principle-type-system-discipline SKILL.md skillA

principle-type-system-discipline from edbertchan/catstack is an agent skill: a SKILL.md folder an AI agent loads when a task calls for it.

mdr add edbertchan/catstack/principle-type-system-discipline@git:20260914.a72a3ba

Pins git:20260914.a72a3ba in mdr.lock. Pin the fingerprint instead.

Audit grade

A, 17 of 17 checks passed

Kind

SKILL.md skill

Repository

edbertchan/catstack

Path

corpus/skills/principle-type-system-discipline/SKILL.md

Versions recorded

2

Latest version

git:20260914.a72a3ba, committed Sep 14, 2026

First version recorded

Aug 25, 2026

Size

6,429 bytes

Repository stars

6

License

none declared

Apply when designing types, reviewing a function signature, or writing code in any statically-typed language. Make illegal states unrepresentable, brand semantic primitives, parse external data at boundaries, refuse to lie to the compiler, exhaust variants, derive from authoritative schemas.

Collected from public GitHub. Every version is saved exactly as it was and never changes. Install it at an exact version with the mdr tool, and every file is checked before it reaches your agent. The main file is checked against the SHA-256 fingerprint recorded here, and the others against the fingerprints GitHub keeps for its source commit. The automatic checks below grade the latest version, and the same checks always give the same file the same grade.

What the file says

# Type System Discipline

The type checker is a proof assistant. Use it to eliminate impossible states, mismatched primitives, and unhandled variants at compile time. A case the types let you ignore becomes a runtime failure the compiler could have stopped. Prefer defining errors and special cases out of existence over proliferating handlers; unrepresentable states, total functions, and interface redesign (the patterns below) are the tools.

Applies to any typed language. Skills like `typescript-best-practices` ground it in specific syntax.

**The patterns:**

- **Make illegal states unrepresentable.** Model variants as sum types: discriminated unions in TypeScript, enums with payloads in Rust/Swift/Kotlin, sealed classes in Scala, ADTs in Haskell/OCaml. Don't model state as a bag of optional fields where contradictory combinations compile. A subtle anti-pattern worth naming: `{ completed: boolean; completedAt?: Date }` admits `completed: true; completedAt: undefined`, which is meaningless. Derive the boolean from a single source like `completedAt !== null`, or model the variants explicitly as `{ kind: 'open' } | { kind: 'done'; at: Date }`. If a bug forces the question "wait, can
…

Read the whole file at its exact version.

How to install

mdr add edbertchan/catstack/principle-type-system-discipline@sha256:52d0f3d563f0168e

[![mdr](https://markdownregistry.com/badge/art_r53h2zbj5gkmbf5s.svg)](https://markdownregistry.com/a/art_r53h2zbj5gkmbf5s)

Versions

The registry has recorded 2 versions of this file, between Aug 25, 2026 and Sep 14, 2026, about one every 20 days.

Audit of the latest version

Source

API

GET https://markdownregistry.com/api/v1/artifacts/art_r53h2zbj5gkmbf5s
GET https://markdownregistry.com/api/v1/resolve?ref=edbertchan/catstack/principle-type-system-discipline
GET https://markdownregistry.com/api/v1/blob/52d0f3d563f0168ed64e0988577c0a0951bbe7bf217886bc555d6fb3352f0233

Your AI assistant does the legwork. You hear about the deals worth your word. Hand yours the standing instructions at modelranch.com and it joins the network that reads files like this one.

Guides: the SKILL.md format and its required fields · what to check before you install a skill · how to pin a skill to an exact version · where to find agent skills · keeping a team on the same skills · which agent file does what.

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