add list.scan

3 files changed, 54 insertions, 1 deletions

diff --git a/CHANGELOG.md b/CHANGELOG.md
index 6d0cc1c..57a3f4c 100644
--- a/CHANGELOG.md
+++ b/CHANGELOG.md
@@ -8,7 +8,7 @@
   functions and their typed equivalents `typed_tuple3`, `typed_tuple4`,
   `typed_tuple5`, `typed_tuple6`.
 - The `list` module gains the `drop_while`, `map_fold`, `take_while`, `reduce`,
-  `chunk` and `sized_chunk` functions.
+  `chunk`, `sized_chunk` and `scan` functions.
 - The `iterator` module gains the `index`, `iterate`, `zip`, `scan`,
   `take_while`, `drop_while`, `chunk`, `sized_chunk`, `intersperse`, `reduce`, `any` and `all` functions.
 - Breaking change in `iterator.take`. Now it returns an iterator instead of a list.
diff --git a/src/gleam/list.gleam b/src/gleam/list.gleam
index dc03ec1..9f88063 100644
--- a/src/gleam/list.gleam
+++ b/src/gleam/list.gleam
@@ -1401,3 +1401,33 @@ pub fn reduce(over list: List(a), with fun: fn(a, a) -> a) -> Result(a, Nil) {
       |> Ok
   }
 }
+
+fn do_scan(
+  list: List(a),
+  accumulator: b,
+  accumulated: List(b),
+  fun: fn(a, b) -> b,
+) -> List(b) {
+  case list {
+    [] -> reverse(accumulated)
+    [x, ..xs] -> {
+      let next = fun(x, accumulator)
+      do_scan(xs, next, [next, ..accumulated], fun)
+    }
+  }
+}
+
+/// Similar to `fold`, but yields the state of the accumulator at each stage.
+///
+/// ## Examples
+///
+///    > scan(over: [1, 2, 3], from: 100, with: fn(i, acc) { acc + i })
+///    [101, 103, 106]
+///
+pub fn scan(
+  over list: List(a),
+  from initial: b,
+  with fun: fn(a, b) -> b,
+) -> List(b) {
+  do_scan(list, initial, [], fun)
+}
diff --git a/test/gleam/list_test.gleam b/test/gleam/list_test.gleam
index bf93203..572812c 100644
--- a/test/gleam/list_test.gleam
+++ b/test/gleam/list_test.gleam
@@ -622,3 +622,26 @@ pub fn reduce_test() {
   |> list.reduce(with: fn(x, y) { x + y })
   |> should.equal(Ok(15))
 }
+
+pub fn scan_test() {
+  []
+  |> list.scan(from: 0, with: fn(i, acc) { i + acc })
+  |> should.equal([])
+
+  [1, 2, 3, 4]
+  |> list.scan(from: 0, with: fn(i, acc) { 2 * i + acc })
+  |> should.equal([2, 6, 12, 20])
+
+  [1, 2, 3, 4]
+  |> list.scan(
+    from: "",
+    with: fn(i, acc) {
+      case int.is_even(i) {
+        True -> "Even"
+        False -> "Odd"
+      }
+      |> string.append(acc, _)
+    },
+  )
+  |> should.equal(["Odd", "OddEven", "OddEvenOdd", "OddEvenOddEven"])
+}