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ADR 0009: Nullability with platform types and boxed optionals

Status: Accepted; smart casts of locals amended by 0031 (locals that are never reassigned)

Context

Section titled “Context”

Nil dereference is Go’s most common panic. Kotlin-style null safety helps, but every Go API returns pointers and interfaces of unknown nullability.

Decision

Section titled “Decision”

Pointers, interfaces, function types, and channels are non-null by default; T? is nullable (same Go type). Slices and maps are never null (nil is empty). Go-originated values have platform nullability (usable as either, no inserted checks). Value types get optional forms T? represented as *T and never mutated through. Smart casts work for vals and parameters. ?., ?:, !!. The keyword is null (lowers to nil).

Alternatives

Section titled “Alternatives”
  • No null safety (Go semantics): forgoes a major safety win.
  • Treat all Go pointers as nullable: every interop call needs !!. Unusable.
  • Insert runtime checks at platform boundaries (Kotlin’s approach): hidden branches and panics. Kept as a future opt-in mode.
  • Generic Option[T] struct runtime type: needs a runtime package and fights idiomatic Go APIs (*T for optional JSON/SQL values).

Consequences

Section titled “Consequences”

Optional values interoperate with JSON and SQL natively. Boxing costs an allocation where escape analysis cannot avoid it (documented). T? on type parameters is deferred. Platform types are a deliberate soundness gap.