Question

我有此代码：

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"

此行if (digitToInt(x)) == 0检查数字是否为0（如果为0），则它将随字符串的其余部分一起扩展：

示例：

输入：foo "00004a5b"

输出："aaaabbbbb"

我添加了else if (digitToInt(x)) /= 0来检查其他情况。

但是它给了我

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

错误。我在这里错过了哪些情况？

Answer 1

函数foo中的非穷举模式

此错误在运行时发生，因为尚未根据评估中给出的模式声明该函数。这种特殊情况不考虑列表尾部的结尾。

我在下面介绍的解决方案旨在归纳所介绍的用例。

请注意函数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

这里是考虑到以下问题的解决方案 foo函数的一般化。

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

分步：

提供一个条目“ 003aA3b4vX10z”
按数字分组：["003","a","A","3","b","4","v","X","10","z"]
按NoDigits分组：["003","aA","3","b","4","vX","10","z"]
复制与合并："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"

haskell函数Exception中的非穷举模式

1 个答案: