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