Question

为什么func4无效？我尝试加载它时遇到异常。

-- Works fine
func::Integer
func = sum . takeWhile (<10000) . filter odd . map (^2) $ [1..]

-- Works fine
func2::Integer
func2 = sum . takeWhile (<10000) . filter odd $ map (^2) [1..]

-- Works fine
func3::Integer
func3 = _func [1..] 
    where _func = sum . takeWhile (<10000) . filter odd . map (^2)

--Doesn't work!
func4::Integer
func4 = _func (^2) [1..] 
    where _func = sum . takeWhile (<10000) . filter odd . map

异常消息：

src.hs:3:63:
    Couldn't match type `[a] -> [b]' with `[c]'
    Expected type: (a -> b) -> [c]
      Actual type: (a -> b) -> [a] -> [b]
    Relevant bindings include
      _func :: (a -> b) -> c (bound at src.hs:3:15)
    Probable cause: `map' is applied to too few arguments
    In the second argument of `(.)', namely `map'
    In the second argument of `(.)', namely `filter odd . map'
Failed, modules loaded: none.

Answer 1

_func (^2) [1..]

与

相同

(sum . takeWhile (<10000) . filter odd . map) (^2) [1..]

而不是

sum . takeWhile (<10000) . filter odd . map (^2) $ [1..]

有点明显（对我来说）原因。

其中一个可能的解决方案是让_func获取映射函数：

func4::Integer
func4 = _func (^2) [1..] 
    where _func f = sum . takeWhile (<10000) . filter odd . map f

Answer 2

如果你想把第三个例子放在pointfree表示法中，你可以使用无点程序

> pointfree "_func x = sum . takeWhile (<10000) . filter odd . map x"

产量

_func = ((sum . takeWhile (< 10000) . filter odd) .) . map

老实说，我认为这是一个让无点符号成为坏名字的例子之一，我只是保留变量。

没有最后一个参数的函数组成

2 个答案: