mutable

A Haskell coding guideline that favors immutable records instead of variables whose contents can change, while allowing STM variables for thread communication.

In plain words
What is it for?
Use it when writing or reviewing Haskell code involving mutable references, immutable record updates, or communication between concurrent threads.
Why use it?
It reduces hidden state changes that can make functional code harder to understand and review.

Skill for Claude CodeCodex

Install

Getting it into your agent

One page per mod, every tool's command on it. A separate URL per tool would split the same page into five that compete with each other.

agentmods
npx agentmods add skills/ncaq/konoka/mutable
Any agent
npx skills add ncaq/konoka --skill mutable
Clone the repo
git clone --depth 1 https://github.com/ncaq/konoka

Made for: Claude Code, Codex.

Per session 48 Skills are progressive disclosure: only the name and description are preloaded; the body loads when the skill is used.
When invoked 329 The whole file, excluding the scripts and references it only reads on demand.
Security scan A 0 findings. Scan, not verified.
Origin original No closer match found in the catalogue.
Token cost

What it costs to keep this loaded

Counted locally with the o200k_base tokenizer, which is exact for GPT models; Claude uses its own tokenizer and its counts differ. Treat this as one consistent yardstick across the catalogue rather than a bill. Prices are per million input tokens.

ModelPer sessionOnce invoked
Fable 5 $0.00048 $0.00329
Opus 5 $0.00024 $0.00164
Sonnet 5 $0.00010 $0.00066
Haiku 4.5 $0.00005 $0.00033

Measured 2d ago against content hash 43cb5de0b507, method: parsed. Prices are Anthropic first-party input rates as of 2026-08-30, from the pricing page.

Security

Grade A, and why

mutable scanned grade A with 0 findings against 26 rules in 11 categories — prompt injection, anti-refusal, data exfiltration, privilege escalation, supply chain, agent snooping, system-prompt leakage, SSRF and excessive agency — measured 2d ago.

A static scan of the body, not an audit. Every finding is printed with the line that produced it so you can judge whether it matters here. A mod is markdown that instructs an agent; that is exactly why what it instructs is worth reading.

Nothing flagged

None of the 26 patterns this scan looks for appear in this file: no shell pipes, no recursive deletes, no credential paths, no hidden text, no instruction-override or anti-refusal phrasing, no agent-config snooping. That is not a guarantee, it is the absence of the things that are checkable.

plugins/haskell-tasuke/skills/mutable/SKILL.md · 42 lines

What it actually says

mutableな変数

普段はmutableな変数の使用を避ける

一般的なプログラミングにおいても、 mutableな変数はできるだけ避けるべきだとされています。

特にHaskellは純粋関数型言語です。

immutableにレコードを差分更新することを効率よく行えるようになっているので、 mutableな変数が欲しいことは滅多にありません。

mutableな変数を実現する機構は原則禁止です。

例えば以下の機構は避けるべきです。

  • IORef
  • STRef
  • ST

スレッド間の通信にはSTMのミュータブルな変数を使う

スレッド間の通信を行いたい場合はミュータブルな変数を使った方がスムーズなことがあります。

例えば以下の型が該当します。

  • TVar
  • TMVar
  • TChan
  • TQueue
  • TBQueue

しかしこれらも低レイヤーな機構なので、 そのまま深く入り組ませずに、 抽象化して使うべきです。

Changes

What this file has done since we first saw it

Hashed on every crawl. A supply-chain change to an agent config is a question of when, not whether, so the history is kept rather than the latest state alone.

  1. 2d ago First seen · 42 lines · 48 tokens per session scan A 43cb5de0b507

Subscribe to this mod's changes

mutable is a skill published in the GitHub repository ncaq/konoka (3 stars, last pushed 4d ago), licensed Apache-2.0. It adds 48 tokens to every session and 329 once invoked, about $0.0002 per session on Opus 5. A static security scan graded it A with 0 findings. No closer match exists in the catalogue, so it is treated as the original; first seen 2026-08-31.

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