hlist_question.hs

module A where

import Data.HList.CommonMain

data A = A deriving (Show)

ls = hCons "hi" $ hCons A $ hNil

upls = hUpdateAtType "bye" ls

-- shouldn't upls = HCons "bye" (HCons A HNil) ?
-- problem is i get this error:

-- /Users/polypus/Documents/creations/software/haskell/genesis/A.hs:10:7:
--     No instances for (TypeEq [Char] [Char] b,
--                       HType2HNatCase b [Char] (HCons A HNil) n,
--                       HUpdateAtHNat n [Char] (HCons [Char] (HCons A HNil)) l')
--       arising from a use of `hUpdateAtType'
--                    at /Users/polypus/Documents/creations/software/haskell/genesis/A.hs:10:7-28
--     Possible fix:
--       add an instance declaration for
--       (TypeEq [Char] [Char] b,
--        HType2HNatCase b [Char] (HCons A HNil) n,
--        HUpdateAtHNat n [Char] (HCons [Char] (HCons A HNil)) l')
--     In the expression: hUpdateAtType "bye" ls
--     In the definition of `upls': upls = hUpdateAtType "bye" ls

you also have to import some instances for TypeEq, try with Data.HList.TypeEqGeneric1