具有不同参数的Haskell类型签名

Question

欧拉计划4：回文数在两个方向上都相同。 由两个2位数字的乘积构成的最大回文为9009 = 91×99。找到由两个3位数字的乘积构成的最大回文。

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：鉴于两者都有不同的参数排列方式[[]，[x]，（x：xs）），您可以为where子句中的函数分配类型签名吗？ 坚持isPalindrome :: (Eq a) -> [a] -> Bool引发错误。

编辑：我正在尝试插入类型签名，如下所示：

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

你有错字 [应该] (Eq a) =>... （箭头应带有等号）– Michal Seweryn

类约束与其使用=>约束的类型分开。