Re: [Haskell-cafe] typeclass to select a list element

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 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

...

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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