函数无法识别模式匹配

Question

我的函数 paralellenCheck 每次我调用它抱怨非详尽模式时都会引发异常

paralellenCheck定义如下：

paralellenCheck :: (Eq a, Num a ) => [a] -> [a] -> Bool
paralellenCheck [] [] = True
paralellenCheck [_] [] = True
paralellenCheck [] [_] = True
paralellenCheck (x:xs) (y:ys)
  | intervall `elem`  [0,5,7,12]  = False
  | not $ paralellenCheck (x:xs) ys  = False 
  | not $ paralellenCheck xs (y:ys)  = False
  | otherwise  = True
  where intervall = abs (x - y)

在应用了 -Wall 的 GHCI 中运行函数只会返回

<interactive>:10:1:
Warning: Defaulting the following constraint(s) to type `Integer'
           (Eq a0)
             arising from a use of `paralellenCheck' at <interactive>:10:1-15
           (Num a0)
             arising from a use of `paralellenCheck' at <interactive>:10:1-15
In the expression: paralellenCheck [] [2, 3, 4, 5]
In an equation for `it': it = paralellenCheck [] [2, 3, 4, ....]

<interactive>:10:1:
Warning: Defaulting the following constraint(s) to type `Integer'
           (Eq a0)
             arising from a use of `paralellenCheck' at <interactive>:10:1-15
           (Num a0)
             arising from a use of `paralellenCheck' at <interactive>:10:1-15
In the expression: paralellenCheck [] [2, 3, 4, 5]
In an equation for `it': it = paralellenCheck [] [2, 3, 4, ....]
*** Exception: haskelltest.hs:(6,1)-(14,31): Non-exhaustive patterns in function paralellenCheck

我对 haskell 比较陌生，而且很困惑。我认为与[_] [] 和[] [_] 的模式匹配应该可以解决这个问题。

Answer 1

[_] 在模式中的意思是“任何一个元素的列表”。您似乎打算“根本没有任何清单”。现在，例如，[] (a:b:[]) 不匹配。对任何列表使用_。