• Rust

let c = items.iter().unique().count();

• Dart

var c = items.toSet().length;

• Pascal

function CountUniqueItems(Items: TStrings): Integer;
var
  List: TStringList;
begin
  List := TStringList.Create;
  List.Duplicates := dupIgnore;
  List.Sorted := True;
  List.AddStrings(Items);
  Result := List.Count;
  List.Free;
end;

begin
 ...
 c := CountUniqueItems(items);
 ...
end.

• Clojure

(def items [1 2 3 4 4 5 5 5])

(def c (count (set items)))

• C#

int c = items.Distinct().Count();

• Elixir

c = items |> Enum.uniq |> length

• Go

distinct := make(map[T]bool)
for _, v := range items {
	distinct[v] = true
}
c := len(distinct)

• Go

func count[T comparable](items []T) int {
	distinct := make(map[T]bool)
	for _, v := range items {
		distinct[v] = true
	}
	return len(distinct)
}

• Haskell

c = count . nub $ items

• JS

const c = new Set(items).size;

• Java

long c = items.stream().distinct().count();

• Kotlin

val c = items.distinct().size

• Lua

local c=#items

• PHP

$c = count(array_unique($items));

• Perl

my $c = scalar(uniq @items);

• Python

c = len(set(items))

• Python

c = []
for x in items:
    if x not in c:
        c.append(x)
c = len(c)

• Python

c = 0
for a, x in enumerate(items):
    if x not in items[a + 1:]:
        c = c + 1

• Python

c = len({*items})

• Ruby

c = items.uniq.count