Promise then_try

3 files changed, 32 insertions, 1 deletions

diff --git a/CHANGELOG.md b/CHANGELOG.md
index e99cc60..77f90e2 100644
--- a/CHANGELOG.md
+++ b/CHANGELOG.md
@@ -8,4 +8,4 @@
   `to_list`, `from_list`, `map`, `fold`, `fold_right`, `get`, and `length`
   functions.
 - The `javascript/promise` module was created with the `Promise(value)` type and
-  `resolve`, `tap`, `map_try`, `map`, and `then` functions.
+  `resolve`, `tap`, `map_try`, `map`, `then_try` and `then` functions.
diff --git a/src/gleam/javascript/promise.gleam b/src/gleam/javascript/promise.gleam
index 7e3670a..8407c8c 100644
--- a/src/gleam/javascript/promise.gleam
+++ b/src/gleam/javascript/promise.gleam
@@ -31,3 +31,16 @@ pub fn map_try(
     }
   })
 }
+
+pub fn then_try(
+  promise: Promise(Result(a, e)),
+  callback: fn(a) -> Promise(Result(b, e)),
+) -> Promise(Result(b, e)) {
+  promise
+  |> then(fn(result) {
+    case result {
+      Ok(a) -> callback(a)
+      Error(e) -> resolve(Error(e))
+    }
+  })
+}
diff --git a/test/gleam/javascript/promise_test.gleam b/test/gleam/javascript/promise_test.gleam
index 9d37984..ea7c3d3 100644
--- a/test/gleam/javascript/promise_test.gleam
+++ b/test/gleam/javascript/promise_test.gleam
@@ -53,3 +53,21 @@ pub fn map_try_error_test() -> Promise(Result(Int, Int)) {
   |> promise.map_try(fn(a) { Ok(a + 1) })
   |> promise.tap(fn(a) { assert Error(1) = a })
 }
+
+pub fn then_try_ok_ok_test() -> Promise(Result(Int, Int)) {
+  promise.resolve(Ok(1))
+  |> promise.then_try(fn(a) { promise.resolve(Ok(a + 1)) })
+  |> promise.tap(fn(a) { assert Ok(2) = a })
+}
+
+pub fn then_try_ok_error_test() -> Promise(Result(Int, Int)) {
+  promise.resolve(Ok(1))
+  |> promise.then_try(fn(a) { promise.resolve(Error(a + 1)) })
+  |> promise.tap(fn(a) { assert Error(2) = a })
+}
+
+pub fn then_try_error_test() -> Promise(Result(Int, Int)) {
+  promise.resolve(Error(1))
+  |> promise.then_try(fn(a) { promise.resolve(Ok(a + 1)) })
+  |> promise.tap(fn(a) { assert Error(1) = a })
+}