Question

项目欧拉＃4：回文数字两种方式相同。由两个2位数字的乘积制成的最大回文是9009 = 91×99。找到由两个3位数字的乘积制成的最大回文。

此解决方案有效：

p004largestPalindrome :: Integer
p004largestPalindrome = largest [ a * b | a <- [100..999], b <- [100..999], isPalindrome $ show(a*b) ]
    where
        isPalindrome [] = True
        isPalindrome [_] = True
        isPalindrome (x:xs) = if x == last xs then isPalindrome (init xs) else False
        largest [] = 0
        largest [x] = x
        largest (x:xs) = if x > head xs then largest (x:(tail xs)) else largest xs

我的问题是：你能否为where子句中的函数分配类型签名，因为两者都有不同的参数排列（[]，[x]，（x：xs））？坚持isPalindrome :: (Eq a) -> [a] -> Bool会引发错误。

编辑：我试图像这样插入一个类型签名：

p004largestPalindrome :: Integer
p004largestPalindrome = largest [ a * b | a <- [100..999], b <- [100..999], isPalindrome $ show(a*b) ]
    where
        isPalindrome :: (Eq a) -> [a] -> Bool
        isPalindrome [] = True
        isPalindrome [_] = True
        isPalindrome (x:xs) = if x == last xs then isPalindrome (init xs) else False
        largest [] = 0
        largest [x] = x
        largest (x:xs) = if x > head xs then largest (x:(tail xs)) else largest xs

Answer 1

你有一个错字。 [应]为(Eq a) =>...（箭头应以等号表示） - Michal Seweryn

类约束与它们用=>约束的类型分开。

具有不同参数的Haskell类型签名

1 个答案: