code-that-fits-in-your-head

# Code That Fits in Your Head Engineering heuristics for sustainable software, based on Mark Seemann's 2021 book. ## Philosophy (Why This Skill Exists) Software development is principally a **design activity**, not construction. Code is written by people, read by people, and endures far longer than any "project". These heuristics are rules of thumb (not scientific laws) that make software sustainable — that is, cheap to change and resilient to the next 10,000 small decisions. Core mental model from Chapter 1: | Metaphor | What it gets right | What it misses | |----------|-------------------|-----------------| | **Building a house** | Plans, structure | Software endures; there's no construction phase (compiling is free); dependencies can start anywhere | | **Growing a garden** | Pruning, refactoring, tending | Code doesn't grow by itself — someone writes it | | **Art / craft** | Skill, mastery, situational knowledge | Doesn't scale; leaves newcomers without guidance | | **Engineering** (the target) | Heuristics, review, sign-off, checklists | We're not there yet — physical-construction calculations don't apply | > "The act of describing a program in unambiguous detail and the act of programming are one and the same." — Kevlin Henney **Practical implications for a code agent:** 1. **Successful software endures.** Prefer changes that don't lock the future; avoid "finish-and-forget" thinking. 2. **Heuristics, not laws.** Every rule in this skill has context. Understand *w...