Add singleton function to immut/list

immut/list/list.mbt

@@ -1025,3 +1025,7 @@ pub impl[X : @quickcheck.Arbitrary] @quickcheck.Arbitrary for T[X] with arbitrar
 ) {
  @quickcheck.Arbitrary::arbitrary(size, rs) |> from_array
 }
+
+pub fn singleton[A](x : A) -> T[A] {
+ Cons(x, Nil)
+}

immut/list/list.mbti

@@ -6,6 +6,8 @@ alias @moonbitlang/core/quickcheck as @quickcheck
 // Values
 fn repeat[A](Int, A) -> T[A]
 
+fn singleton[A](A) -> T[A]
+
 fn unfold[A, S]((S) -> (A, S)?, ~init : S) -> T[A]
 
 fn unzip[A, B](T[(A, B)]) -> (T[A], T[B])

immut/list/list_test.mbt

@@ -21,7 +21,8 @@ test "from_array" {
 
 test "length" {
  let ls = @list.of([1, 2, 3, 4, 5])
- inspect!(ls.length(), content="5")
+ @json.inspect!(ls.length(), content=5)
+ @json.inspect!(@list.singleton(11), content=[11])
 }
 
 test "iter" {