获取错误 "Parse error in pattern: length Failed, modules loaded: none"

Question

 --To check the list is in ascending order or not
 ascending::[Int]->Bool
 ascending [] =True
 ascending ((length l) == 1) =True 
 ascending l  =((head l)<=l !! 1) && ascending(tail l)

这是我的 Haskell 代码，当我尝试在我的 GHCI 解释器中运行这段代码时，出现以下错误：

  ascending.hs(File_name):4:13: Parse error in pattern: length
  Failed, modules loaded: none.

谁能告诉我错误在哪里？

Answer 1

您不能在参数模式中放置条件。长度为 1 的列表的模式为 hd::[].

Answer 2

您似乎在尝试添加守卫。您需要将输入列表绑定到 l 并将条件分隔为 |:

ascending :: [Int]->Bool
ascending [] =True
ascending l | ((length l) == 1) =True 
ascending l = ((head l)<=l !! 1) && ascending(tail l)

您可以使用模式匹配代替对守卫 head 和 tail 的使用：

ascending :: [Int]->Bool
ascending [] = True
ascending [_] = True 
ascending (x:y:xs) = (x <= y) && ascending (y:xs)

获取错误 "Parse error in pattern: length Failed, modules loaded: none"

Getting error "Parse error in pattern: length Failed, modules loaded: none"

haskell

parse-error