---

name: bb80-invariant-construction description: "Extract invariants from specification, encode in types/structure, enable single-pass construction." allowed_tools: "Read, Write, Edit"

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:

Type-Safe Design

Copy// ✅ 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)