# XO0436: `for` over `next()` with a `?` that does not match it

A value whose type has `next() -> T?` is iterated by `for x in v`, which calls next until it returns None. When next returns `T? ! E`, each step can fail, and the loop must say where the error goes: `for x? in v` propagates it as `?` does. Write the `?` when next can fail and leave it out when it cannot; to handle the error inside the loop, write `loop { match v.next() { Ok(Some(x)) => ...  Ok(None) => break  Err(e) => ... } }`. See core 3.5 and decision 0494.

## Example

```xo
struct Pages {
  left: Int
}

fn (var p: Pages) next() -> Int? ! Str {
  if p.left == 0 { return Ok(None) }
  p.left -= 1
  Ok(Some(p.left))
}

fn total(var p: Pages) -> Int ! Str {
  var n = 0
  for x in p { n += x }
  Ok(n)
}
```

## Fix

```xo
struct Pages {
  left: Int
}

fn (var p: Pages) next() -> Int? ! Str {
  if p.left == 0 { return Ok(None) }
  p.left -= 1
  Ok(Some(p.left))
}

fn total(var p: Pages) -> Int ! Str {
  var n = 0
  for x? in p { n += x }
  Ok(n)
}
```

Run `xo explain XO0436` for this text in a terminal, or see
[section 10.6 of the specification](../../spec/core/#106-diagnostic-codes).