swift-formatstyle

# Swift FormatStyle

Format values for human-readable display using the `FormatStyle` protocol
and Foundation's concrete format styles. Replaces legacy `Formatter` subclasses
with a type-safe, composable, cacheable API.

Locale-aware display is an i18n concern even when the app is not adding new languages. For user-facing formatted output, always recommend previewing or testing representative locales such as `en_US`, `de_DE`, `ar_SA`, and `ja_JP`; check separators, numbering systems, calendars, currency and unit conventions, text direction, and layout-sensitive output. Keep this skill focused on `FormatStyle`, `ParseableFormatStyle`, parsing, and reusable formatter API design; route broader localization work such as String Catalogs, bundles, plurals, localized copy, and RTL layout review to `ios-localization`. Do not use "not adding languages" as the reason to skip `ios-localization`; locale-sensitive formatting can be a localization review issue without translation work.

Docs: [FormatStyle](https://sosumi.ai/documentation/foundation/formatstyle)

## Contents

- [Quick Reference](#quick-reference)
- [Numbers](#numbers)
- [Currency](#currency)
- [Percentages](#percentages)
- [Dates](#dates)
- [Durations](#durations)
- [Measurements](#measurements)
- [Person Names](#person-names)
- [Lists](#lists)
- [Byte Counts](#byte-counts)
- [URLs](#urls)
- [SwiftUI Integration](#swiftui-integration)
- [Custom FormatStyle](#custom-formatstyle)
- [Common Mistakes](#common-mistakes)
- [Review Checklist](#review-checklist)

## Quick Reference

| Type | Style Access | Example |
|------|-------------|---------|
| `Int`, `Double` | `.number` | `42.formatted(.number.precision(.fractionLength(2)))` → `"42.00"` |
| Currency | `.currency(code:)` | `29.99.formatted(.currency(code: "USD"))` → `"$29.99"` |
| Percent | `.percent` | `0.85.formatted(.percent)` → `"85%"` |
| `Date` | `.dateTime` | `Date.now.formatted(.dateTime.month().day().year())` |
| Date range | `.interval` | `(date1..<date2).formatted(.interval)` |
| Relative date | `.relative(presentation:unitsStyle:)` | `date.formatted(.relative(presentation: .named))` → `"yesterday"` |
| `Duration` | `.time(pattern:)` | `Duration.seconds(3661).formatted(.time(pattern: .hourMinuteSecond))` → `"1:01:01"` |
| `Duration` | `.units(allowed:width:)` | `Duration.seconds(90).formatted(.units(allowed: [.minutes, .seconds]))` → `"1 min, 30 sec"` |
| `Measurement` | `.measurement(width:)` | `Measurement(value: 72, unit: UnitTemperature.fahrenheit).formatted(.measurement(width: .abbreviated))` |
| `PersonNameComponents` | `.name(style:)` | `name.formatted(.name(style: .short))` → `"Tom"` |
| `[String]` | `.list(type:width:)` | `["A","B","C"].formatted(.list(type: .and))` → `"A, B, and C"` |
| Byte count | `.byteCount(style:)` | `Int64(1_048_576).formatted(.byteCount(style: .memory))` → `"1 MB"` |
| `URL` | `.url` | `url.formatted(.url.scheme(.never).host().path())` |

## Numbers

```swift
// Default locale-aware formatting
let n = 1234567.formatted()  // "1,234,567" (en_US)

// Precision
1234.5.formatted(.number.precision(.fractionLength(0...2)))  // "1,234.5"
1234.5.formatted(.number.precision(.significantDigits(3)))    // "1,230"

// Rounding
1234.formatted(.number.rounded(rule: .down, increment: 100)) // "1,200"

// Grouping
1234567.formatted(.number.grouping(.never))                   // "1234567"

// Notation
1_200_000.formatted(.number.notation(.compactName))           // "1.2M"
42.formatted(.number.notation(.scientific))                    // "4.2E1"

// Sign display
(-42).formatted(.number.sign(strategy: .always()))            // "+42" / "-42"

// Locale override
42.formatted(.number.locale(Locale(identifier: "de_DE")))     // "42"
```

Docs: [IntegerFormatStyle](https://sosumi.ai/documentation/foundation/integerformatstyle),
[FloatingPointFormatStyle](https://sosumi.ai/documentation/foundation/floatingpointformatstyle)

## Currency

```swift
29.99.formatted(.currency(code: "USD"))   // "$29.99"
29.99.formatted(.currency(code: "EUR"))   // "€29.99"
29.99.formatted(.currency(code: "JPY"))   // "¥30"

// Customize precision
let style = FloatingPointFormatStyle<Double>.Currency(code: "USD")
    .precision(.fractionLength(0))
1234.56.formatted(style)  // "$1,235"
```

## Percentages

```swift
0.85.formatted(.percent)                                      // "85%"
0.8567.formatted(.percent.precision(.fractionLength(1)))       // "85.7%"
42.formatted(.percent)                                         // "42%"  (integer)
```

## Dates

```swift
let now = Date.now

// Components
now.formatted(.dateTime.year().month().day())           // "Apr 22, 2026"
now.formatted(.dateTime.hour().minute())                // "4:30 PM"
now.formatted(.dateTime.weekday(.wide).month(.wide).day()) // "Wednesday, April 22"

// Predefined styles
now.formatted(date: .long, time: .shortened)            // "April 22, 2026 at 4:30 PM"
now.formatted(date: .abbreviated, time: .omitted)       // "Apr 22, 2026"

// ISO 8601
now.formatted(.iso8601)                                 // "2026-04-22T16:30:00Z"

// Relative
let yesterday = Calendar.current.date(byAdding: .day, value: -1, to: .now)!
yesterday.formatted(.relative(presentation: .named))    // "yesterday"
yesterday.formatted(.relative(presentation: .numeric))  // "1 day ago"

// Interval
(date1..<date2).formatted(.interval.month().day().hour().minute())

// Components (countdown-style)
(date1..<date2).formatted(.components(style: .wide, fields: [.day, .hour]))
// "2 days, 5 hours"
```

Docs: [Date.FormatStyle](https://sosumi.ai/documentation/foundation/date/formatstyle),
[Date.RelativeFormatStyle](https://sosumi.ai/documentation/foundation/date/relativeformatstyle),
[Date.IntervalFormatStyle](https://sosumi.ai/documentation/foundation/date/intervalformatstyle)

### Anchored Relative Dates (iOS 18+)

`Date.AnchoredRelativeFormatStyle` formats relative to a fixed anchor date
rather than the current moment.

Docs: [Date.AnchoredRelativeFormatStyle](h