Use swapRemove

src/Specular/Internal/Incremental/MutableArray.js

@@ -11,8 +11,17 @@ export function push(self, x) {
 // remove :: forall a. EffectFn2 (MutableArray a) a Unit
 export function remove(self, x) {
   var index = self.indexOf(x);
+
+  // The removed element is replaced by the last element of the array.
+  // This does not preserve ordering, but is *O*(1). 
   if (index !== -1) {
-    self[index] = null;
+    var lastIdx = self.length - 1;
+
+    if (index !== lastIdx) {
+      self[index] = self[lastIdx];
+    }
+
+    self.length = lastIdx;
   }
 }
 
@@ -23,20 +32,7 @@ export function length(self) {
 
 // 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];
-    if (value !== null) {
-      fn(value);
-      if (writeIndex !== i) {
-        self[writeIndex] = value; // Move non-null values to the left
-      }
-      writeIndex++;
-    }
+    fn(self[i]);
   }
-
-  // Trim the array to remove null values
-  self.length = writeIndex;
 }