{-# LANGUAGE FlexibleInstances, FunctionalDependencies #-}

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

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

instance QMul Float (Qty a) (Qty a) where
  x .* (Qty y) = Qty (x*y)

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

main :: IO ()
main = return ()