---
name: go-data-structures
description: >
  Correct use of Go's built-in data structures: slices (nil vs empty, append
  semantics, aliasing, preallocation), maps (comma-ok, sets, iteration
  order), arrays, and choosing between them. Use when: "slice vs array",
  "nil slice", "empty slice", "preallocate", "map iteration", "use a set in
  Go", "slice aliasing", "append gotcha", "copy a slice", "sync.Map or
  mutex".
  Not for: structures shared across goroutines (go-concurrency-review),
  allocation profiling (go-performance-review), generic containers
  (go-design-patterns).
user-invocable: true
license: MIT
compatibility: Designed for Claude Code or similar AI coding agents working on Go projects. Requires the Go toolchain.
allowed-tools: Read Edit Write Glob Grep Bash(go:*) Bash(gofmt:*)
metadata:
  author: eduardo-sl
  version: "1.2.0"
---

# Go Data Structures

Slices and maps look simple and hide the sharpest edges in the language.
These rules prevent the aliasing, nil, and iteration bugs that survive
code review.

Detailed reference material, loaded on demand:

- `references/containers.md` — `container/heap` priority queue, LRU with
  `container/list`, reclaiming map memory, a `weak.Pointer` cache.

Read a reference file only when the summary below is not enough.

## 1. Nil Slice vs Empty Slice

```go
var a []int          // nil slice — len 0, cap 0, no allocation
b := []int{}         // empty slice — len 0, allocated header
c := make([]int, 0)  // empty slice — same as b
```

- `len`, `cap`, `range`, and `append` treat all three identically.
- **Prefer the nil slice** as the "no elements" value; don't allocate
  just to return "empty".
- Exception: JSON. With `encoding/json`, `nil` marshals to `null` and empty
  to `[]`; if the contract requires `[]`, return an empty slice. The Go
  1.27 `encoding/json/v2` API marshals nil as `[]` by default.
- Never distinguish nil from empty in logic — check `len(s) == 0`.

## 2. Append Semantics and Aliasing

`append` MAY return the same backing array or a new one. Both cases
bite:

```go
// ❌ Bad — result may alias the input
func addSuffix(base []string) []string {
    return append(base, "suffix") // if cap(base) > len(base),
}                                 // this WRITES INTO base's array

// ✅ Good — force a copy when the input must not be touched
func addSuffix(base []string) []string {
    out := make([]string, len(base), len(base)+1)
    copy(out, base)
    return append(out, "suffix")
}
```

```go
// ❌ Bad — subslice keeps the whole 64 MB alive
func header(big []byte) []byte {
    return big[:512] // backing array is still the full big
}

// ✅ Good — copy the window you keep
func header(big []byte) []byte {
    return slices.Clone(big[:512]) // Go 1.21+; or copy() manually
}
```

Rule: a function either owns a slice or copies it. Returning a subslice
of a caller's slice, or appending to one, silently shares memory.

## 3. Preallocation

When the final size is known or bounded, allocate once:

```go
// ✅ Good — one allocation
names := make([]string, 0, len(users))
for _, u := range users {
    names = append(names, u.Name)
}

// ❌ Bad — repeated growth and copying
var names []string
for _, u := range users {
    names = append(names, u.Name)
}
```

Same for maps: `make(map[string]int, len(items))`.
Don't preallocate when the size is unknown — a wrong large cap wastes
memory; `append` growth is fine for cold paths.

## 4. Map Essentials

```go
// Comma-ok distinguishes "missing" from "zero value"
count, ok := hits[key]
if !ok { /* key absent */ }

// Zero value reads are safe; writes to a nil map PANIC
var m map[string]int
_ = m["x"]      // 0, fine
m["x"] = 1      // panic: assignment to entry in nil map — make() first

// Iteration order is RANDOM and differs between runs.
// Sort keys when output must be deterministic:
keys := slices.Sorted(maps.Keys(m)) // Go 1.23+
for _, k := range keys {
    fmt.Println(k, m[k])
}
```

- Map values are not addressable: `m[k].Field = v` doesn't compile for
  struct values. Use a map of pointers, or read-modify-write.
- Deleting during `range` is safe; inserting during `range` is
  unspecified (the new key may or may not be visited).

## 5. Sets

The idiomatic set is a map with empty-struct values:

```go
seen := make(map[string]struct{}, len(items))
for _, it := range items {
    if _, dup := seen[it.ID]; dup {
        continue
    }
    seen[it.ID] = struct{}{}
    process(it)
}
```

`struct{}` occupies zero bytes; `map[string]bool` also works and reads
better when you'll test membership with `if seen[id]`.

## 6. Arrays vs Slices

- Arrays (`[4]byte`) are values: assignment and passing copy the whole
  array. Comparable with `==` when elements are comparable.
- Use arrays for fixed-size data with value semantics: hashes
  (`[32]byte`), IPv4 addresses, fixed matrices, map keys.
- Everything else is a slice. A function taking `[100]int` copies 800
  bytes per call — almost always wrong.

## 7. Choosing a Structure

| Need | Use |
|---|---|
| Ordered collection, growable | `[]T` |
| Membership / dedup | `map[K]struct{}` |
| Key→value lookup | `map[K]V` |
| Fixed size, value semantics, comparable | `[N]T` array |
| FIFO queue (single goroutine) | slice with head index, or `container/list` for heavy churn |
| Stack | slice + `append` / `s[:len(s)-1]` |
| Concurrent map, write-once read-many keys | `sync.Map` — otherwise mutex + map |
| Priority queue, top-K, scheduler | `container/heap` |
| Fixed-size rolling window | slice used as a ring (index mod len), or `container/ring` |
| LRU cache | `map[K]*list.Element` + `container/list` |
| Cache that must not keep values alive | `map[K]weak.Pointer[V]` (Go 1.24+) |
| Deduplicate many equal strings | `unique.Make` (Go 1.23+) |
| Key type for a custom hash table | `maphash.Comparable` / `maphash.Hasher` (1.24 / 1.27) |

`sync.Map` is a special-case tool (append-only caches, disjoint key
sets). Default to `map` + `sync.RWMutex`; see the concurrency skill for
locking patterns.

## 8. Memory Behaviour

- **Maps never shrink.** `delete` and `clear` keep the buckets. After a
  map that once held millions of entries is mostly empty, rebuild it
  (`m = maps.Clone(m)`) to give the memory back.
- **`clear(s)`** zeroes a slice's elements but keeps its length; `s = s[:0]`
  keeps the backing array for reuse. Neither frees memory.
- **Popping from a slice of pointers** — nil the vacated slot
  (`s[n-1] = nil`) or the GC still sees the object as reachable.
- **`strings.Builder`** builds strings without a final copy; it must not be
  copied once written to. **`bytes.Buffer`** is the type for byte-oriented
  I/O (it is both an `io.Reader` and an `io.Writer`); its `String()` copies.

## 9. Copy Semantics

| Type | Assignment copies | Independent copy |
|---|---|---|
| Basic types, arrays, structs of values | Everything | Already independent |
| Slice | Header (ptr, len, cap) | `slices.Clone` |
| Map | Reference | `maps.Clone` (shallow) |
| Pointer, channel, func | Reference | — |
| Interface | Type + value word | Depends on the dynamic type |

Clones are shallow: a cloned `[]*User` still shares every `User`.
Examples for sections 7-9: `references/containers.md`.

## Verification Checklist

1. No logic distinguishes nil slice from empty slice; `len()` used for emptiness
2. JSON-facing slices explicitly empty (not nil) where the contract requires `[]`
3. No `append` to a slice the function doesn't own; copies made explicit
4. No long-lived subslices of large arrays without `slices.Clone`/`copy`
5. Slices and maps preallocated with capacity when size is known
6. Comma-ok used wherever "missing" differs from zero value
7. No writes to possibly-nil maps
8. Deterministic output paths sort map keys before iteration
9. Sets built as `map[K]struct{}` (or `map[K]bool` for readability)
10. Arrays only where value semantics or comparability is the point
11. Long-lived maps that shrink drastically are rebuilt, not just deleted from
12. Priority queues and LRUs use `container/heap` / `container/list`, not ad-hoc sorting