Option then function

3 files changed, 46 insertions, 1 deletions

diff --git a/CHANGELOG.md b/CHANGELOG.md
index 48415df..08acb07 100644
--- a/CHANGELOG.md
+++ b/CHANGELOG.md
@@ -15,7 +15,7 @@
 - The `dynamic.tuple2_of` function has been renamed to `dynamic.typed_tuple2`.
 - The `list.traverse` function has been renamed to `list.try_map`.
 - The `list.traverse` first argument gains the label `over`.
-- The `option` module gains the the `map` and `flatten` functions.
+- The `option` module gains the the `map`, `flatten` and `then` functions.
 
 ## 0.9.0 - 2020-05-26
 
diff --git a/src/gleam/option.gleam b/src/gleam/option.gleam
index 9a2e479..2c01b4a 100644
--- a/src/gleam/option.gleam
+++ b/src/gleam/option.gleam
@@ -126,3 +126,34 @@ pub fn flatten(option: Option(Option(a))) -> Option(a) {
     None -> None
   }
 }
+
+/// Update a value held within the Some of an Option by calling a given function
+/// on it, where the given function also returns an Option. The two Options are
+/// then merged together into one Option.
+///
+/// If the Option is a None rather than Some the function is not called and the
+/// Option stays the same.
+///
+/// This function is the equivalent of calling `map` followed by `flatten`, and
+/// it is useful for chaining together multiple functions that return Options.
+///
+/// ## Examples
+///
+///    > then(Some(1), fn(x) { Some(x + 1) })
+///    Some(2)
+///
+///    > then(Some(1), fn(x) { Some(tuple("a", x)) })
+///    Some(tuple("a", 1))
+///
+///    > then(Some(1), fn(x) { None })
+///    None)
+///
+///    > then(None, fn(x) { Some(x + 1) })
+///    None
+///
+pub fn then(option: Option(a), apply fun: fn(a) -> Option(b)) -> Option(b) {
+  case option {
+    Some(x) -> fun(x)
+    None -> None
+  }
+}
diff --git a/test/gleam/option_test.gleam b/test/gleam/option_test.gleam
index 03f4186..7271769 100644
--- a/test/gleam/option_test.gleam
+++ b/test/gleam/option_test.gleam
@@ -68,3 +68,17 @@ pub fn flatten_option_test() {
   |> option.flatten()
   |> should.equal(None)
 }
+
+pub fn then_option_test() {
+  Some(1)
+  |> option.then(fn(x) { Some(x + 1) })
+  |> should.equal(Some(2))
+
+  Some(1)
+  |> option.then(fn(x) { Some("2") })
+  |> should.equal(Some("2"))
+
+  None
+  |> option.then(fn(x) { Some(x + 1) })
+  |> should.equal(None)
+}