Super simple arithmetic expressions in haskell

arithmetic.hs

{-# LANGUAGE GADTs #-}
module Main where

data Expr a where
    N :: Float -> Expr Float
    (:+) :: Expr Float -> Expr Float -> Expr Float
    (:*) :: Expr Float -> Expr Float -> Expr Float
    (:-) :: Expr Float -> Expr Float -> Expr Float
    (:/) :: Expr Float -> Expr Float -> Expr Float
    App :: (Float -> Float) -> Expr Float -> Expr Float 

instance Show (Expr a) where
    show (N i) = show i
    show (i :+ j) = "("++show i ++ "+" ++ show j++")"
    show (i :- j) = "("++show i ++ "-" ++ show j++")"
    show (i :* j) = "("++show i ++ "*" ++ show j++")"
    show (i :/ j) = "("++show i ++ "/" ++ show j++")"
    show (App _ x) = "f(" ++ show x ++ ")"

eval :: Expr a -> a
eval (N i) = i
eval (i :+ j) = eval i + eval j
eval (i :- j) = eval i - eval j
eval (i :* j) = eval i * eval j
eval (i :/ j) = eval i / eval j
eval (App f x) = f (eval x)

comp :: Expr Float
comp = ((N 10 :+ N 12) :* N 10) :- N 15 

comp2 :: Expr Float
comp2 = App (\x -> x*x) (((N 10 :+ N 12) :* N 10) :- N 15)

comp3 :: Expr Float
comp3 = (N 10 :* N 10) :/ N 3

computations :: [Expr Float]
computations = [comp, comp2, comp3]

main :: IO ()
main = mapM_ (\expr -> print (show expr ++ " = " ++ show (eval expr))) 
    computations