具有不同参数的Haskell类型签名

Haskell type signatures with varying parameters

Project Euler_4：回文数字的读写方式相同。由两个两位数的乘积产生的最大回文是9009=91×99。找出两个3位数的乘积所产生的最大回文。

此解决方案有效：

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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：你能给where子句中的函数分配类型签名吗，前提是这两个子句的参数([]，[x]，(x:xs))排列不同？卡在isPalindrome :: (Eq a) -> [a] -> Bool中会产生错误。

编辑：我试图插入一个类型签名，如下所示：

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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

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