add min/max to gleam/list

2 files changed, 88 insertions, 0 deletions

diff --git a/src/gleam/list.gleam b/src/gleam/list.gleam
index 5f537b9..2d5b624 100644
--- a/src/gleam/list.gleam
+++ b/src/gleam/list.gleam
@@ -2311,3 +2311,55 @@ fn do_shuffle_by_pair_indexes(
     float.compare(a_pair.0, b_pair.0)
   })
 }
+
+/// Takes a list and a comparator, and returns the maximum element in the list
+///
+///
+/// ## Example
+///
+/// ```gleam
+/// range(1, 10) |> list.max(int.compare)
+/// // -> Ok(10)
+/// ```
+///
+/// ```gleam
+/// ["a", "c", "b"] |> list.max(string.compare)
+/// // -> Ok("c")
+/// ```
+pub fn max(
+  over list: List(a),
+  with compare: fn(a, a) -> Order,
+) -> Result(a, Nil) {
+  reduce(over: list, with: fn(acc, other) {
+    case compare(acc, other) {
+      order.Gt -> acc
+      _ -> other
+    }
+  })
+}
+
+/// Takes a list and a comparator, and returns the minimum element in the list
+///
+///
+/// ## Example
+///
+/// ```gleam
+/// range(1, 10) |> list.int(int.compare)
+/// // -> Ok(1)
+/// ```
+///
+/// ```gleam
+/// ["a", "c", "b"] |> list.int(string.compare)
+/// // -> Ok("a")
+/// ```
+pub fn min(
+  over list: List(a),
+  with compare: fn(a, a) -> Order,
+) -> Result(a, Nil) {
+  reduce(over: list, with: fn(acc, other) {
+    case compare(acc, other) {
+      order.Lt -> acc
+      _ -> other
+    }
+  })
+}
diff --git a/test/gleam/list_test.gleam b/test/gleam/list_test.gleam
index 79795e0..0e7e4de 100644
--- a/test/gleam/list_test.gleam
+++ b/test/gleam/list_test.gleam
@@ -1281,3 +1281,39 @@ pub fn shuffle_test() {
   list.range(0, recursion_test_cycles)
   |> list.shuffle()
 }
+
+pub fn max_test() {
+  []
+  |> list.max(int.compare)
+  |> should.equal(Error(Nil))
+
+  [1, 3, 2]
+  |> list.max(int.compare)
+  |> should.equal(Ok(3))
+
+  [-1.0, 1.2, 1.104]
+  |> list.max(float.compare)
+  |> should.equal(Ok(1.2))
+
+  ["a", "c", "b"]
+  |> list.max(string.compare)
+  |> should.equal(Ok("c"))
+}
+
+pub fn min_test() {
+  []
+  |> list.min(int.compare)
+  |> should.equal(Error(Nil))
+
+  [1, 3, 2]
+  |> list.min(int.compare)
+  |> should.equal(Ok(1))
+
+  [-1.0, 1.2, 1.104]
+  |> list.min(float.compare)
+  |> should.equal(Ok(-1.0))
+
+  ["a", "c", "b"]
+  |> list.min(string.compare)
+  |> should.equal(Ok("a"))
+}