# Libtmux.Query.exactly_one

- **Module:** Libtmux
- **Package:** libtmux
- **Language:** OCaml
- **Kind:** function
- **Source:** https://github.com/libtmux/libtmux-ocaml/blob/205dfda49742592b479247049d186956e4a4cfe9/lib/libtmux.mli#L701
- **Public owner:** Libtmux
- **Page:** https://libtmux.org/en/ocaml/latest/reference/libtmux-query-exactly_one/

```
val exactly_one : 'a Seq.t -> ('a, Cardinality.error) result
```

Return the unique value. Stop when a second value is found; proving uniqueness requires exhausting the remaining sequence. The input's effects and exceptions propagate. This does not execute a mutation.

## Parameters

- `arg1` (`'a Seq.t`)

## Guides

Pages that discuss `exactly_one`.

- [Query captured data](/en/ocaml/latest/guides/queries/)