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Lyth

ADR 0018: Typed `catch` clauses match with `errors.AsType`

Status: Accepted (extends 0008)

Context

Section titled “Context”

Backend code handles specific errors all the time: a missing row becomes a 404, a unique-constraint violation a 409, a timeout a retry. Go code writes var nf *NotFound; if errors.As(err, &nf) { ... }, or since Go 1.26 if nf, ok := errors.AsType[*NotFound](https://github.com/lyth-lang/lyth/tree/main/docs/adr/err); ok { ... }, and must remember to return every other error. With only catch (err), Lyth code wrote the same dispatch by hand inside the handler, including the easy-to-forget else { throw err }.

Decision

Section titled “Decision”
  • e catch (x: T) { ... } handles errors of type T, a non-null type that implements Error, and binds x as a T. Several clauses may follow one another and are tried in order.
  • Matching uses errors.AsType[T], so it sees through wrapping. That differs from is (ADR 0016) on purpose: is asks about a value’s dynamic type, while catch asks about an error, and Go inspects errors through their chain.
  • Errors that no typed clause handles are still errors. The expression stays fallible and must end with ? (propagate the rest) or with an untyped catch (handle the rest). Otherwise it is E0303.
  • The chain lowers to one if err != nil holding an if/else if dispatch on errors.AsType, ending in the final handler or in return ..., err.

Alternatives

Section titled “Alternatives”
  • Unmatched errors propagate implicitly, as in Java or Kotlin: shorter, but it hides an exit point. In Lyth every propagation is visible as ? (ADR 0008).
  • when over errors with is/errors.Is conditions: more general, but it moves the handling away from the call and needs the error bound first. It remains possible later for sentinel values.
  • Type assertions (err.(T)): miss wrapped errors, which are the common case once code uses wrap.
  • Sentinel patterns (catch (ErrNotFound)): sentinels compare with errors.Is, and a value in the binder position would be ambiguous. An untyped catch with errors.Is covers them.

Consequences

Section titled “Consequences”

The generated code is the dispatch a Go programmer writes, with no helper and no allocation beyond errors.AsType’s own. It needs Go 1.26, which Lyth already requires. A typed clause whose binder is unused names it _ (catch (_: *NotFound) { fallback }?), following the rule for every unused name.