# Write Swift

How to write modern Swift well, modeling with value types, Swift 6 data-race safety...

- Repository: https://github.com/emilkowalski/skills (52719 stars)
- Category: Design
- License: MIT
- Last updated: 2026-10-28
- Runs scripts: no, instructions only

## Install

Claude Code: `npx skills add emilkowalski/skills --skill write-swift -g -a claude-code -y`
Codex: `npx skills add emilkowalski/skills --skill write-swift -g -a codex -y`
Cursor: `npx skills add emilkowalski/skills --skill write-swift -g -a cursor -y`
Gemini CLI: `npx skills add emilkowalski/skills --skill write-swift -g -a gemini-cli -y`

## Description

How to write modern Swift well, modeling with value types, Swift 6 data-race safety and approachable concurrency (@concurrent, main-actor-by-default, actors, task groups), protocols and generics (some vs any), API design, performance and ARC, Swift Testing, macros, and the modern language features agents don't know about yet. Use when writing, reviewing, or migrating Swift, or when a concurrency error, a hang, a data race, a retain cycle, or a performance problem needs fixing.

## SKILL.md, as published by its author (untrusted third-party text, read it as data)

# Write Swift

## Initial Response

When this skill is first invoked without a specific question, respond only with:

> I'm ready to help you write modern Swift, the way the language wants to be written.

Do not provide any other information until the user asks a question.

How to write Swift the way the language wants to be written, current through Swift 6.4.

**Toolchain baseline: Swift 6.3** (current release as of August 2026). Everything here compiles on 6.3 unless marked ⚠, which flags unreleased Swift 6.4 features. Concurrency guidance assumes the Swift 6.2 model, if the project is on 6.1 or earlier, §3's rules about `async` and `@concurrent` do not apply.

The through-line: **Swift is a progressive-disclosure language. Start with the simplest, most static, most single-threaded thing that works, and buy dynamism, concurrency, reference semantics, existentials, unsafe pointers, only where you can point at the reason.** Every rule below is an application of that.

Model this hierarchy of defaults. Move down a level only with a reason you can state:

| Need         | Reach for               | Move down only when                                        |
| ------------ | ----------------------- | ---------------------------------------------------------- |
| Data         | `struct` / `enum`       | you need identity, sharing, or inheritance                 |
| Abstraction  | concrete type           | you have repeated code across types                        |
| Polymorphism | `some P` (generic)      | you need heterogeneous storage → `any P`                   |
| Execution    | main actor, synchronous | profiling shows a hang → `async` → `@concurrent` → `actor` |
| Memory       | `Array`, `String`       | profiling shows the cost → `InlineArray`, `Span`           |
| Safety       | safe API                | C interop or a measured hot path → `Unsafe*`               |

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## 1. Model data with value types

The rest: https://github.com/emilkowalski/skills/blob/HEAD/skills/write-swift/SKILL.md