Add list.flat_map (#202)

3 files changed, 20 insertions, 0 deletions

diff --git a/CHANGELOG.md b/CHANGELOG.md
index f27a2eb..5c26e43 100644
--- a/CHANGELOG.md
+++ b/CHANGELOG.md
@@ -2,6 +2,7 @@
 
 ## Unreleased
 
+- The `list` module gains the `flat_map` function.
 - The `option` module gains the `values` function.
 - The `result` module gains the `values` function.
 - All modules now use the new `#(a, b, ...)` tuple syntax.
diff --git a/src/gleam/list.gleam b/src/gleam/list.gleam
index ea67ba5..a0b3b16 100644
--- a/src/gleam/list.gleam
+++ b/src/gleam/list.gleam
@@ -452,6 +452,20 @@ pub fn flatten(lists: List(List(a))) -> List(a) {
   do_flatten(lists, [])
 }
 
+/// Map and flatten the result
+///
+/// ## Examples
+///
+/// ```
+/// > flat_map([2, 4, 6], fn(x) { [x, x + 1] })
+/// [2, 3, 4, 5, 6, 7]
+/// ```
+///
+pub fn flat_map(over list: List(a), with fun: fn(a) -> List(b)) -> List(b) {
+  map(list, fun)
+  |> flatten
+}
+
 /// Reduces a list of elements into a single value by calling a given function
 /// on each element, going from left to right.
 ///
diff --git a/test/gleam/list_test.gleam b/test/gleam/list_test.gleam
index 1e90583..c7e2a34 100644
--- a/test/gleam/list_test.gleam
+++ b/test/gleam/list_test.gleam
@@ -165,6 +165,11 @@ pub fn flatten_test() {
   |> should.equal([1, 2, 3, 4])
 }
 
+pub fn flat_map_test() {
+  list.flat_map([1, 10, 20], fn(x) { [x, x + 1] })
+  |> should.equal([1, 2, 10, 11, 20, 21])
+}
+
 pub fn fold_test() {
   [1, 2, 3]
   |> list.fold([], fn(x, acc) { [x, ..acc] })