Prefer val

Default to Immutable

• Properties and locals default to val — declare var only when mutation is required and the mutation is intentional
• var in a data class signals you're modelling mutable state by accident; convert to val and reconstruct via copy() instead
• The reviewer test: "would I be surprised if this changed?" — if yes, it's a val

When var Is Actually Right

• Accumulators inside a function body where the alternative would obscure intent
• State machines whose entire purpose is to advance
• Backing fields for computed properties that must memoize across calls

Reconstruction Over Mutation

• Update a value type via someValue.copy(field = newValue), not someValue.field = newValue
• This composes cleanly with ?.let { … } and stream-style transformations
• Mutating a shared instance through a property setter is the bug factory val exists to prevent