I try to understand phantom types in Haskell. In main, try eur 12 <+> usd 23 and see the compilation error, saying that you cannot add euros and US dollars. Neat ;)

currency.hs

data EUR
data USD

data Amount a = Amount Float deriving Show

eur :: Float -> Amount EUR
eur a = Amount a

usd :: Float -> Amount USD
usd a = Amount a

(<+>) :: Amount a -> Amount a -> Amount a
(Amount a) <+> (Amount b) = Amount (a + b)

main = do 
    print $ eur 12 <+> eur 23

newtype.hs

{-# LANGUAGE GeneralizedNewtypeDeriving #-}

newtype Feet = Feet Double deriving (Num, Show, Eq)
newtype Cm   = Cm   Double deriving (Num, Show, Eq)

main = 
    print $ Feet 12 + Feet 12

duplication

GeneralizedNewtypeDeriving is providing derivation for numeric behaviour (Num typeclass)
It is less verbose than phantom types, no need for "smart constructors" (eur and usd), and all numeric functions for free.
I guess GeneralizedNewtypeDeriving is one of the many reasons ghc is so big :P

Deriving Num means that an expression like 12 + Feet 34 is valid, because Feed is now like a Num. It may or may not be what you want, depending of your application. I think if you define some new types that must not mix, then you don't want to mix them with Num.