> {-# 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
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