Haskell 确定线的凸度或凹度的长度和过滤器

Question

我正在阅读 Real World Haskell，试图解决 Ch3, Q10 using ghc online。

到目前为止我有以下代码：

data Direction point = MyLeft point | MyRight point | Straight deriving (Show)

getDirectionFromTriple :: Direction p -> Direction p -> Direction p -> Direction p
getDirectionFromTriple p1 p2 p3 
  | (length . filter (== MyLeft) [p1, p2, p3]) > 1 = MyLeft p3
  | (length . filter (== MyRight) [p1, p2, p3]) > 1 = MyRight p3
  | otherwise = Straight

我在尝试编译此代码时收到以下错误（仅发布了部分，同样的错误多次弹出）：

[1 of 1] Compiling Main             ( jdoodle.hs, jdoodle.o )


jdoodle.hs:17:15: error:
    * Couldn't match expected type `a0 -> t0 a1'
                  with actual type `[point0 -> Direction point0]'
    * Possible cause: `filter' is applied to too many arguments
      In the second argument of `(.)', namely
        `filter (== MyLeft) [p1, p2, p3]'
      In the first argument of `(>)', namely
        `(length . filter (== MyLeft) [p1, p2, p3])'
      In the expression: (length . filter (== MyLeft) [p1, p2, p3]) > 2
   |
17 |   | (length . filter (== MyLeft) [p1, p2, p3]) > 2 = MyLeft p3
   |               ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^

jdoodle.hs:17:35: error:
    * Couldn't match expected type `point0 -> Direction point0'
                  with actual type `Direction p'
    * In the expression: p1
      In the second argument of `filter', namely `[p1, p2, p3]'
      In the second argument of `(.)', namely
        `filter (== MyLeft) [p1, p2, p3]'
    * Relevant bindings include
        p3 :: Direction p (bound at jdoodle.hs:16:30)
        p2 :: Direction p (bound at jdoodle.hs:16:27)
        p1 :: Direction p (bound at jdoodle.hs:16:24)
        getDirectionFromTriple :: Direction p
                                  -> Direction p -> Direction p -> Direction p
          (bound at jdoodle.hs:16:1)
   |
17 |   | (length . filter (== MyLeft) [p1, p2, p3]) > 2 = MyLeft p3
   |                                   ^^

jdoodle.hs:17:39: error:
    * Couldn't match expected type `point0 -> Direction point0'
                  with actual type `Direction p'
    * In the expression: p2
      In the second argument of `filter', namely `[p1, p2, p3]'
      In the second argument of `(.)', namely
        `filter (== MyLeft) [p1, p2, p3]'
    * Relevant bindings include
        p3 :: Direction p (bound at jdoodle.hs:16:30)
        p2 :: Direction p (bound at jdoodle.hs:16:27)
        p1 :: Direction p (bound at jdoodle.hs:16:24)
        getDirectionFromTriple :: Direction p
                                  -> Direction p -> Direction p -> Direction p
          (bound at jdoodle.hs:16:1)
   |
17 |   | (length . filter (== MyLeft) [p1, p2, p3]) > 2 = MyLeft p3
   |                                       ^^

无论是关于如何修复我的代码的建议，还是关于从三重点确定主导方向的更简洁解决方案的建议，我都将不胜感激。

Answer 1

同最近的问题。当你写

length . filter f xs

这被解析为

length . (filter f xs)

这显然是不对的：filter f xs 是一个列表，而不是一个函数，所以用 length 组合它是没有意义的。相反，你可以写

(length . filter f) xs

尽管更流行的拼写方式是

length . filter f $ xs

Answer 2

您可以创建函数来告诉您它是右还是左，而不是使用 ==

import Data.List 

data Direction point = MyLeft point | MyRight point | Straight deriving (Show)

getDirectionFromTriple :: Direction p -> Direction p -> Direction p -> Direction p
getDirectionFromTriple p1 p2 p3 
  | isDirection isLeft  [p1, p2, p3]  = MyLeft (getValue p3)
  | isDirection isRight [p1, p2, p3]  = MyRight (getValue p3)
  | otherwise = Straight

isDirection :: (Direction p -> Bool) -> [Direction p] -> Bool
isDirection f ps = (length . filter f) ps > 1

getValue (MyLeft a) = a
getValue (MyRight a) = a
getValue Straight  = error "No value in Straight"

isLeft  (MyLeft _)  = True
isLeft   _          = False
isRight (MyRight _) = True
isRight  _          = False

main = do
  putStrLn $ show $ getDirectionFromTriple (MyRight 2) Straight (MyLeft 1)
  putStrLn $ show $ getDirectionFromTriple (MyRight 2) (MyRight 3) (MyLeft 1)
  putStrLn $ show $ getDirectionFromTriple (MyLeft 1) (MyLeft 1) (MyRight 2)

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