Add iterator.index (#169)

2 files changed, 33 insertions, 0 deletions

diff --git a/src/gleam/iterator.gleam b/src/gleam/iterator.gleam
index f441859..9146833 100644
--- a/src/gleam/iterator.gleam
+++ b/src/gleam/iterator.gleam
@@ -495,6 +495,32 @@ pub fn find(
   }
 }
 
+fn do_index(
+  continuation: fn() -> Action(element),
+  next: Int,
+) -> fn() -> Action(tuple(Int, element)) {
+  fn() {
+    case continuation() {
+      Continue(e, continuation) ->
+        Continue(tuple(next, e), do_index(continuation, next + 1))
+      Stop -> Stop
+    }
+  }
+}
+
+/// Wraps values yielded from an iterator with indices, starting from 0.
+///
+/// ## Examples
+///
+///    > from_list(["a", "b", "c"]) |> index |> to_list
+///    [tuple(0, "a"), tuple(1, "b"), tuple(2, "c")]
+///
+pub fn index(over iterator: Iterator(element)) -> Iterator(tuple(Int, element)) {
+  iterator.continuation
+  |> do_index(0)
+  |> Iterator
+}
+
 /// Creates an iterator that inifinitely applies a function to a value.
 ///
 /// ## Examples
diff --git a/test/gleam/iterator_test.gleam b/test/gleam/iterator_test.gleam
index af38508..098b1b9 100644
--- a/test/gleam/iterator_test.gleam
+++ b/test/gleam/iterator_test.gleam
@@ -251,6 +251,13 @@ pub fn find_test() {
   |> should.equal(Ok(Cat(id: 10)))
 }
 
+pub fn index_test() {
+  iterator.from_list(["a", "b", "c"])
+  |> iterator.index
+  |> iterator.to_list
+  |> should.equal([tuple(0, "a"), tuple(1, "b"), tuple(2, "c")])
+}
+
 pub fn iterate_test() {
   fn(x) { x * 3 }
   |> iterator.iterate(from: 1)