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

// ✅ 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)