如何修复'Eq a'在haskell中有类似的'GHC.Prim.Constraint'错误？

Question

我正在Haskell中编写一个小函数来检查列表是否是回文,并将其与反向进行比较.

checkPalindrome :: [Eq a] -> Bool
checkPalindrome l = (l == reverse l)
                    where
                        reverse :: [a] -> [a]
                        reverse xs
                            | null xs = []
                            | otherwise = (last xs) : reverse newxs
                                  where
                                      before = (length xs) - 1
                                      newxs = take before xs

我知道我应该在函数定义中使用[Eq a],因为我稍后使用了相等运算符,但是在编译时遇到了这个错误:

Expected kind ‘*’, but ‘Eq a’ has kind ‘GHC.Prim.Constraint’
In the type signature for ‘checkPalindrome’:
  checkPalindrome :: [Eq a] -> Bool

如果我的缩进做错了,请随意纠正我,我对这门语言很新.

Answer 1

除非Haskell采用新语法,否则您的类型签名应为:

checkPalindrome :: Eq a => [a] -> Bool

在fat-arrow的左侧声明约束,然后在右侧使用它.