void-functional

Use a Result<T, E> discriminated union with ok and err constructors, then chain operations with map and flatMap helpers. Expected failures like validation errors become typed return values, while exceptions are reserved for truly unexpected bugs like database outages.

It is a pattern where all decisions happen in pure functions (the core) and all side effects happen at the edges (the shell). The shell reads external input, calls the pure core, and writes results back out, making domain logic deterministic and trivially testable.

This discipline rejects fp-ts and Effect-TS as default dependencies, favoring a minimal Result, Option, and pipe helper set under 50 lines. Adopt those libraries only as an opt-in pack when you need structured concurrency or rich combinators across multiple use cases.

Define each state as a variant with a kind literal tag, then write transition functions that accept only specific variants. The compiler rejects invalid transitions, and exhaustive switches guarantee every case is handled.

Use T | undefined for optional configuration or properties that may not be set yet. Use Option<T> with some/none variants for domain optionality where you want exhaustive switch enforcement so the none case cannot be silently ignored.

Null duplicates the optionality concept and creates null versus undefined ambiguity in TypeScript. The discipline uses T | undefined or Option<T> instead, with a companion hook that warns on null literals in domain code.