空列表的非穷举模式错误 [英] Non-exhaustive patterns error with empty list

问题描述

我正在声明一种新的内存类型，然后使用一个函数对其进行更新.当我向列表中添加值时，该程序可以编译并运行良好，但是如果列表为空，则会出现错误函数update中的非穷举模式".这是我的代码，如果可以的话请帮助我:

I am declaring a new type for memory and then use a function to update it. The program compiles and works fine when I add values to the list, but if my list is empty I get the error "non-exhaustive patterns in function update". Here is my code, if you can please help me:

type Name = [Char] 
type Memory = [(Name,Integer)] 

update :: Name ->Integer -> Memory -> Memory
update n x (h:t)
    |(fst h==n) =(n,x):t
    |((h:t) == []) = (n,x):[]
    |(fst t==x) = h:(n,t)
    |(t==[]) =  h:(n,x):[]
    |otherwise = h:update n x t 

推荐答案

这是因为您的代码没有涵盖空列表的情况. 特别是:h:t == []永远不会等于True. h:t是仅匹配非空列表的模式:它将h绑定到列表的开头，将t绑定到列表的其余部分.

This is because your code doesn't cover the empty list case. In particular this: h:t == [] will never evaluate to True. h:t is a pattern which will only match a non-empty list: it binds h to the head of the list and t to the rest of the list.

因此您的函数需要处理三种情况:

So your function needs to handle three cases:

update n x [] = (n,x):[]                        -- empty list
update n x (h:t) | n == fst h = (n,x):t         -- key equal to n
                 | otherwise  = h:update n x t  -- key not equal to n

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