# XO1404: recursion among real-time functions

The call graph of real-time functions (core 6.5) must be acyclic, so that their stack depth is bounded. A function that calls itself, directly or through other real-time functions, is reported at the call that closes the cycle. Since real-time functions cannot call function values or interface methods, recursion through them is ruled out too. Rewrite the recursion as a loop with `bound N` and an explicit stack in a preallocated List.

## Example

```xo
fn depth(n: Int) -> Int uses realtime {
  if n == 0 { 0 } else { 1 + depth(n - 1) }
}
```

## Fix

```xo
fn depth(n: Int) -> Int uses realtime {
  var d = 0
  var i = n
  while i > 0 bound 1000 {
    d += 1
    i -= 1
  }
  d
}
```

Run `xo explain XO1404` for this text in a terminal, or see
[section 10.6 of the specification](../../spec/core/#106-diagnostic-codes).