Question

关于较早的问题Find if Duplicates Exist SML NJ，如果我想要相反的结果：

[1,2,2,3,4,5,6] should return false
[1,2,3,4,5,6,7] should return true
[1,2,3,4,5,6,1] should return false

我怎么能用它：

fun duplicated [] = false
| duplicated (x::xs) = (List.exists (fn y => x = y) xs) orelse (duplicated xs)

例如，

fun non_duplicated ps =
case ps of
[] => false
| x::xs' => (List.exists (fn y => x<>y) xs') andalso (non_duplicated xs')

不起作用。

为什么??? 感谢。

Answer 1

如果您想获得相反的结果，只需按如下方式定义函数：

fun non_duplicated xs = not (duplicated xs)

也就是说，您可以使用De Morgan laws将not函数的正文推向duplicated：

not (a orelse b) <=> (not a) andalso (not b)
not exists equal <=> forall (not equal)
not false <=> true

然后你到达相反的版本：

fun non_duplicated [] = true
  | non_duplicated (x::xs) = 
      (List.forall (fn y => x <> y) xs) andalso (non_duplicated xs)