Haskell 嵌套 where 子句和 "let ... in"语法

Question

我刚开始使用 Haskell。

这个嵌套的 where 子句有什么问题？

length' a = fromIntegral (length a)

isPalin1 xs = fstHalf == reverse sndHalf
    where
        fstHalf = take halfLength xs
        sndHalf = drop halfLength xs
            where halfLength = (length' xs) / 2

我得到的错误: isPalin1.hs:5:32: Not in scope: ‘halfLength’
以下也是错误的，希望有人能告诉我为什么:

length' a = fromIntegral (length a)

isPalin2 xs = fstHalf == reverse sndHalf
    where
        let
            halfLength = (length' xs) / 2
        in
            fstHalf = take halfLength xs
            sndHalf = drop halfLength xs

错误信息:

isPalin2.hs:7:17:
    parse error (possibly incorrect indentation or mismatched brackets)

Answer 1

where只适用于一个绑定(bind)，所以 halfLength在您的第一个片段中仅对 sndHalf 可用.你应该这样写:

length' a = fromIntegral (length a)

isPalin1 xs = fstHalf == reverse sndHalf
    where
        halfLength = (length' xs) / 2
        fstHalf = take halfLength xs
        sndHalf = drop halfLength xs

这是因为 where本身就是一个作用域，所以绑定(bind)可以相互引用。

至于你的第二个片段，那是完全不正确的。一个 let ⋯ in statement 严格来说是一个表达式，不能在 where 的绑定(bind)上下文中使用。 .

如果你在哪里使用 let ⋯ in ，你会像这样使用它:

length' a = fromIntegral (length a)

isPalin1 xs =
    let halfLength = (length' xs) / 2
        fstHalf = take halfLength xs
        sndHalf = drop halfLength xs
    in  fstHalf == reverse sndHalf