# XO1402: loop without a provable bound

Every loop of a real-time function (core 6.5) needs a bound known at compile time: a `for` over a range whose ends are constants is bounded by its length; any other `for` and every `while` needs a `bound N` clause after the iterated value or the condition, N a constant Int. A bound is a contract (core 3.5): when the iteration count is not constant, the loop faults with XO-F006 when it would start iteration N + 1, and `xo test` fuzzes the function. This code is also reported in any function when a bound is not a constant, is negative, or is smaller than a constant range it bounds.

## Example

```xo
fn sum(xs: List[Int]) -> Int uses realtime {
  var t = 0
  for x in xs { t += x }
  t
}
```

## Fix

```xo
const MAX = 64

fn sum(xs: List[Int]) -> Int uses realtime {
  var t = 0
  for x in xs bound MAX { t += x }
  t
}
```

Run `xo explain XO1402` for this text in a terminal, or see
[section 10.6 of the specification](../../spec/core/#106-diagnostic-codes).