Runtime Behavior
Runtime Behavior
This chapter describes runtime behavior in Rue, including error conditions and panics.
Certain operations trap at runtime. A trap is not a value, and no surrounding expression can consume it; it abandons the rest of the evaluation and halts the program with exit code 101 (the panic exit code of Appendix B). The core calculus fixes five trap categories — overflow (arithmetic, negation, and min_T / -1), div-zero, rem-zero, bounds (a negative or out-of-range array index), and user (an explicit @panic) — together with their propagation and exit code (core calculus docs/formal/01-core-calculus.md §6.12, and the (Panic-Lift) rule of §6.2 by which a trap in any subexpression aborts the whole evaluation).
Two further operations trap under the same discipline but are not yet carried by the core's taxonomy: decoding a byte sequence that is not well-formed UTF-8 (4.8:28) and an out-of-range @intCast (4.13:28). The core records this gap itself. The set above is therefore the core's, not an exhaustive enumeration of every operation in this language that can trap; a rule needing the complete set must state it rather than cite this paragraph as closed. Each is total, deterministic, and observable, so a conforming compiler reproduces the same trap on the same input. The following sections state each category normatively.
A panic terminates the program immediately. No language-level destructor or other cleanup runs for the panicking callee, its callers, or any surrounding scope, and no user code after the trap executes. Rue provides no language construct that observes, catches, unwinds, recovers, or resumes a panic. This rule applies to @panic, failed @assert/@assert_eq/@assert_ne, integer overflow, bounds violations, division or remainder by zero, and every other runtime trap defined by this chapter. Recoverable failures are represented as typed values such as Result, Option, or an application enum and propagated with ? (ADR-0038); a host that must survive a panic isolates the work in a separate process (ADR-0083).