Interrupts (resumable conditions)

Status: Implemented - skip + recover, for top-level functions, entry, and class methods. retry, value-producing signals, and checking of the interrupts T annotation (it is currently parsed but not enforced) remain future work (see Limitations). Runnable: examples/interrupts/interrupt_demo.xi.

Summary

interrupt introduces resumable conditions to Xi. When a function signals an interrupt, that function is suspended at the signal site - it does not unwind. A handler in an enclosing try/catch runs while the suspended frame is still alive and decides what happens next:

• recover - the suspended function resumes: it runs the inline recover { } block at the signal site and continues from there.
• skip - the suspended function is abandoned; control returns after the try.

It is checked: a function that may signal declares interrupts T, and the compiler verifies that callers either handle or re-declare it.

This is the Common Lisp condition/restart model (also seen in Smalltalk's resumable exceptions and algebraic effects), adapted to Xi's C backend and its pure/impure function-kind system.

Execution model (the key idea)

The running method that raises an interrupt gets interrupted, and only continues once the interruption is managed at an upper stack frame.

foo() running ──signal T──▶ foo SUSPENDED here (frame kept alive)
                                  │
                                  ▼
                  handler search up the stack → matching catch
                                  │
                  catch body runs (no unwinding yet), decides:
                     ├─ recover ─▶ run foo's recover{} block, foo CONTINUES
                     └─ skip ────▶ unwind to the try, foo ABANDONED

Contrast with Xi's existing Result and with classic exceptions:

Mechanism	Use	Stack behaviour
Result (T!, ?)	expected, local errors handled as values	none; caller inspects
Interrupt	recoverable conditions; an outer policy decides	handler runs with the signalling frame intact, then resumes or abandons
(classic exceptions)	-	would unwind before the handler; not in Xi

Syntax

// 1. Declare an interrupt type (a condition with a payload).
interrupt FooCalcInt { x: Integer }

// 2. A function that may raise it declares `interrupts`. `signal` raises it;
//    the `recover { }` block is the inline restart - it runs only if a handler
//    chooses to recover, in this (suspended) function's own frame.
consumer foo(n: Integer) interrupts FooCalcInt {
    if n > 20 {
        signal FooCalcInt { x: n } recover {
            system.stdout.writeln("recovering; clamped " + n)
        }
    }
    system.stdout.writeln("foo continues normally")   // reached after recover
}

// 3. Handle it. The catch body DECIDES; it does not contain the recovery code.
try {
    foo(24)
} catch e: FooCalcInt {
    if e.x > 100 { skip }      // abandon the rest of foo; resume after `try`
    else         { recover }   // resume foo: run its recover{} block, continue
}

Grammar (sketch)

interrupt_decl ::= "interrupt" Ident "{" field ("," field)* "}"
signal_stmt    ::= "signal" Type "{" fields "}" "recover" block
func_decl      ::= ... ("interrupts" Type ("," Type)*)? block
try_stmt       ::= "try" block ("catch" Ident ":" Type block)+
resolution     ::= "skip" | "recover"

catch is paren-free (like if/for); the payload uses the compound-type literal { field: value }.

Semantics

Resolutions

• recover - run the recover { } block at the signal site, then continue the signalling function at the statement after the signal. The recovery logic lives with the code that knows how to recover; the handler only opts in.
• skip - unwind from the signal site back to the try; the signalling function's remaining work is abandoned. Control resumes after the try/catch.
• retry - deferred (not in the first version): would re-execute the interrupting operation.

A catch body must select exactly one resolution (skip or recover) on every path. It may run other statements first (e.g. logging) - subject to the restriction below.

Handler lookup

signal T searches the dynamically-enclosing handler stack for the nearest try whose catch matches T (by type). The matching handler runs without unwinding and returns a resolution, which the signal site then enacts. If no handler matches, the signal is unhandled (see checked signatures).

The interrupts annotation

• A function that may signal declares interrupts T (multiple: interrupts A, B).
• An interrupt that reaches main with no matching handler is a panic (xc: unhandled interrupt: T).

The annotation is currently parsed but not enforced - the compiler does not yet verify that every signal site is declared, nor that callers handle or re-declare. That effect-checking pass is future work; today interrupts T is documentation that the runtime backs up with the unhandled-panic.

Purity

Signalling is an effect, permitted only in the impure kinds - consumer, producer, creator (and entry). The pure kinds - mapper, projector, predicate, reducer - may neither signal nor call an interrupts function. This reuses Xi's existing purity line.

The catch-as-function restriction

To run a handler before unwinding (so recover can resume the suspended frame), the catch body is compiled as a function over the payload. A catch body may therefore read:

• the interrupt payload (e),
• module-level / global state,
• and call functions,

but may not capture try-scope local variables (Xi has no closures). This is the simplification that makes resumption implementable without continuations or coroutines. (Future work could lift it with explicit captures.)

Worked example

interrupt RateLimited { retryAfter: Integer }

producer fetch(url: String) interrupts RateLimited {
    let r = http.get(url)?
    if r.status == 429 {
        signal RateLimited { retryAfter: 5 } recover {
            system.stdout.writeln("backing off, then continuing")
        }
    }
    // ... use r ...
}

consumer run() {
    try {
        fetch("http://example.com/")
    } catch e: RateLimited {
        system.stdout.writeln("rate limited; retryAfter=" + e.retryAfter)
        recover            // resume fetch: it runs its backoff and continues
    }
}

Static checks / diagnostics

• signal T in a function not declaring interrupts T → error (suggest adding it or wrapping in try).
• Calling an interrupts T function without handling or re-declaring T → error.
• signal / calling an interrupts function from a pure kind → error.
• catch body capturing a try-local → error (restriction explained).
• catch body that does not select a resolution on all paths → error.

Limitations & future work

• retry resolution (re-execute the interrupting operation).
• Value-producing signals (use-value): let y = signal T {..} recover {..} where the recover block yields the value of the signal expression.
• Value-producing try (the try block yielding a result; today a statement).
• Multiple named restarts at one site (beyond a single recover).
• Closures in catch to lift the local-capture restriction.
• Interaction with async.

Decisions on record

• Raise with signal; declare capability with interrupts T; type keyword is interrupt.
• Resolutions are skip (abandon) and recover (resume); retry deferred.
• Recovery code is an inline recover { } restart at the signal site; the handler only chooses.
• Execution model: the signalling function suspends and resumes only after an enclosing handler decides.
• Interrupts are checked; pure kinds may not signal.
• First version implements skip + recover. Ship this proposal first.