haskell中的let和where

haskell中有两种定义局部变量的方法let和where，方法分别如下

roots a b c = ((-b + det) / (a2), (-b - det) / (a2))
      where det = sqrt(b*b-4*a*c)
               a2 = 2*a

roots a b c = let det = sqrt (b*b - 4*a*c)
                        a2 = 2*a
                   in ((-b + det) / a2, (-b - det) / a2)

这两种方法都可以使全局变量定义失效

det = "Hello World"
roots a b c =((-b + det) / (2*a), (-b - det) / (2*a))
    where det = sqrt(b*b-4*a*c)
roots' a b c=
    let det = sqrt(b*b-4*a*c)
    in ((-b + det) / (2*a), (-b - det) / (2*a))
f _ = det
*Main> roots 1 3 2
(-1.0,-2.0)
*Main> roots' 1 3 2
(-1.0,-2.0)
*Main> f 'a'
"Hello World"

对于let和where混用，那么是let的优先级高，也就是说优先使用let定义的变量，而不是where定义的变量，比如

f x =
    let y = x+1
    in y
    where y = x+2
*Main> f 10
11

等于11而不是12

对于let和where，同样也支持模式匹配，例如

showmoney2 a b c=
    (create a 0 0)+(create 0 b 0)+(create 0 0 c)
    where create 1 _ _=1
          create _ 1 _=2
          create _ _ 1=3

showmoney a b c=
    let create 1 _ _=1
        create _ 1 _=2
        create _ _ 1=3
    in (create a 0 0) + (create 0 b 0) + (create 0 0 c)
*Main> showmoney 1 1 1
6
*Main> showmoney2 1 1 1
6