Replace Derived Variable with Query

Removes smells

Symptom

The agent finds a field whose value mirrors a computation on other fields; every writer of the source field must update the derived field consistently or the values drift.

Goal

Derived values are computed on demand; the agent reasons about state by reading source fields and trusting derivations.

Before the refactoring

class Order {
  items;
  total;
  add(item) { this.items.push(item); this.total += item.price; }
}

After the refactoring

class Order {
  items;
  add(item)  { this.items.push(item); }
  total()    { return this.items.reduce((s, i) => s + i.price, 0); }
}

Example source: Illustrative example written for this site, not a quotation from any source.

Pressure

The agent must trace every writer of the source field to verify the derived field stays in sync; bugs hide where one writer forgot to update the derivation.

Tradeoff

Recomputing on every read can multiply cost if the derivation is expensive and the source rarely changes; the agent verifying performance must measure before deciding.

Relief

Mutation scope shrinks to source fields; the agent reasons about state without modeling derivation update timing; consistency is by construction.

Trap

Replacing every derived field with a query — including ones wrapping expensive computations called many times — trades runtime cost for correctness without measuring the impact.