# Typed Racket mutable hash sets

**URL:** <https://racket.discourse.group/t/typed-racket-mutable-hash-sets/3339>\
**Category:** Questions & Answers\
**Created:** [November 19, 2024, 7:06pm UTC](https://racket.discourse.group/t/typed-racket-mutable-hash-sets/3339 "2024-11-19T19:06:59Z")\
**Posts on this page:** 1\
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**Author:** ![NoahStoryM](https://yyz2.discourse-cdn.com/free1/user_avatar/racket.discourse.group/noahstorym/32/17_2.png) [@NoahStoryM](https://racket.discourse.group/u/NoahStoryM)\
**Post date:** [November 23, 2024, 9:01am UTC](https://racket.discourse.group/t/typed-racket-mutable-hash-sets/3339/7 "2024-11-23T09:01:35Z")

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I think we can use `Parameter` and `struct` to define mutable types that support contravariance. See [A new way to type mutable data structures with opposite variance in typed racket](https://racket.discourse.group/t/a-new-way-to-type-mutable-data-structures-with-opposite-variance-in-typed-racket/1846).

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