done getting player's projectiles to collide with enemies successfully, and tested (although the code of the enemy reacting to the player is not the best)

fcore/projectile.sml

@@ -0,0 +1,18 @@
+structure Projectile =
+struct
+  fun helpGenerateTree (pos, projectiles, acc) =
+    if pos = Vector.length projectiles then
+      acc
+    else
+      let
+        val size = Player.defeatedSizeInt
+
+        val {x, y, facing = _} = Vector.sub (projectiles, pos)
+        val acc = QuadTree.insert (x, y, size, size, 0, 0, 1920, 1080, pos, acc)
+      in
+        helpGenerateTree (pos + 1, projectiles, acc)
+      end
+
+  fun generateTree projectiles =
+    helpGenerateTree (0, projectiles, QuadTree.empty)
+end