---
title: "New Flow Language Rule: Constrained Writes"
short-title: "Introducing Constrained Writes"
authors: [jordan-brown]
medium-link: "https://medium.com/flow-type/new-flow-language-rule-constrained-writes-4c70e375d190"
---

:::info[Historical]
This post announced the constrained writes rule when it was introduced. For current syntax and behavior, see the [Variables](/en/docs/lang/variables) documentation.
:::

Flow is releasing a new language rule that determines the type of an unannotated variable at its initialization. Along with these new rules come several fixes to soundness bugs that were causing refinements to not be invalidated.

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