功能noThirds中没有详尽的模式

问题描述

所以，我的问题是，我必须编写一个程序来过滤列表中的所有3 * x（3,6,9 ...）元素。我的程序看起来像：

  length':: [a]  - > Int 
 length'= foldr（\_  - >（+1））0 
 
 help_ :: [a]  - > [a]  - > [a] 
 help_（x：xs）[] = help_（xs）[x] 
 help_ []（x）=（x）
 help_（x：xs） （y）
 |（（length'[xs]）== 0）=（y）
 |（（length'[y]）`mod` 2 == 0）= help_（xs） 
 |否则= help_（xs）（y ++ [x]）
 
 noThirds :: [a]  - > [a] 
 noThirds [x] = help_ [x] [] 
  

但是当我输入
noThirds [1,2,3,4,5,6,7,8]
时会出现错误函数noThirds中的非穷举模式。我想这是因为我缺少各种help_ ..，但我不明白。感谢每一个帮助！
btw预定义列表和算术函数是不允许的。

解决方案

这是因为 noThirds 只有一个模式， [x] ，它只能匹配单个元素列表。
[x] 完全等价于（x：[]）。我认为你的意思是



  noThirds :: [a]  - > [a] 
 noThirda xs = help_ xs [] 
  




So, my Problem is, that I have to write a program that filters all 3 * x (3,6,9...) elements from a list. My program looks like:

length'  :: [a] -> Int
length' = foldr (\_ -> (+1)) 0

help_ :: [a] -> [a] -> [a]
help_ (x:xs) [] = help_ (xs) [x]
help_ [] (x) = (x)
help_ (x:xs) (y)
    |((length' [xs]) ==0) = (y)
    |((length' [y]) `mod` 2 ==0) = help_ (xs) (y)
    |otherwise = help_ (xs) (y++[x])

noThirds :: [a] -> [a]
noThirds [x] = help_ [x] []

The compiler accepts this but gives the error "Non exhaustive pattern in function noThirds" when I enter "noThirds [1,2,3,4,5,6,7,8]". I guess it's cause im missing a variety of "help_ .." but I don't get it. Im grateful for every help!Btw predefined list and arithmetic functions are not allowed.

解决方案

It's because noThirds only has one pattern, [x] which only matches against a single element list.[x] is exactly equivalent to (x : []). What I think you meant was

noThirds :: [a] -> [a]
noThirda xs = help_ xs []

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