design-patternslisted

# Design Patterns Apply patterns from two authoritative sources: - **GoF**: *Design Patterns* — Gamma, Helm, Johnson, Vlissides - **PoEAA**: *Patterns of Enterprise Application Architecture* — Martin Fowler **Key principle**: patterns describe solutions to recurring problems — use them when the problem recurs, not to demonstrate pattern knowledge. In Go, first-class functions and interfaces make several GoF patterns unnecessary or simpler than in OOP languages. --- > For encoding pattern invariants into the type system (Value Objects, sum types, smart constructors), apply the `type-driven-design` skill. ## GoF Creational Patterns ### Factory Method / Abstract Factory **Problem**: Create objects without hardcoding concrete types; decouple creation from use. ```go func NewReader(format string) Reader { switch format { case "json": return &JSONReader{} case "xml": return &XMLReader{} default: return &TextReader{} } } ``` - **Use when**: multiple concrete types implement the same interface; creation logic varies by context - **Avoid when**: only one concrete type exists (just use a direct constructor) - **Go note**: return interfaces, not concrete types; constructor functions are more idiomatic than factory objects ### Builder / Functional Options **Problem**: Construct complex objects step-by-step, managing many optional parameters cleanly. ```go // Idiomatic Go: functional options type Option func(*Server) func WithTimeout(d time.Duration) Optio