{-# LANGUAGE FlexibleInstances, FunctionalDependencies #-}

newtype Qty a = Qty Float
-- Dimensions
data DMass
data DMassFrac
-- Quantities
type Mass = Qty DMass
type MassFrac = Qty DMassFrac

class QMul a b c | a b -> c where (.*) :: a -> b -> c

-- instance QMul Float (Qty a) (Qty a) where
instance float ~ Float => QMul float (Qty a) (Qty a) where
  x .* (Qty y) = Qty (x*y)
instance QMul Mass MassFrac Mass where
  (Qty x) .* (Qty y) = Qty (x*y)

g = Qty 1 :: Mass
kg = 1000 .* g :: Mass
-- kg = (1000 :: Float) .* g -- Works fine

main :: IO ()
main = return ()