長方形を表現

SICPの問題2.3。長方形を表し、周囲の長さと面積を計算する手続きを作る。
今回は、問題2.2とほぼ同じなので、以下にソースと実行結果を示します。　

$cat rectangle.hs
data Point = Point Integer Integer

instance Show Point where
    show (Point x y ) = "( "++show(x)++" , " ++ show(y)++" )"

data Rectangle = Rectangle Point Point

instance Show Rectangle where
    show (Rectangle (Point x y) (Point x' y')) 
         = show (Point x y) ++ " - " ++ show (Point x' y)
                ++ "\n" ++ show (Point x y') 
                     ++ " - " ++ show (Point x' y')

makeRectangle:: Integer->Integer->Integer->Integer->Rectangle
makeRectangle x y x' y' = Rectangle (Point x y) (Point x' y')

getCircuit::Rectangle->Integer
getCircuit (Rectangle (Point x y) (Point x' y'))
        = double $( getLength x x') + (getLength y y')
        where
                double = (*) 2

getDimension (Rectangle (Point x y) (Point x' y'))
        = (*) (getLength  x x') (getLength  y y')

getLength::Integer->Integer->Integer
getLength x x' = abs ( x' -x )

$ghci rectangle.hs
Loading package base-1.0 ... linking ... done.
Compiling Main             ( rectangle.hs, interpreted )
Ok, modules loaded: Main.
*Main> makeRectangle 3 5 6 1
( 3 , 5 ) - ( 6 , 5 )
( 3 , 1 ) - ( 6 , 1 )
*Main> let rect = makeRectangle 3 5 6 1
*Main> getCircuit rect
14
*Main> getDimension rect
12