# XO0165: syntax that is not a pattern

Where a pattern is expected (a `match` arm, `let`, `if let`, `for`, or after `is`), the parser found something that is not one: an operator, a missing pattern after `let`, or a non literal in a range pattern. A pattern after `is` tests without binding, so it may not bind names: write `_`, or bind with `if let` or `match`. See core 3.5 for the pattern table.

## Example

```xo
fn f(r: Result[Int, Str]) -> Bool {
  r is Ok(v)
}
```

## Fix

```xo
fn f(r: Result[Int, Str]) -> Bool {
  r is Ok(_)
}
```

Run `xo explain XO0165` for this text in a terminal, or see
[section 10.6 of the specification](../../spec/core/#106-diagnostic-codes).