{-# LANGUAGE DeriveDataTypeable, ScopedTypeVariables #-}
Hi MagicCloud, A worse, but perhaps simpler alternative to Oleg's solution uses Data.Dynamic:
import Data.Dynamic
data LongDec a = LongDec a a a a a a a a deriving (Show, Typeable)
values = "abcdefgh"
mkLongDec :: forall a. Typeable a => [a] -> Maybe (LongDec a) mkLongDec = (fromDynamic =<<) . foldl (\f x -> do f' <- f dynApply f' (toDyn x)) (Just (toDyn (\x -> LongDec (x :: a))))
main = do print (mkLongDec values) print (mkLongDec [1 .. 8 :: Integer])
*Main> main Just (LongDec 'a' 'b' 'c' 'd' 'e' 'f' 'g' 'h') Just (LongDec 1 2 3 4 5 6 7 8) There is no check that all arguments of LongDec are the same type (in this case a specific instance of Typeable): you'd only be able to get Nothing out of mkLongDec was defined as: data LongDec a = LongDec a Int a a a Char Regards, Adam Vogt