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-*a*c)
               a2 = *a

roots a b c = let det = sqrt (b*b - *a*c)
                        a2 = *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) / (*a), (-b - det) / (*a))
    where det = sqrt(b*b-*a*c)
roots' a b c=
    let det = sqrt(b*b-*a*c)
    in ((-b + det) / (*a), (-b - det) / (*a))
f _ = det
*Main> roots
(-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+
    in y
    where y = x+
*Main> f

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  )+(create  b )+(create   c)
    where create  _ _=
          create _  _=
          create _ _ =

showmoney a b c=
    let create  _ _=
        create _  _=
        create _ _ =
    in (create a  ) + (create  b ) + (create   c)
*Main> showmoney   

*Main> showmoney2