# Sequence.exactlyOne(_:)

- **Module:** Sequence
- **Package:** libtmux-swift
- **Language:** Swift
- **Kind:** method
- **Source:** https://github.com/libtmux/libtmux-swift/blob/f02a4668570e1cc5198c941413750e021f42c214/Sources/LibTmux/FilterExpr.swift#L333
- **Page:** https://libtmux.org/reference/swift/sequence-exactlyone(_-)/

```
Sequence.exactlyOne(_:)(expression: FilterExpr<Element>) -> Self.Element
```

The one element the filter matches.

Distinguishes "nothing matched" from "several matched", because a caller
that meant to address one object needs to know which mistake it made.

## Raises

- `FilterSelectionError`