Generate both a Haskell function and a SQL expression from a data structure

{-# LANGUAGE GADTs, KindSignatures, TypeOperators #-}

data InvoiceLine = InvoiceLine
    { ilDescription :: String
    , ilQuantity    :: Int
    , ilAmount      :: Int
    }

data (:->:) :: * -> * -> * where
    Sum        :: e :->: Int -> [e] :->: Int
    (:-:)      :: e :->: Int -> e :->: Int -> e :->: Int
    (:*:)      :: e :->: Int -> e :->: Int -> e :->: Int
    Constant   :: Int -> e :->: Int
    ILQuantity :: InvoiceLine :->: Int
    ILAmount   :: InvoiceLine :->: Int

toSQL :: (e :->: a) -> String
toSQL (Sum xs)     = "coalesce(sum(" ++ toSQL xs ++ "), 0)"
toSQL (a :-: b)    = "(" ++ toSQL a ++ ") - (" ++ toSQL b ++ ")"
toSQL (a :*: b)    = "(" ++ toSQL a ++ ") * (" ++ toSQL b ++ ")"
toSQL (Constant n) = show n
toSQL ILQuantity   = "invoice_lines.quantity"
toSQL ILAmount     = "invoice_lines.amount"

toHaskell :: (e :->: a) -> (e -> a)
toHaskell (Sum xs)     e = sum (map (toHaskell xs) e)
toHaskell (a :-: b)    e = toHaskell a e - toHaskell b e
toHaskell (a :*: b)    e = toHaskell a e * toHaskell b e
toHaskell (Constant n) _ = n
toHaskell ILQuantity   e = ilQuantity e
toHaskell ILAmount     e = ilAmount e

invoiceLineTotal :: InvoiceLine :->: Int
invoiceLineTotal = ILQuantity :*: (ILAmount :-: Constant 50)

invoiceTotal :: [InvoiceLine] :->: Int
invoiceTotal = Sum invoiceLineTotal

main :: IO ()
main = do
    let computation = invoiceTotal
    putStrLn $ toSQL computation
    print $ toHaskell computation [InvoiceLine "Banana" 2 99, InvoiceLine "Pineapple" 1 139]