# Sequence.exactlyOne(_:)

- **Module:** Sequence
- **Package:** libtmux-swift
- **Language:** Swift
- **Kind:** method
- **Source:** https://github.com/libtmux/libtmux-swift/blob/254f8b2be7eb60cacc3ffcb3ea8e456784f582df/Sources/LibTmux/FilterExpr.swift#L347
- **Page:** https://libtmux.org/en/swift/latest/reference/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`