Strategy

interface PricingStrategy { price(o: Order): Money }
class StandardPricing implements PricingStrategy { price(o) { return o.subtotal } }
class DiscountPricing implements PricingStrategy { price(o) { return o.subtotal.times(0.9) } }
class PremiumPricing implements PricingStrategy { price(o) { ... } }

class PricingStrategyFactory {
  static create(tier: Tier): PricingStrategy {
    switch (tier) { case 'standard': return new StandardPricing() /* ... */ }
  }
}
// Four files, one method each, plus a factory whose body is
// the switch we set out to remove.

type PricingRule = (o: Order) => Priced

const rules: Record<Tier, PricingRule> = {
  standard: (o) => priced('standard', o.subtotal),
  discount: (o) => priced('discount', o.subtotal.times(0.9)),
  premium:  (o) => priced('premium', o.subtotal.minus(o.loyaltyCredit)),
}

export const priceFor = (o: Order) => rules[o.tier](o)

// Adding a tier: one line in the record. Same change cost,
// no factory, and the whole rule set fits on one screen.

New tier: four edits, found by grep, and the quote module was already inconsistent. New operation: one edit per module — actually cheap, because the branches are already colocated.

New tier: one entry, nothing else opened, and the four call sites cannot drift because they all go through selection. New operation: every rule signature changes at once — five edits and a compile error at each, which is safe but not cheap.