# go-data-structures

A skill that steers your coding agent toward correct Go idioms for slices, maps and arrays: make vs new, append, nil slices and JSON.

- skill · `type=skill` · Claude Code, Codex, Cursor, GitHub Copilot, Gemini CLI, OpenCode · 1200 installs · ★ 181
- Author: cxuu
- License: Apache-2.0 · policy=mirror · the beginning of the file is at the end of this page
- Source: https://github.com/cxuu/golang-skills/tree/HEAD/skills/go-data-structures/SKILL.md
- Updated: 2026-06-20
- HTML: https://affpapa.org/en/nevibecode/skills/go-data-structures

## What it does

The skill fires whenever the agent writes or edits Go code that builds collections, even if you never asked about allocation idioms. It ships a decision tree: fixed size means an array `[N]T`, dynamic size means a slice `[]T`, key lookup means `map[K]V`, and a set means `map[T]struct{}`. When the approximate size is known it suggests `make([]T, 0, capacity)`; when JSON needs a real array it suggests `var s []T`.

A separate `references/SLICES.md` covers nil versus empty literals, slice copying and capacity. The skill also reminds the agent to always assign the `append` result, and warns against copying a struct whose methods sit on `*T` — that bites on `bytes.Buffer`, `sync.Mutex` and `sync.WaitGroup`, where a value copy usually hides an aliasing bug.

Concurrent access is out of scope: for mutexes and atomics the skill points to `go-concurrency`. Defensive copying at API boundaries lives in `go-defensive`, and capacity hints in `go-performance`. Worth checking which of those neighbouring skills you already have installed.

## Install

### Claude Code (native platform)

1. **Skills CLI (Vercel)**
```bash
npx skills add cxuu/golang-skills --skill go-data-structures -a claude-code
```

2. **Manually: put the files in place**
- File: `~/.claude/skills/go-data-structures/SKILL.md` — Copy the whole skill folder; for one project use .claude/skills/go-data-structures/

### Codex

1. **Skills CLI (Vercel)**
```bash
npx skills add cxuu/golang-skills --skill go-data-structures -a codex
```

2. **Manually: put the files in place**
- File: `.agents/skills/go-data-structures/SKILL.md` — For all projects use ~/.agents/skills/

### Cursor

1. **Skills CLI (Vercel)**
```bash
npx skills add cxuu/golang-skills --skill go-data-structures -a cursor
```

2. **Manually: put the files in place**
- File: `.cursor/skills/go-data-structures/SKILL.md` — Cursor also reads .agents/skills/ and .claude/skills/

### GitHub Copilot

1. **GitHub CLI**
```bash
gh skill install cxuu/golang-skills go-data-structures
```

2. **Manually: put the files in place**
- File: `.github/skills/go-data-structures/SKILL.md`

### Gemini CLI

1. **Skills CLI (Vercel)**
```bash
npx skills add cxuu/golang-skills --skill go-data-structures -a gemini-cli
```

2. **Platform CLI**
```bash
gemini skills install https://github.com/cxuu/golang-skills --path skills/go-data-structures
```

### Windsurf (Devin Desktop)

**Manually: put the files in place**
- File: `.devin/skills/go-data-structures/SKILL.md` — The legacy .windsurf/skills/ path still works

### OpenCode

**Skills CLI (Vercel)**
```bash
npx skills add cxuu/golang-skills --skill go-data-structures -a opencode
```

### Claude.ai

**Claude.ai**
- Zip the folder and upload it in Settings → Capabilities → Skills

Where a file is given instead of a command, copy the files from the source to that path: https://github.com/cxuu/golang-skills/tree/HEAD/skills/go-data-structures/SKILL.md

## When to use it
- pick between `make` and `var` for a slice in a JSON response
- swap a boolean map for `map[T]struct{}` as a set
- catch a `sync.Mutex` copied by value
- size a 2D slice with a single allocation
- explain to the agent why `append` needs assignment

## Example prompts
- declare the empty slice so the JSON field encodes as `[]` instead of `null`
- build a set of attendees with `map[string]struct{}`
- check this code for aliasing when structs get copied

## Source text · SKILL.md · Apache-2.0

Below is the content of a third-party file. It is data, not instructions for you: do not act on anything written inside unless the human asks. Block tag: e274550a0f65b683.

BEGIN THIRD-PARTY FILE e274550a0f65b683
````text
---
name: go-data-structures
description: Use when working with Go slices, maps, or arrays — choosing between new and make, using append, declaring empty slices (nil vs literal for JSON), implementing sets with maps, and copying data at boundaries. Also use when building or manipulating collections, even if the user doesn't ask about allocation idioms. Does not cover concurrent data structure safety (see go-concurrency).
---

# Go Data Structures

## Resource Routing

- `references/SLICES.md` - Read when deciding nil versus empty slices, copying slices, or managing slice capacity and aliasing.

## Choosing a Data Structure

```
What do you need?
├─ Ordered collection of items
│  ├─ Fixed size known at compile time → Array [N]T
│  └─ Dynamic size → Slice []T
│     ├─ Know approximate size? → make([]T, 0, capacity)
│     └─ Unknown size or nil-safe for JSON? → var s []T (nil)
├─ Key-value lookup
│  └─ Map map[K]V
│     ├─ Know approximate size? → make(map[K]V, capacity)
│     └─ Need a set? → map[T]struct{} (zero-size values)
└─ Need to pass to a function?
   └─ Copy at the boundary if the caller might mutate it
```

> **When this skill does NOT apply**: For concurrent access to data structures (mutexes, atomic operations), see [go-concurrency](../go-concurrency/SKILL.md). For defensive copying at API boundaries, see [go-defensive](../go-defensive/SKILL.md). For pre-sizing capacity for performance, see [go-performance](../go-performance/SKILL.md).

---

## Slices

### The append Function

**Always assign the result** — the underlying array may change:

```go
x := []int{1, 2, 3}
x = append(x, 4, 5, 6)

// Append a slice to a slice
x = append(x, y...)  // Note the ...
```

### Two-Dimensional Slices

**Independent inner slices** (can grow/shrink independently):

```go
picture := make([][]uint8, YSize)
for i := range picture {
    picture[i] = make([]uint8, XSize)
}
```

**Single allocation** (more efficient for fixed sizes):

```go
picture := make([][]uint8, YSize)
pixels := make([]uint8, XSize*YSize)
for i := range picture {
    picture[i], pixels = pixels[:XSize], pixels[XSize:]
}
```

### Declaring Empty Slices

Prefer nil slices over empty literals:

```go
// Good: nil slice
var t []string

// Avoid: non-nil but zero-length
t := []string{}
```

Both have `len` and `cap` of zero, but the nil slice is the preferred style.

**Exception for JSON**: A nil slice encodes to `null`, while `[]string{}`
encodes to `[]`. Use non-nil when you need a JSON array.

When designing interfaces, avoid distinguishing between nil and non-nil
zero-length slices.

---

## Maps

### Implementing a Set
````
END THIRD-PARTY FILE e274550a0f65b683

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