I am building on this proposal, but I am again stuck on the following problems:

- The second proposal introduces 'Content' to the 'IsSchema' type signature. Is it necessary? The problem is when I try to extend it to other types, like 'Variation' and 'Item' (see below).

- I was able to implement instances (using the first proposed solution), but I am getting error on the call site (that is: on the 'main' function. The error says: ‘SchemaVal’ is a non-injective type family. I don't know how to get around it. I would appreciate if you have another look.

Here is an updated example:

{-# LANGUAGE AllowAmbiguousTypes #-}
{-# LANGUAGE DataKinds #-}
{-# LANGUAGE TypeFamilies #-}
{-# LANGUAGE UndecidableInstances #-}

module Test where

import Data.Kind
import Data.Proxy
import GHC.TypeLits

data Content n s b
  = ContentRaw
  | ContentTable [(n, s)]
  | ContentBool b

deriving instance (Show n, Show s, Show b) => Show (Content n s b)

data Variation n s b
  = Element n (Content n s b)
  | Group [Item n s b]

deriving instance (Show n, Show s, Show b) => Show (Variation n s b)

data Item n s b
  = Spare n
  | Item s (Variation n s b)

deriving instance (Show n, Show s, Show b) => Show (Item n s b)

-- Convert from types to values.
class IsSchema t where
  type SchemaVal t :: Type
  schema :: SchemaVal t

-- Instances implementation...
-- to be defined...

-- Test
type V1 = 'Element 1 'ContentRaw
type I1 = 'Item "title1" V1
type I2 = 'Item "title2" V1

type G0 = 'Group '[]
type G1 = 'Group (I1 ': '[])
type G2 = 'Group (I1 ': I2 ': '[])

main :: IO ()
main = do
  print $ schema @V1
  print $ schema @I1
  print $ schema @I2
  print $ schema @G0
  print $ schema @G1
  print $ schema @G2
  print (schema @G1 /= schema @G2)