Exceptions and Backtraces

Contributor implementation note: source-level guarantees are defined in Statements and Control Flow.

Runtime Model

Hosted DQ exceptions are object references derived from the runtime exception base type. Polymorphic type information supports handler matching. Raising an exception transfers control through compiler-generated cleanup and handler regions.

A raise captures a backtrace before propagation. Re-raising preserves the active exception context rather than constructing an unrelated exception.

Generated Cleanup

The compiler tracks initialized locals that own cleanup behavior. Every exit edge from a protected scope—normal fallthrough, return, loop transfer, or unwind—runs the required cleanup exactly once in reverse construction order.

finally is represented on all exits from its protected region. Contributor changes must test both normal control flow and exception replacement from within cleanup/finally code.

Matching and Top-Level Handling

Handlers are considered in source order. Runtime type-chain checks implement base/derived matching. If none match, propagation continues.

The hosted top-level handler prints the exception message and captured frames, then terminates with failure. Signal-to-exception behavior is platform/runtime specific and must not be generalized to bare targets without an implemented runtime contract.

Verification Areas

The exception autotests cover typed matching, propagation and rethrow, finally on all control transfers, local cleanup, runtime-error catching, top-level reporting, backtraces, and supported signal handling. Any change to scope cleanup or object lifetime should run these tests in addition to its local test group.