Rust design patterns for RTK. Newtype, Builder, RAII, Trait Objects, State Machine. Applied to CLI filter modules. Use when designing new modules or refactoring existing ones.

RTK Rust Design Patterns Patterns that apply to RTK's filter module architecture. Focused on CLI tool patterns, not web/service patterns. Pattern 1: Newtype (Type Safety) Use when: wrapping primitive types to prevent misuse (command names, paths, token counts). Pattern 2: Builder (Complex Configuration) Use when: a struct has 4+ optional fields, many with defaults. When NOT to use Builder: if the struct has 1 3 fields with obvious meaning. Over engineering for simple cases. Pattern 3: State Machine (Parser/Filter Flows) Use when: parsing multi section output (test results, build output) where context changes behavior. Pattern 4: Trait Object (Command Dispatch) Use when: different command families need the same interface. Avoids massive match arms. Note: RTK's current match dispatch in main.rs is intentional — static dispatch, zero overhead. Only move to trait objects if the match arm count exceeds ~20 commands. Pattern 5: RAII (Resource Management) Use when: managing resources that need cleanup (temp files, SQLite connections). Pattern 6: Strategy (Swappable Filter Logic) Use when: a command has multiple filtering modes (e.g., compact vs. verbose). Pattern 7: Extension Trait (Add Methods to External Types) Use when: you need to add methods to types you don't own (like &str for RTK specific parsing). RTK Pattern Selection Guide Situation Pattern Avoid New cmd.rs filter module Standard module pattern (see CLAUDE.md) Over abstracting 4+ optional config fields Builder Struct literal Multi phase output parsing State Machine Nested if/else Type safe wrapper around string Newtype Raw String Adding methods to &str Extension Trait Free functions Resource with cleanup RAII / Drop Manual cleanup Dynamic filter registry Trait Object Match sprawl Anti Patterns in RTK Context

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