Hi Paolino,

There are some functions similar to that in HList (Data.HList.HArray).
Check the repo http://code.haskell.org/HList for a version that uses
more type families / gadts.

Maybe there is a way to take advantage of the fact that you've
labelled the elements of the list, but extract isn't too bad if you
don't: http://lpaste.net/94210.

Regards,
Adam

On Sat, Oct 12, 2013 at 4:41 AM, Paolino <paolo.verone...@gmail.com> wrote:
> Hello everyone,
>
> I'm still trying to resolve my problem. I try to restate it in a simpler
> way.
> Is it possible to write extract and update functions for L ?
>
> import Data.Nat
>
>
> data family X (n::Nat) :: *
>
> data L (n::Nat) where
>     Q :: L (Succ n) -> X n -> L n
>     E :: L n
>
> extract :: L Zero -> X n
> extract = undefined
>
> update :: L Zero -> (X n -> X n) -> L Zero
> update = undefined
>
> Thanks for hints and help
>
> paolino
>
>
>
> 2013/10/7 Paolino <paolo.verone...@gmail.com>
>>
>> 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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