# Filterable.filterFormatField(_:)

- **Module:** Filterable
- **Package:** libtmux-swift
- **Language:** Swift
- **Kind:** method
- **Source:** https://github.com/libtmux/libtmux-swift/blob/254f8b2be7eb60cacc3ffcb3ea8e456784f582df/Sources/LibTmux/FilterExpr.swift#L32
- **Page:** https://libtmux.org/en/swift/latest/reference/filterable-filterformatfield(_-)/

```
Filterable.filterFormatField(_:)(id: String) -> String?
```

The tmux format field an id reads, or `nil` when it has no single one.

This is what lets a filter be answered by tmux instead of by listing
everything and discarding most of it. Returning `nil` is always safe:
the field drops out of the `-f` predicate and is evaluated in Swift.