Apply Design Patterns

Turns a design smell into the smallest GoF pattern that removes it, adapted to the stack's idiom. Catalogue, what-varies table and the when-to-use guard: patterns-catalogue.md. Principles behind the choice: engineering-principles.md.

1. Audit (change nothing)

cat .claude/stack-profile.md 2>/dev/null | head -40
grep -rn "switch\|if.*type\|instanceof\|typeof" --include="*.ts" --include="*.rs" --include="*.go" --include="*.py" . 2>/dev/null | head -30

Name the smell precisely: a conditional over a type discriminator (→ Strategy/State), a class hierarchy that exists only to vary configuration (→ composition or Bridge), a change that ripples across many files (→ Facade/Mediator), a new-object family (→ Abstract Factory), a one-to-many notification (→ Observer). Every smell maps to a what varies in patterns-catalogue.md.

2. Decide

3. Apply the idiom

Translate the classic class diagram into the stack's idiom — the GoF book predates first-class functions, generators, structural typing and discriminated unions:

• Strategy = a function type or Record<Kind, (x) => y>, never an interface + class tree.
• Observer = state + effects + TanStack Query, or useSyncExternalStore; never a subject/observer class.
• Iterator = a generator or [Symbol.iterator]; never a hand-rolled cursor class.
• Singleton = a module-level const; a lazy class factory only when init cost is real.
• Adapter/Facade/Proxy = a structural interface plus one wrapper function, not a hierarchy.
• Decorator = a function/HOC for runtime wrapping; @ decorators for metadata only.
• Command = closures/thunks or {type, payload} actions; add {execute, undo} only for real undo.

4. Name the trade-off

State what the pattern buys and what it costs (one extra indirection, one new file, a perf hop) before applying. If the cost exceeds the flexibility gained, leave the simpler version and record the decision.

Verify

The smell is gone: the conditional over the discriminator is a lookup, the change that used to touch N files touches one, and the new indirection is justified by a second caller or a measured need. A second pass finds no leftover instanceof/typeof switch that the pattern was meant to replace.

When to deviate

• Throwaway or prototype code: skip the pattern entirely.
• A pattern already exists in the codebase: reuse it, do not add a second spelling of the same idea.
• The framework owns the mechanism (routing, state, DI): use the framework's form, not a hand-built one.