Question 1

How do I create a domain primitive with validation in Rust?

Accepted Answer

Use the newtype pattern with a private field and a Smart Constructor that returns Result. Implement TryFrom for parsing, so invalid values like malformed emails or negative amounts cannot be constructed.

Question 2

What is the Always-Valid domain model principle?

Accepted Answer

Always-Valid means an object's existence guarantees its validity, enforced by the type system. Validation happens once at construction, so downstream code never re-checks invariants.

Question 3

How do branded types work in TypeScript for domain modeling?

Accepted Answer

Branded types intersect a primitive with a unique symbol tag, creating a nominal type like Email that plain strings cannot be assigned to. A parse function validates the string and returns the branded type.

Question 4

When should I not wrap a primitive type in a domain type?

Accepted Answer

Avoid wrapping temporary local variables, private implementation details, and values at external library boundaries. Convert to primitives only at domain boundaries like JSON serialization, database persistence, and external APIs.

Question 5

Why do primitive-typed function parameters cause production bugs?

Accepted Answer

Same-typed parameters can be swapped without compiler errors, and unvalidated primitives accept invalid values like negative amounts or SQL injection strings. Tests rarely cover these cases, so bugs surface only in production.

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