feat(builtin): add Iter2::concat()

builtin/builtin.mbti

@@ -278,6 +278,7 @@ impl[T : Show] Show for Iter[T]
 
 type Iter2
 impl Iter2 {
+  concat[A, B](Self[A, B], Self[A, B]) -> Self[A, B]
   each[A, B](Self[A, B], (A, B) -> Unit) -> Unit
   iter[A, B](Self[A, B]) -> Iter[(A, B)]
   iter2[A, B](Self[A, B]) -> Self[A, B]

builtin/iter2.mbt

@@ -88,3 +88,35 @@ pub fn Iter2::to_array[A, B](self : Iter2[A, B]) -> Array[(A, B)] {
   }
   arr
 }
+
+///|
+/// Combines two iterators into one by appending the elements of the second iterator to the first.
+///
+/// # Arguments
+///
+/// * `self` - The first input iterator.
+/// * `other` - The second input iterator to be appended to the first.
+///
+/// # Returns
+///
+/// Returns a new iterator that contains the elements of `self` followed by the elements of `other`.
+///
+/// # Examples
+///
+/// ```moonbit
+/// test "Iter2::concat" {
+///   inspect!(
+///     "abc".iter2().concat("def".iter2()).to_array(),
+///     content="[(0, 'a'), (1, 'b'), (2, 'c'), (0, 'd'), (1, 'e'), (2, 'f')]",
+///   )
+/// }
+/// ```
+pub fn Iter2::concat[A, B](
+  self : Iter2[A, B],
+  other : Iter2[A, B]
+) -> Iter2[A, B] {
+  fn(yield_) {
+    guard self.run(yield_) == IterContinue else { IterEnd }
+    other.run(yield_)
+  }
+}

builtin/iter2_test.mbt

@@ -12,6 +12,7 @@
 // See the License for the specific language governing permissions and
 // limitations under the License.
 
+///|
 test "iter2_to_iter" {
   let numbers = [1, 2, 3]
   let iter2_numbers = numbers.iter2() // creates Iter2[Int, Int] from Array
@@ -20,6 +21,7 @@ test "iter2_to_iter" {
   inspect!(result, content="[(0, 1), (1, 2), (2, 3)]")
 }
 
+///|
 test "iter2 identity function should return its input" {
   let m : @builtin.Map[String, Int] = @builtin.Map::new()
   m.set("x", 1)
@@ -29,3 +31,25 @@ test "iter2 identity function should return its input" {
   let result = same_iter2.to_array()
   inspect!(result, content="[(\"x\", 1), (\"y\", 2)]")
 }
+
+///|
+test "Iter2::concat" {
+  inspect!(
+    "abcde".iter2().concat("fghij".iter2()).to_array(),
+    content="[(0, 'a'), (1, 'b'), (2, 'c'), (3, 'd'), (4, 'e'), (0, 'f'), (1, 'g'), (2, 'h'), (3, 'i'), (4, 'j')]",
+  )
+}
+
+///|
+test "Iter2::concat with early return" {
+  let first = Iter2::new(fn(y) {
+    for i, v in "abcde" {
+      ignore(y(i, v))
+    }
+    IterEnd
+  })
+  inspect!(
+    first.concat("fghij".iter2()).to_array(),
+    content="[(0, 'a'), (1, 'b'), (2, 'c'), (3, 'd'), (4, 'e')]",
+  )
+}