a bit of refactoring for quad tree's 'toTriangles' function

temp-squares/fcore/quad-tree.sml

@@ -213,7 +213,7 @@ struct
         NODE {tl = tl, bl = bl, tr = tr, br = br}
       end
 
-  fun foldWithDuplicates (f, tree, acc) = 
+  fun foldWithDuplicates (f, tree, acc) =
     case tree of
       LEAF item => f (item, acc)
     | NODE {tl, tr, bl, br} =>
@@ -226,34 +226,37 @@ struct
         end
 
   fun insertItemIntoTree (item, acc) =
-    if #data item = 0 then 
+    if #data item = 0 then
       (* ignore specific data by not inserting it into tree.
        * May later functorise this quad tree,
        * and allow different types of data
        * to be ignored/stored in #data field. *)
       acc
-    else 
+    else
       BinTree.insert (item, acc)
 
   fun toList qtree =
-    let
-      val tree = foldWithDuplicates (insertItemIntoTree, qtree, BinTree.empty)
-    in
-      BinTree.toList (tree, [])
+    let val tree = foldWithDuplicates (insertItemIntoTree, qtree, BinTree.empty)
+    in BinTree.toList (tree, [])
     end
 
-  fun toTriangles (windowWidth, windowHeight, squares, acc) =
-    case squares of
-      {x, y, ex, ey, data = _} :: tl =>
-        let
-          val startX = Ndc.fromPixelX (x, windowWidth, windowHeight)
-          val endX = Ndc.fromPixelX (ex, windowWidth, windowHeight)
-          val startY = Ndc.fromPixelY (y, windowWidth, windowHeight)
-          val endY = Ndc.fromPixelY (ey, windowWidth, windowHeight)
+  local
+    fun loop (windowWidth, windowHeight, squares, acc) =
+      case squares of
+        {x, y, ex, ey, data = _} :: tl =>
+          let
+            val startX = Ndc.fromPixelX (x, windowWidth, windowHeight)
+            val endX = Ndc.fromPixelX (ex, windowWidth, windowHeight)
+            val startY = Ndc.fromPixelY (y, windowWidth, windowHeight)
+            val endY = Ndc.fromPixelY (ey, windowWidth, windowHeight)
 
-          val acc = Ndc.ltrbToVertex (startX, startY, endX, endY) :: acc
-        in
-          toTriangles (windowWidth, windowHeight, tl, acc)
-        end
-    | [] => Vector.concat acc
+            val acc = Ndc.ltrbToVertex (startX, startY, endX, endY) :: acc
+          in
+            loop (windowWidth, windowHeight, tl, acc)
+          end
+      | [] => Vector.concat acc
+  in
+    fun toTriangles (windowWidth, windowHeight, squares) =
+      loop (windowWidth, windowHeight, squares, [])
+  end
 end