# XO0166: `while cond` used as an expression; only `while { }` is one

A `while` loop with a condition may end without producing a value, so it is a statement. Only the infinite loop `while { ... }` is an expression, whose value comes from `break`. Move the loop into its own statement and compute the value separately. See core 3.5.

## Example

```xo
fn f(a: Bool) {
  let x = while a { }
}
```

## Fix

```xo
fn f(a: Bool) {
  while a { }
}
```

Run `xo explain XO0166` for this text in a terminal, or see
[section 10.6 of the specification](../../spec/core/#106-diagnostic-codes).