{-# language GHC2024, AllowAmbiguousTypes, TemplateHaskell, TypeAbstractions #-}

module Dispatchable where

import GHC.TypeLits
import Language.Haskell.TH


data Dict c = c => Dict

-- since TH needs to run code during compilation it introduces order-dependence.
-- declaration splices split the file up into chunks that are checked
-- separately, and `reify` (for example) can only see chunks that have already
-- been done. so that `pure []` is to cause `Dispatchable` to be processed and
-- be visible

class Dispatchable d where dispatch :: String
instance Dispatchable "a" where dispatch = "A"
instance Dispatchable "b" where dispatch = "B"

pure []

data DispatchableInstance = forall s. DI (SSymbol s) (Dict (Dispatchable s))

instances :: Q Exp
instances = do
  ClassI _ insts <- reify ''Dispatchable
  listE $ map fromInstance insts
 where
  fromInstance :: Dec -> Q Exp
  fromInstance (InstanceD _ _ (AppT _ str) _) =
    [| DI (SSymbol @($(pure str))) Dict |]