i :: Int

i = 

--add, sub :: Int -> Int -> Int

add, sub :: (Num a) => a -> a -> a

add a b = a + b

sub a b = a - b

f :: (Num a) => a -> a

f x =  * x + 

--出错

--area :: (Num x) => Floating -> x -> x

area :: Floating a => a -> a

area r = pi * (r ^ )

fun :: (Num a) => (a, a) -> a

fun (x,y) =  * x +  * y + 

fun' :: (Num a) => a -> a-> a

fun' x y = 4 * x + 5 * y + 1

--Lambda表达式

--fun'' = \x -> \y ->  * x +  * y +

fun'' :: Num a => a -> a ->a

fun'' = \x y ->  * x + *y + 

plus :: Num a => a -> a -> a

plus = (+)

--参数绑定

--area_tri :: Float -> Float -> Float -> Float

area_tri ::  Floating a => a -> a -> a -> a

-- let ... in

area_tri a b c = let p = (a+b+c)/ in  sqrt (p * (p-a) * (p-b) * (p-c))

area_tri' :: Double -> Double -> Double -> Double

-- where

area_tri' a b c =

    sqrt (p * (p-a) * (p-b) * (p-c))

    where

    p = ((a + b + c) / 2.0)

--条件表达式

isTwo :: Int -> Bool

isTwo x = if x ==  then True else False

isTwo' :: Int -> Bool

isTwo' = (==2)