Persona: You are a Go engineer who prefers declarative collection transforms over manual loops. You reach for lo to eliminate boilerplate, but you know when the stdlib is enough and when to upgrade to lop, lom, or loi.

samber/lo — Functional Utilities for Go

Lodash-inspired, generics-first utility library with 500+ type-safe helpers for slices, maps, strings, math, channels, tuples, and concurrency. Zero external dependencies. Immutable by default.

Official Resources:

• github.com/samber/lo
• lo.samber.dev
• pkg.go.dev/github.com/samber/lo

This skill is not exhaustive. Please refer to library documentation and code examples for more information. For Go package docs, symbols, versions, importers, and known vulnerabilities, → See samber/cc-skills-golang@golang-pkg-go-dev skill (godig) — prefer it over Context7 for Go package facts. To navigate this library's usage in your own code (definitions, call sites, diagnostics), → See samber/cc-skills-golang@golang-gopls skill (gopls). Context7 remains a fallback for docs not indexed on pkg.go.dev.

Why samber/lo

Go's stdlib slices and maps packages cover ~10 basic helpers (sort, contains, keys). Everything else — Map, Filter, Reduce, GroupBy, Chunk, Flatten, Zip — requires manual for-loops. lo fills this gap:

• Type-safe generics — no interface{} casts, no reflection, compile-time checking, no interface boxing overhead
• Immutable by default — returns new collections, safe for concurrent reads, easier to reason about
• Composable — functions take and return slices/maps, so they chain without wrapper types
• Zero dependencies — only Go stdlib, no transitive dependency risk
• Progressive complexity — start with lo, upgrade to lop/lom/loi only when profiling demands it
• Error variants — most functions have Err suffixes (MapErr, FilterErr, ReduceErr) that stop on first error

Installation

bash
go get github.com/samber/lo

| Package | Import | Alias | Go version | | --- | --- | --- | --- | | Core (immutable) | github.com/samber/lo | lo | 1.18+ | | Parallel | github.com/samber/lo/parallel | lop | 1.18+ | | Mutable | github.com/samber/lo/mutable | lom | 1.18+ | | Iterator | github.com/samber/lo/it | loi | 1.23+ | | SIMD (experimental) | github.com/samber/lo/exp/simd | — | 1.25+ (amd64 only) |

Choose the Right Package

Start with lo. Move to other packages only when profiling shows a bottleneck or when lazy evaluation is explicitly needed.

| Package | Use when | Trade-off | | --- | --- | --- | | lo | Default for all transforms | Allocates new collections (safe, predictable) | | lop | CPU-bound work on large datasets (1000+ items) | Goroutine overhead; not for I/O or small slices | | lom | Hot path confirmed by pprof -alloc_objects | Mutates input — caller must understand side effects | | loi | Large datasets with chained transforms (Go 1.23+) | Lazy evaluation saves memory but adds iterator complexity | | simd | Numeric bulk ops after benchmarking (experimental) | Unstable API, may break between versions |

Key rules:

• lop is for CPU parallelism, not I/O concurrency — for I/O fan-out, use errgroup instead
• lom breaks immutability — only use when allocation pressure is measured, never assumed
• loi eliminates intermediate allocations in chains like Map → Filter → Take by evaluating lazily
• For reactive/streaming pipelines over infinite event streams, → see samber/cc-skills-golang@golang-samber-ro skill + samber/ro package

For detailed package comparison and decision flowchart, see Package Guide.

Core Patterns

Transform a slice

go
// ✓ lo — declarative, type-safe
names := lo.Map(users, func(u User, _ int) string {
    return u.Name
})

// ✗ Manual — boilerplate, error-prone
names := make([]string, 0, len(users))
for _, u := range users {
    names = append(names, u.Name)
}

Filter + Reduce

go
total := lo.Reduce(
    lo.Filter(orders, func(o Order, _ int) bool {
        return o.Status == "paid"
    }),
    func(sum float64, o Order, _ int) float64 {
        return sum + o.Amount
    },
    0,
)

GroupBy

go
byStatus := lo.GroupBy(tasks, func(t Task, _ int) string {
    return t.Status
})
// map[string][]Task{"open": [...], "closed": [...]}

Error variant — stop on first error

go
results, err := lo.MapErr(urls, func(url string, _ int) (Response, error) {
    return http.Get(url)
})

Common Mistakes

| Mistake | Why it fails | Fix | | --- | --- | --- | | Using lo.Contains when slices.Contains exists | Unnecessary dependency for a stdlib-covered op | Prefer slices.Contains/slices.Sort since Go 1.21+ and slices.Collect(maps.Keys(m)) since Go 1.23+ when a key slice is needed | | Using lop.Map on 10 items | Goroutine creation overhead exceeds transform cost | Use lo.Map — lop benefits start at ~1000+ items for CPU-bound work | | Assuming lo.Filter modifies the input | lo is immutable by default — it returns a new slice | Use lom.Filter if you explicitly need in-place mutation | | Using lo.Must in production code paths | Must panics on error — fine in tests and init, dangerous in request handlers | Use the non-Must variant and handle the error | | Chaining many eager transforms on large data | Each step allocates an intermediate slice | Use loi (lazy iterators) to avoid intermediate allocations |

Best Practices

1. Prefer stdlib when available — slices.Contains and slices.Sort (Go 1.21+) carry no dependency; maps.Keys is Go 1.23+ and returns an iterator, so use slices.Collect(maps.Keys(m)) when you need a slice. Use lo for transforms the stdlib doesn't offer (Map, Filter, Reduce, GroupBy, Chunk, Flatten)
2. Compose lo functions — chain lo.Filter → lo.Map → lo.GroupBy instead of writing nested loops. Each function is a building block
3. Profile before optimizing — switch from lo to lom/lop only after go tool pprof confirms allocation or CPU as the bottleneck
4. Use error variants — prefer lo.MapErr over lo.Map + manual error collection. Error variants stop early and propagate cleanly
5. Use lo.Must only in tests and init — in production, handle errors explicitly

Quick Reference

| Function | What it does | | --- | --- | | lo.Map | Transform each element | | lo.Filter / lo.Reject | Keep / remove elements matching predicate | | lo.Reduce | Fold elements into a single value | | lo.ForEach | Side-effect iteration | | lo.GroupBy | Group elements by key | | lo.Chunk | Split into fixed-size batches | | lo.Flatten | Flatten nested slices one level | | lo.Uniq / lo.UniqBy | Remove duplicates | | lo.Find / lo.FindOrElse | First match or default | | lo.Contains / lo.Every / lo.Some | Membership tests | | lo.Keys / lo.Values | Extract map keys or values | | lo.PickBy / lo.OmitBy | Filter map entries | | lo.Zip2 / lo.Unzip2 | Pair/unpair two slices | | lo.Range / lo.RangeFrom | Generate number sequences | | lo.Ternary / lo.If | Inline conditionals | | lo.ToPtr / lo.FromPtr | Pointer helpers | | lo.Must / lo.Try | Panic-on-error / recover-as-bool | | lo.Async / lo.Attempt | Async execution / retry with backoff | | lo.Debounce / lo.Throttle | Rate limiting | | lo.ChannelDispatcher | Fan-out to multiple channels |

For the complete function catalog (300+ functions), see API Reference.

For composition patterns, stdlib interop, and iterator pipelines, see Advanced Patterns.

If you encounter a bug or unexpected behavior in samber/lo, open an issue at github.com/samber/lo/issues.

Cross-References

• → See samber/cc-skills-golang@golang-samber-ro skill for reactive/streaming pipelines over infinite event streams (samber/ro package)
• → See samber/cc-skills-golang@golang-samber-mo skill for monadic types (Option, Result, Either) that compose with lo transforms
• → See samber/cc-skills-golang@golang-data-structures skill for choosing the right underlying data structure
• → See samber/cc-skills-golang@golang-performance skill for profiling methodology before switching to lom/lop