haskell中函数Exception中的非穷举模式

Question

我有这个代码：

import Data.Char
foo  :: String -> String
foo (x:xs) = if (digitToInt(x)) == 0 
then foo xs
else if (digitToInt(x)) /= 0   
then replicate (digitToInt(x)) (head $ take 1 xs) ++ foo (tail xs )
else ""    

输入：foo "4a5b"

输出："aaaabbbbb"

这行if (digitToInt(x)) == 0检查数字是否为0，如果是，则它将与字符串的其余部分展开：

示例：

输入：foo "00004a5b"

输出："aaaabbbbb"

我添加了CCD_ 6来检查其他情况。

然而，它给了我：

*Main> foo "4a5b"
"aaaabbbbb*** Exception: test.hs:(89,1)-(93,28): Non-exhaustive patterns in function foo

错误。我错过了哪些案例？

Answer 1

函数foo中的非穷举模式

此错误发生在运行时，因为尚未根据评估中给定的模式声明函数。这种特殊情况不考虑列表尾部的末尾。

我在下面提出的解决方案旨在概括所提出的用例。

注意函数reproduce的定义。

reproduce []        .. --Empty list
reproduce (x:[])    .. --A single value in the list
reproduce (x:y:[])  .. --A list with only two elements x and y
reproduce (x:y:xs)  .. --A list with the first two elements x and y and the rest of the list xs

以下是考虑foo函数的推广。

代码:

--We import the functions to use
import Data.List (replicate, group, groupBy, all)
import Data.Char (isDigit)
--Grouping by digits
gbDigit :: String -> [String]
gbDigit = groupBy (x y -> (isDigit x) && (isDigit y)) --ETA reduce form
--Grouping by nor digits
gbNoDigit :: String -> [String]
gbNoDigit s = fmap concat $ groupBy (x y -> not (all isDigit x) && not (all isDigit y)) (gbDigit s)
--Prepare applying grouping 
--by digit first and not by digit afterwards, therefore:
prepare = gbNoDigit
foo :: String -> String
foo x = concat $ reproduce (prepare x)
reproduce :: [String] -> [String]
reproduce [] = []        --Empty list, nothing to do
reproduce (x:[]) = []    --A numerical value and nothing to replicate, nothing to do
reproduce (x:y:[]) = (replicate (read x::Int)) y --A numeric value and a string to replicate, so we replicate
reproduce (x:y:xs) = (replicate (read x::Int)) y <> reproduce xs --A numeric value, a string to replicate, and a tail to continue

逐步：

• 给定条目"003aA3b4vX10z">
• 按数字分组：["003","a","A","3","b","4","v","X","10","z"]
• 按数字分组：["003","aA","3","b","4","vX","10","z"]
• 再现&凹面："aAaAaAbbbvXvXvXvXzzzzzzzzzz"

测试用例：

Prelude> foo "00004a5b"
Prelude> "aaaabbbbb"
Prelude> foo "4a"
Prelude> "aaaa"
Prelude> foo "4a10B"
Prelude> "aaaaBBBBBBBBBB"
Prelude> foo "3w1.1w6o1l4y1.1com"
Prelude> "www.woooooolyyyy.com"