Design Patterns (Gang of Four)

Field guide to the 23 patterns from Design Patterns: Elements of Reusable Object-Oriented Software (Gamma, Helm, Johnson, Vlissides, 1994), updated for modern TypeScript/Python/Rust/Swift practice. Each pattern lives in its own reference file — this SKILL.md is the index and the meta-level (principles, categories, decision tree, criticism).

Why use this skill

• You want the vocabulary — naming a shape ("this is a Decorator chain") beats a paragraph of prose in PR review.
• You're about to introduce an abstraction and want to sanity-check whether it earns its keep.
• You're reading a codebase that leans on patterns and need a map.
• You're translating a C++/Java pattern into a language with first-class functions and sum types.

This skill is opinionated. In 2026, a lot of GoF ceremony dissolves into language features (closures, discriminated unions, generators, pattern matching, DI containers). The patterns that survived kept their names because the shape is useful; the class hierarchies often aren't.

The two core principles (the spine of the book)