Using Equality Constraints to put Constraints on elemts in a Type Family's CoDomain

eq-const-type-fam-codo.hs

-- | I wanted to encode the json key for a payload type as type-level strings
-- one can use equality constraints to resolve type family mappings

data SomeData = SomeData

type family Key (a :: *) :: Symbol where
    Key SomeData = "some_data"

-- | here we use equality constrains to resolve `k` to the type-level mapping
-- of SomeData a. then we can restrict `k` to some other constraint (in this
-- case to be a `KnownSymbol`.
data Payload a where
    Payload :: (k ~ SomeData a, KnownSymbol k)
            => Proxy (Key a)
            -> a
            -> Payload a

instance (ToJSON a) => ToJSON (Payload a) where
    toJSON (Payload p a) = object [ "payload_type" .= symbolVal p
                                  , "data"         .= toJSON a
                                  ]