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Mutable (Grade A)Security-tested data-ai skill for Claude AI. Grade A. Avoid mutable variables like IORef, STRef, and ST in Haskell and prefer immutable records. Use STM variables such as TVar for thread communication

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Daniel NikulshynReviewed by Daniel Nikulshyn·Updated July 2026

Overview

Avoid mutable variables like IORef, STRef, and ST in Haskell and prefer immutable records. Use STM variables such as TVar for thread communication. This advice applies when writing or reviewing Haskell code that uses mutable state or concurrency. In general programming, mutable variables should be avoided whenever possible, especially in Haskell, a purely functional language. Immutable records can be efficiently updated with differences, rendering mutable state unnecessary in most cases. In order to achieve mutable state, Haskell provides low-level mechanisms which should be avoided, such as IORef, STRef, and ST. Instead, for thread communication, it is recommended to use STM's mutable variables, including TVar, TMVar, TChan, TQueue, and TBQueue. However, these should not be used directly and should be abstracted and utilized properly to maintain a clean and organized codebase.

Key features

  • Immutable record recommendations
  • STM variable usage guidelines
  • Avoidance of IORef, STRef, and ST

Pricing

Model
Free
Category
Skills
Rating
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Use cases

Haskell Code Review

Use Mutable to review Haskell code and ensure it follows best practices for mutable variable usage and concurrent programming.

Concurrent Programming in Haskell

Use Mutable to guide the implementation of concurrent Haskell code, ensuring safe and efficient use of mutable variables and thread communication.

Pros & Cons

Pros

  • Encourages best practices for mutable variable usage in Haskell
  • Helps prevent common pitfalls in concurrent programming
  • Provides clear guidelines for thread communication

Cons

  • Limited to Haskell-specific advice
  • May not cover all edge cases or complex scenarios

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Q&A

What’s included in initial troubleshooting?

For initial troubleshooting, the assigned Premium Support Engineer or Customer Reliability Engineer will review and acknowledge your ticket. They may request additional information such as screenshots, error messages, log files, diagnostics files, support bundles, or command output, and may collaborate with other support staff, engineering, or the regional incident commander. If a callback was requested, they will determine if screen sharing is the most effective way to drive ticket resolution and invite you to join a session.

Asked by Noelia Campos · Nov 29, 2025

What are the limitations?

Mutable is limited to Haskell-specific advice and may not cover all edge cases or complex scenarios.

Asked by Lena Fischer · Nov 13, 2025

What are the key features?

Immutable record recommendations, STM variable usage guidelines, and avoidance of IORef, STRef, and ST are the key features of Mutable.

Asked by Tobias Hartmann · Nov 11, 2025

What is crisis prevention?

Crisis prevention allows GitHub Enterprise Server customers to prepare for—and experience—an incident without risk. Your Customer Reliability Engineer (CRE) guides your team through an incident simulation in a safe, controlled environment. Crisis Prevention consists of up to four sessions covering reliability best practices, incident preparation, and efficient interaction with GitHub Support. After the simulation, your CRE runs a detailed retrospective, uncovering lessons learned and improvement suggestions.

Asked by Lindiwe Mahlangu · Nov 11, 2025

How long does GitHub Premium Support take?

GitHub Premium Support and GitHub Premium Plus Support customers have service‑level agreements (SLAs) for initial response. For urgent priority tickets, the initial response SLA guarantees a 30‑minute response time, which includes troubleshooting. For high‑priority tickets, the SLA provides a four‑hour response. Resolution times are not estimated because ticket complexity varies, but we review metrics regularly and aim to reduce times.

Asked by Qiu Yan · Nov 8, 2025

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