LIMITS-005: Closure mutation blocked for outer let — leverage ceiling for stateful recursion #177

Summary

Nodus closures cannot assign to variables captured from an enclosing let scope:

let count = 0i
let inc = fn() {
    count = count + 1i  // ERROR: cannot assign to captured variable
}

The documented workaround is a quoted-key map:

let state = {"count": 0i}
let inc = fn() {
    state["count"] = state["count"] + 1i  // works
}

This workaround is functional but adds significant cognitive overhead for any program that accumulates state across recursive calls — schema validators, tree traversals, parser state machines.

Impact

This is the primary leverage ceiling for stateful data processing programs. The audit identifies "a stateful recursive data processor" as the program type hardest to write in Nodus despite being within its stated scope.

Why the restriction exists

Nodus closures capture variables by value at closure creation time. Assignment to a captured variable in Python's closure model requires nonlocal; Nodus has no equivalent keyword. The map workaround works because maps are mutable objects captured by reference.

Fix direction (v5)

Two options:

1. Add a mut keyword to let that signals the variable is assignable from inner closures: let mut count = 0i
2. Allow outer let assignment via upvalue mutation — detect at compile time that a variable is being assigned from an inner scope and emit a SET_UPVALUE opcode instead of SET_LOCAL

Option 2 is lower surface area; option 1 is more explicit. Either would make the closure mutation workaround unnecessary for the common case.

Affected versions

v4.0.0 (current). Noted in System Limits Audit §5 (Leverage Boundary).