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

import Data.Maybe
import Data.Type.Equality
import GHC.TypeLits
import System.Environment

main = do
  [s] <- getArgs
  putStrLn $ fromMaybe "some default value" $ dispatchDyn s


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

dispatchDyn :: String -> Maybe String
dispatchDyn m = withSomeSSymbol m $ \(SSymbol @m) ->
  case stringDispatchable @m of
    Just Dict -> Just $ dispatch @m
    Nothing   -> Nothing


stringDispatchable :: KnownSymbol m => Maybe (Dict (Dispatchable m))
stringDispatchable @m
  | Just Refl <- is @"a" @m = Just Dict
  | Just Refl <- is @"b" @m = Just Dict
  | otherwise = Nothing
  where is :: (KnownSymbol a, KnownSymbol b) => Maybe (a :~: b)
        is @a @b = sameSymbol (SSymbol @a) (SSymbol @b)


data Dict c = c => Dict