Sandbox limits

Two profiles via VM::with_limits, same compiler.wasm runs unsandboxed in trusted contexts, clamped in untrusted.

Integer width

Two-tier:

• Inline (fast): 47-bit signed in a NaN-boxed Val. Range +/-2^47 (+/-140_737_488_355_327). One ALU op per arithmetic, no allocation.
• Wide (slow): i128 in HeapObj::LongInt. Range +/-2^127 - 1. Auto-used when a literal exceeds 47-bit or inline arithmetic overflows.

Outside +/-2^127 raises OverflowError. Promotion is automatic, user code doesn’t see the boundary.

print(140737488355327) # inline, fast path
print(2 ** 47) # 140737488355328: auto-promotes to LongInt
print(2 ** 100) # 1267650600228229401496703205376
try:
    print(2 ** 127) # past the i128 cap
except OverflowError:
    print("overflow")

140737488355327
140737488355328
1267650600228229401496703205376
overflow

Caveats

• pow(a, b, m) modular: modulus must be < 2^63 (larger overflows i128 in the multiply). Hard cap without arbitrary-precision arithmetic.
• No CPython-style unbounded ints: by design, edge workloads don’t need wider than 128 bits; crypto-scale math is out of scope.
• Float vs LongInt mixing: == works (LongInt -> f64), but dict/set hashing follows raw Val bits, {long_int: x} indexed by a same-magnitude float misses. Coerce explicitly.

Triggering limits

# Recursion depth
def loop(n):
    return loop(n + 1)
 
try:
    loop(0)
except RecursionError:
    print("hit max depth")

hit max depth

# Heap quota, a tight loop allocating new objects
try:
    xs = []
    while True:
        xs = xs + [0] * 1000
except MemoryError:
    print("hit heap limit")

Source size

Source must be under 10 MiB; larger rejected at lex time.

Token limits

Prevent asymmetric DoS, small input producing an exponentially large parse tree.

Error types

Compile-time

Reported as Diagnostic { start, end, msg }, byte offsets into source; line/column computed lazily by render(). Caught before any code runs.

Runtime

Raised as VmErr; most catchable with try / except.

Exception hierarchy

Flat tree rooted at BaseException -> Exception. except walks parent links, except Exception catches RuntimeError, ValueError, KeyError, AssertionError, etc.; except RuntimeError catches RecursionError, NotImplementedError. SystemExit sits directly under BaseException, so except Exception does not catch it (use except SystemExit or a bare except).

try:
    raise RuntimeError("oops")
except Exception as e:
    print("caught via parent:", e)
 
try:
    [][0]
except Exception:
    print("caught IndexError as Exception")

caught via parent: oops
caught IndexError as Exception

User-defined classes don’t auto-extend the built-in BaseException tree but support single-level inheritance among themselves, except UserBase catches a raised UserSub when UserSub inherits from UserBase. raise X from Y raises X; the cause is discarded (no __cause__ / __context__ chaining).

Exception arguments

Caught exceptions expose constructor args as e.args (tuple). raise X("msg") and raise X(a, b) carry through; runtime-raised errors carry their message as a single arg; bare raise X produces an empty tuple.

try:
    raise TypeError("bad input")
except TypeError as e:
    print(e.args)
 
try:
    1 / 0
except ZeroDivisionError as e:
    print(e.args)
 
try:
    raise ValueError
except ValueError as e:
    print(e.args)

('bad input',)
('division by zero',)
()

Catching errors

def safe(f, x):
    try:
        return f(x)
    except TypeError:
        return "type"
    except ValueError:
        return "value"
    except ZeroDivisionError:
        return "zero"
    except:
        return "other"
 
print(safe(lambda x: 1 / x, 0))
print(safe(lambda x: int(x), "abc"))
print(safe(lambda x: len(x), 42))

zero
value
type

Environmental errors

Failures surfaced before the source reaches the compiler, no line/column preview, no parsed code to anchor to. Emitted as plain text, uncatchable from Python.

Handle at the embedder layer (path validation, encoding, size check) before invoking the compiler.

Unsupported features at runtime

Parse but raise RuntimeError when executed:

try:
    import os
except RuntimeError as e:
    print("import:", e)

Exist for syntactic compatibility. For code reuse, use higher-order functions.

Determinism

Same source + input -> same output across runs and architectures (x86_64, aarch64, wasm32). No time, randomness, threading, or OS interaction. Heap-pool slot reuse is the only nondeterminism, observable through id(x) only, never ==, repr, or any other operation.