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 alloca

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 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: Two Dimensional Slices Independent inner slices (can grow/shrink independently): Single allocation (more efficient for fixed sizes): Declaring Empty Slices Prefer nil slices over empty literals: 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 Use map[T]struct{} when the map is only a set. The empty struct takes no storage and makes membership intent explicit: Use boolean map values only when the value carries a separate meaning beyond presence. Copying Be careful when copying a struct from another package. If the type has methods on its pointer type ( T ), copying the value can cause aliasing bugs. General rule: Do not copy a value of type T if its methods are associated with the pointer type T . This applies to bytes.Buffer , sync.Mutex , sync.WaitGroup , and types containing them. Quick Reference Topic Key Point Slices Always assign append result; nil slice preferred over []T{} Sets map[T]struct{} for membership only sets Copying Don't copy T if methods are on T ; beware aliasing Related Skills Defensive copying : See [go defensive](../go defensive/SKILL.md) when copying slices or maps at API boundaries to prevent mutation Capacity hints : See [go performance](../go performance/SKILL.md) when pre sizing slices or maps for known workloads Iteration patterns : See [go control flow](../go control flow/SKILL.md) when using range loops over slices, maps, or channels Declaration style : See [go declarations](../go declarations/SKILL.md) when choosing between new , make , var , and composite literals

go-data-structures