Return number of non empty items

src/Specular/Internal/Incremental/MutableArray.js

@@ -1,42 +1,45 @@
 // empty :: forall a. Effect (MutableArray a)
 export function empty() {
-  return [];
+  return { array: [], nullCnt: 0 };
 }
 
 // push :: forall a. EffectFn2 (MutableArray a) a Unit
 export function push(self, x) {
-  self.push(x);
+  self.array.push(x);
 }
 
 // remove :: forall a. EffectFn2 (MutableArray a) a Unit
 export function remove(self, x) {
-  var index = self.indexOf(x);
+  var index = self.array.indexOf(x);
+
   if (index !== -1) {
-    self[index] = null;
+    self.array[index] = null;
+    self.nullCnt++;
   }
 }
 
 // length :: forall a. EffectFn1 (MutableArray a) Int
 export function length(self) {
-  return self.length;
+  return self.array.length - self.nullCnt;
 }
 
 // iterate :: forall a. EffectFn2 (MutableArray a) (EffectFn1 a Unit) Unit
 export function iterate(self, fn) {
   let writeIndex = 0;
 
   // Clean up array using in-place filtering technique
-  for (let i = 0; i < self.length; i++) {
-    const value = self[i];
+  for (let i = 0; i < self.array.length; i++) {
+    const value = self.array[i];
     if (value !== null) {
       fn(value);
       if (writeIndex !== i) {
-        self[writeIndex] = value; // Move non-null values to the left
+        self.array[writeIndex] = value; // Move non-null values to the left
       }
       writeIndex++;
     }
   }
 
   // Trim the array to remove null values
-  self.length = writeIndex;
+  self.array.length = writeIndex;
+  self.nullCnt = 0;
 }