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Lyth

ADR 0007: `val`/`var` and value immutability

Status: Superseded for local variables by 0031 (:= declares mutable locals); parameters, loop variables, and value receivers stay immutable

Context

Section titled “Context”

Go has no immutable local variables. Kotlin’s val freezes only the reference, but Lyth structs are values, so the meaning must be defined for value types.

Decision

Section titled “Decision”

val (and parameters, loop variables, value receivers) are immutable: the value stored in the binding never changes, including fields and array elements reachable without a pointer (Swift let semantics). Mutation through pointers, slices, maps, channels, and interfaces is unaffected. &x and pointer-receiver calls require mutable storage. No :=; it gets a targeted diagnostic.

Alternatives

Section titled “Alternatives”
  • Kotlin semantics (reference-only): meaningless for value types, since u.name = x on a val struct would mutate the binding.
  • Deep immutability: unenforceable across Go code and incompatible with Go APIs.
  • let/var (Swift): visually more distinct, but val/var is the stated preference and familiar to Kotlin users.
  • Keeping :=: a second declaration syntax that Go developers read as mutable.

Consequences

Section titled “Consequences”

Compile-time only, with zero runtime cost. Catches Go’s silent value-receiver write bug. Requires var for Go idioms that mutate locals (json.Unmarshal(&v), var mu: sync.Mutex). Smart casts become sound for vals.