feat: add Map::set_default

builtin/builtin.mbti

@@ -175,6 +175,7 @@ impl Map {
   op_set[K : Hash + Eq, V](Self[K, V], K, V) -> Unit
   remove[K : Hash + Eq, V](Self[K, V], K) -> Unit
   set[K : Hash + Eq, V](Self[K, V], K, V) -> Unit
+  set_default[K : Hash + Eq, V](Self[K, V], K, () -> V) -> V
   size[K, V](Self[K, V]) -> Int
   to_array[K, V](Self[K, V]) -> Array[Tuple[K, V]]
   to_string[K : Show, V : Show](Self[K, V]) -> String

builtin/linked_hash_map.mbt

@@ -193,6 +193,22 @@ pub fn get_or_default[K : Hash + Eq, V](
   }
 }
 
+/// Returns the value for the given key, or sets and returns a default value if the key does not exist.
+pub fn set_default[K : Hash + Eq, V](
+  self : Map[K, V],
+  key : K,
+  default : () -> V
+) -> V {
+  match self.get(key) {
+    Some(v) => v
+    None => {
+      let v = default()
+      self.set(key, v)
+      v
+    }
+  }
+}
+
 /// Check if the hash map contains a key.
 pub fn contains[K : Hash + Eq, V](self : Map[K, V], key : K) -> Bool {
   match self.get(key) {

builtin/linked_hash_map_test.mbt

@@ -62,6 +62,15 @@ test "get_or_default" {
   @assertion.assert_eq(m.get_or_default("d", 42), 42)?
 }
 
+test "set_default" {
+  let m : Map[String, Array[Int]] = Map::new()
+  m.set_default("a", fn() { Array::new() }).push(1)
+  m.set_default("b", fn() { Array::new() }).push(2)
+  m.set_default("a", fn() { Array::new() }).push(3)
+  @assertion.assert_eq(m.get("a"), Some([1, 3]))?
+  @assertion.assert_eq(m.get("b"), Some([2]))?
+}
+
 test "op_set" {
   let m : Map[String, Int] = Map::new()
   m["a"] = 1