Regarding to an older question Find if Duplicates Exist SML NJ, if I want the opposite result:
[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
how can I have it with:
fun duplicated [] = false
| duplicated (x::xs) = (List.exists (fn y => x = y) xs) orelse (duplicated xs)
For example,
fun non_duplicated ps =
case ps of
[] => false
| x::xs' => (List.exists (fn y => x<>y) xs') andalso (non_duplicated xs')
doesn't work.
Why??? Thanks.
If you would like to obtain the opposite result, just define the function as follows:
fun non_duplicated xs = not (duplicated xs)
That said, you can push not inwards the body of duplicated function using De Morgan laws:
not (a orelse b) <=> (not a) andalso (not b)
not exists equal <=> forall (not equal)
not false <=> true
Then you arrive at the opposite version:
fun non_duplicated [] = true
| non_duplicated (x::xs) =
(List.forall (fn y => x <> y) xs) andalso (non_duplicated xs)