I'm having trouble with having subparts be of the same typeclass but not necessarily the same kind of data.

KV.hs

class KVEntity a where
    kvIdentity      :: a -> KVIdentifier
    kvProperties    :: a -> Value
    kvLastModified  :: a -> UTCTime
    kvClass         :: a -> KVClass
    kvRelations     :: a -> [KVLink]
    kvCachedRels    :: a -> [a]

    kvProperties _ = emptyObject
    kvCachedRels _ = []
    kvRelations  _ = []

data (KVEntity a) => KVFullEntity a = KVFullEntity
    { kvFullClass           :: KVClass
    , kvFullProperties      :: Value
    , kvFullIdentity        :: KVIdentifier
    , kvFullLastModified    :: UTCTime
    , kvFullRelations       :: [KVLink]
    , kvFullCachedRels      :: [a]
    }
instance (KVEntity a) => KVEntity (KVFullEntity a) where
    kvIdentity KVFullEntity{..}     = kvFullIdentity
    kvProperties KVFullEntity{..}   = kvFullProperties
    kvLastModified KVFullEntity{..} = kvFullLastModified
    kvClass KVFullEntity{..}        = kvFullClass
    kvRelations KVFullEntity{..}    = kvFullRelations
    -- kvCachedRels KVFullEntity{..}   = kvFullCachedRels
{-
^Notice the above commented line, when uncommented, the following happens
    Couldn't match type `a' with `KVFullEntity a'
      `a' is a rigid type variable bound by
          the instance declaration at Data/Centur/KV.hs:45:20
    Expected type: [KVFullEntity a]
      Actual type: [a]
    In the expression: kvFullCachedRels
    In an equation for `kvCachedRels':
        kvCachedRels (KVFullEntity {..}) = kvFullCachedRels
-}