Question

我不确定这条错误消息是什么让我改变的，因为我看不到代码的问题，但显然有些错误，否则会编译。

错误讯息：

* Couldn't match expected type `(a, a)' with actual type `[(a, a)]'
* In the expression: xs ++ (flips xs)
  In the expression: [xs ++ (flips xs)]
  In an equation for `symClosure': symClosure xs = [xs ++ (flips xs)]
* Relevant bindings include
    xs :: [(a, a)]
symClosure :: [(a, a)] -> [(a, a)]
symClosure xs = [xs ++ (flips xs)]

代码：

heads :: (Eq a) => [(a,a)] -> [a]
heads xs = [x | (x, _) <- xs]

tails :: (Eq a) => [(a,a)] -> [a]
tails xs = [x | (_,x) <- xs]

flips :: [a] -> [(a,a)]
flips xs = tails xs ++ heads xs

symClosure :: (Eq a) => [(a,a)] -> [(a,a)]
symClosure xs = [xs ++ (flips xs)]

旁注：我无法导入任何内容，也无法更改签名。

再一次，非常感谢任何帮助我理解的信息：）

Answer 1

我认为您的flips没有做出其签名所说的内容：

heads :: (Eq a) => [(a,a)] -> [a]
tails :: (Eq a) => [(a,a)] -> [a]
(++) :: [a] -> [a] -> [a]

换句话说，根据您的定义，此签名是正确的：

flips :: [(a, a)] -> [a]
flips xs = tails xs ++ heads xs

请注意，您只能在对名单上调用tails和heads。另请注意，您可以忽略上述所有签名中的Eq约束。

如果您想要反转元组，可以使用zip代替

flips :: [(a, a)] -> [(a, a)]
flips xs = tails xs `zip` heads xs

至于symClosure，取上面flips的定义，表达式

symClosure xs = [xs ++ (flips xs)]

会产生一个包含单个元素的列表，它本身就是一对列表。这就解释了为什么它表示签名中的(a, a)与表达式中的[(a, a)]不匹配。您可能需要将括号括起来。

symClosure xs = xs ++ flips xs

功能symClosure问题

1 个答案: