haskell - 没有最后一个参数的函数组成

Question

func4为什么不起作用？尝试加载时出现异常。

-- 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..]

是相同的

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

并不是

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

因为某种明显的原因(对我而言)。

可能的解决方案之一是使 _func采用映射函数:

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