[Haskell-cafe] typeclass to select a list element

[Paolino]

Hello, I'm trying to use a type class to select an element from a list.
I would like to have a String "CC" as a value for l10'.


{-# LANGUAGE MultiParamTypeClasses, GADTs,FlexibleInstances,  DataKinds
,TypeFamilies, KindSignatures, FlexibleContexts, OverlappingInstances,
StandaloneDeriving, UndecidableInstances #-}



import Data.Nat
import Data.Monoid

data family X (n::Nat) :: *

data L (n::Nat) where
    Q :: (Monoid (X n), Show (X n)) => L (Succ n) -> X n -> L n
    E :: Monoid (X n) => L n

deriving instance Show (L n)
data instance X n = String String

instance Monoid (X n) where
    String x `mappend` String y = String $ x `mappend` y
    mempty = String ""
deriving instance Show (X n)

class Compose n n' where
    compose :: L n  -> L n  -> X n'

instance Compose n n where
    compose (Q _ x) (Q _ y) = x `mappend` y
    compose _ _ = mempty

instance Compose n n' where
    compose (Q x _) (Q y _) = compose x y
    compose _ _ = mempty

l0 :: L Zero
l0 = Q (Q E $ String "C") $ String "A"

l0' :: L Zero
l0' = Q (Q E $ String "C") $ String "B"


l10' :: X (Succ Zero)
l10' = compose l0 l0'

l00' :: X Zero
l00' = compose l0 l0'
{-

*Main> l00'
String "AB"
*Main> l10'
String ""

-}

Thanks for help.

paolino
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