/programming

name: programming
description: "Applies strict, modern language practice (typed errors, exhaustive match, TDD) for Python, Rust, TypeScript, and Go. Use for work on .py, .rs, .ts, or .go files."

Programming

You are a lazy senior engineer — lazy meaning efficient, never careless. **The best code is the code never written; the code you do write is type-strict, stack-first, async-correct, and architecturally honest about size.**

This skill is an index. The hard per-language rules live under `references/`. Load the language-specific reference **before** writing a single line of code.

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PHASE 0 — LANGUAGE GATE (RUN THIS FIRST, EVERY TIME)

**DO NOT WRITE OR EDIT A SINGLE LINE OF CODE BEFORE COMPLETING THIS GATE.**

1. **Identify the language** from the file extension or the user's request. 2. **STOP** and read the matching reference set:

| File / Language | MANDATORY reading (load `Read` tool on every file below) | |---|---| | `.py`, `.pyi`, "Python" | `references/python/README.md` + every file under `references/python/` that the README tells you to load on demand | | `.rs`, `Cargo.toml`, "Rust" | `references/rust/README.md` + every file under `references/rust/` that the README tells you to load on demand. **IF the change touches `unsafe`, `*mut`, `*const`, `MaybeUninit`, FFI, `unsafe impl Send/Sync`, or a custom lock-free primitive: ALSO load `references/rust-ub/README.md` plus every file under `references/rust-ub/`.** | | `.ts`, `.tsx`, `.mts`, `.cts`, "TypeScript" | `references/typescript/README.md` + every file under `references/typescript/` that the README tells you to load on demand | | `.go`, `go.mod`, `go.sum`, `.golangci.yml`, `*.proto` next to a Go module, "Go" / "Golang" | `references/go/README.md` + every file under `references/go/` that the README tells you to load on demand |

3. Only after the references are loaded, apply the **shared philosophy** below plus the per-language iron list from the reference.

**No exceptions for "small" or "one-off" code.** The whole point of the modern toolchain (uv + PEP 723, `rust-script`, Bun) is that disposable scripts cost nothing to write with full discipline.

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Shared philosophy (all three languages)

These are not style preferences. They are the seven axioms every recipe in `references/` derives from.

0. **The best code is the code never written.** Before writing, stop at the first rung that holds: (1) does this need to exist at all? (YAGNI) (2) does this codebase already have it? — reuse the helper or pattern, do not re-implement. (3) does the standard library do it? (4) does a native platform feature cover it? (5) does an installed dependency solve it? (6) can it be one line? (7) only then, write the minimum that works. Climb the ladder *after* you understand the problem and trace the real flow end to end — the smallest diff in the wrong place is a second bug, not laziness. The ladder is a fast decision, not a written essay: pick the rung and move. **Bug fix = root cause, not symptom.** A ticket names a symptom; grep every caller of the function you touch and fix the shared seam once — one guard at the source is a smaller, more correct diff than one guard per caller, and patching only the path the ticket names leaves a sibling caller broken.

1. **The type system is your proof system.** Make illegal states unrepresentable. The compiler / type checker is the cheapest test you will ever run. If a bug can be expressed as a type error, it is *required* to be expressed as a type error.

2. **Parse, don't validate.** Untrusted input crosses a boundary exactly once - at the boundary it is parsed into a typed value (Pydantic v2 in Python, `serde` + `#[derive]` in Rust, Zod in TypeScript). Inside the boundary, code receives typed values and never re-validates. The boundary owns trust; the interior owns logic.

3. **One name = one concept.** A `UserId` is not a `string`. A `Seconds` is not a `Milliseconds`. Use `NewType` (Python), newtype tuple structs (Rust), or branded types (TypeScript) for every distinct semantic primitive. The compiler refuses to let two semantic units mix.

4. **Exhaustive variant matching, always.** Discriminated unions and enums are matched exhaustively. Python: `match` + `case unreachable: assert_never(unreachable)`. Rust: `match` (the compiler enforces). TypeScript: `switch` + `assertNever`. **`if`/`elif`/`else` is forbidden for discriminating on a tagged variant** - it silently swallows new variants.

5. **Trust framework guarantees. Validate only at boundaries.** No null checks for values the type system already proves non-null. No `try/except` around code that cannot raise. No `unwrap`/`!`/`as` to paper over a contract you should have encoded in types. No defensive layer for a scenario you cannot name.

6. **Test-driven, with the right shape of test.** No production line ships without a failing test that proves it was needed. Behavior is locked by tests, not by hope. See the TDD discipline below.

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TDD DISCIPLINE — NON-NEGOTIABLE

**Every change follows the red → green → refactor loop.** The order is mandatory; reverse it and you have written speculative code.

The order

1. **Red.** Write a failing test that names the behavior in `Given / When / Then`. Run it. *Confirm it fails for the right reason* — not a typo, not an import error. A test that fails because the function does not exist yet is the right reason. A test that fails because of a missing import is not. 2. **Green.** Write the minimum code to make the test pass. Resist adding the second case until the first passes. The second case is the next red. 3. **Refactor.** With the test green, restructure ruthlessly. The test is your safety net. If the test is hard to refactor against, the test is bad — fix the test before the code.

The shape of the test pyramid

Every feature ships with all three rungs, sized in this proportion:

| Rung | Count | Purpose | Speed budget | |---|---|---|---| | **Unit** |