a.hs

data Table = Table Bool Symbol Symbol

-- | Term level table information carrier.
data TTable = TTable Bool String String

data instance Sing ('Table opt pre tbl) =
  STable (Sing opt) (Sing pre) (Sing tbl)

instance ( SingI opt, SingI pre, SingI tbl )
         => SingI ('Table opt pre tbl) where
  sing = STable sing sing sing

instance SingKind ('KProxy :: KProxy Table) where
  type DemoteRep ('KProxy :: KProxy Table) = TTable

  fromSing = \case
    (STable opt pre tbl) ->
      TTable (fromSing opt) (fromSing pre) (fromSing tbl)

  toSing (TTable opt pre tbl) =
    STable (toSing opt) (toSing pre) (toSing tbl)


    -- Couldn't match type ‘a’ with ‘'Table a0 a1 a2’
    --   ‘a’ is a rigid type variable bound by
    --       the type signature for fromSing :: Sing a -> DemoteRep 'KProxy
    --       at src/Database/PostgreSQL/Typed/Table.hs:46:3
    -- Expected type: Sing a
    --   Actual type: Sing ('Table a0 a1 a2)
    -- Relevant bindings include
    --   fromSing :: Sing a -> DemoteRep 'KProxy
    --     (bound at src/Database/PostgreSQL/Typed/Table.hs:46:3)
    -- In the pattern: STable opt pre tbl
    -- In a case alternative:
    --     (STable opt pre tbl)
    --       -> TTable (fromSing opt) (fromSing pre) (fromSing tbl)
    -- In the expression:
    --   \case {
    --     (STable opt pre tbl)
    --       -> TTable (fromSing opt) (fromSing pre) (fromSing tbl) }