Add list.flat_map (#202)

Author: sporto

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.

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.
 ///

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] })