# Bb80 Invariant Construction

> Extract invariants from specification, encode in types/structure, enable single-pass construction.

- Skill: `majiayu000/bb80-invariant-construction` (Agent Skill, multi-file: 2 files)
- Install (CLI): `npx skillmds add majiayu000/bb80-invariant-construction`
- Raw SKILL.md: https://api.skillmd.com/api/skills/majiayu000/bb80-invariant-construction/raw
- Safety review: pending
- Works with: Claude Code, Claude.ai, OpenAI Codex
- Category: Coding & Dev Tools
- Author: majiayu000 (https://skillmd.com/u/majiayu000)
- Updated: 2026-09-09
- Page: https://skillmd.com/skills/majiayu000/bb80-invariant-construction

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# Big Bang 80/20: Invariant Construction

## Core Concept

**Big Bang 80/20** means single-pass construction in low-entropy domains.

Key: **Extract invariants from specification, encode in types, never violate.**

## What Are Invariants?

Invariants are **properties that must always be true**:

| Domain | Invariant | Rust Type |
|--------|-----------|-----------|
| Error handling | All errors Result<T,E> | `Result<T, Error>` |
| File paths | No traversal | `SafePath` wrapper |
| Age | 0 ≤ age ≤ 150 | `newtype Age(u8)` |
| Cache size | ≤ 10,000 entries | `BoundedCache<T>` |

## Type-Safe Design

```rust
// ✅ Invariant encoded in type
pub struct Age(u8);  // 0-255 enforced by type

impl Age {
    pub fn new(value: u8) -> Result<Self, Error> {
        if value > 150 {
            Err(Error::InvalidAge)
        } else {
            Ok(Age(value))
        }
    }
}
```

## Reference
See CLAUDE.md sections:
- Constitutional Rules (Type-First)
- Automatic (Type-safe design)
- Holographic Factory Metaphor (Corollary)