use-modern-go

name	use-modern-go
description	Apply modern Go syntax guidelines based on project's Go version. Use when user ask for modern Go code guidelines.

Modern Go Guidelines

Detected Go Version

!awk '/^go /{print $2; exit}' go.mod 2>/dev/null || grep -rh "^go " --include="go.mod" . 2>/dev/null | cut -d' ' -f2 | sort | uniq -c | sort -nr | head -1 | xargs | cut -d' ' -f2 | grep . || echo unknown

How to Use This Skill

DO NOT search for go.mod files or try to detect the version yourself. Use ONLY the version shown above. The detection prefers the root go.mod; it only falls back to scanning nested modules if no root go.mod exists.

If version detected (not "unknown"):

• Say: "This project is using Go X.XX, so I’ll stick to modern Go best practices and freely use language features up to and including this version. If you’d prefer a different target version, just let me know."
• Do NOT list features, do NOT ask for confirmation

If version is "unknown":

• Say: "Could not detect Go version in this repository"
• Use AskUserQuestion: "Which Go version should I target?" → [1.23] / [1.24] / [1.25] / [1.26]

When writing Go code, use ALL features from this document up to the target version:

• Prefer modern built-ins and packages (slices, maps, cmp) over legacy patterns
• Never use features from newer Go versions than the target
• Never use outdated patterns when a modern alternative is available

---

Features by Go Version

Go 1.0+

• time.Since: time.Since(start) instead of time.Now().Sub(start)

Go 1.8+

• time.Until: time.Until(deadline) instead of deadline.Sub(time.Now())

Go 1.13+

• errors.Is: errors.Is(err, target) instead of err == target (works with wrapped errors)

Go 1.18+

• any: Use any instead of interface{}
• bytes.Cut: before, after, found := bytes.Cut(b, sep) instead of Index+slice
• strings.Cut: before, after, found := strings.Cut(s, sep)

Go 1.19+

• fmt.Appendf: buf = fmt.Appendf(buf, "x=%d", x) instead of []byte(fmt.Sprintf(...))
• atomic.Bool/atomic.Int64/atomic.Pointer[T]: Type-safe atomics instead of atomic.StoreInt32

var flag atomic.Bool
flag.Store(true)
if flag.Load() { ... }

var ptr atomic.Pointer[Config]
ptr.Store(cfg)

Go 1.20+

• strings.Clone: strings.Clone(s) to copy string without sharing memory
• bytes.Clone: bytes.Clone(b) to copy byte slice
• strings.CutPrefix/CutSuffix: if rest, ok := strings.CutPrefix(s, "pre:"); ok { ... }
• errors.Join: errors.Join(err1, err2) to combine multiple errors
• context.WithCancelCause: ctx, cancel := context.WithCancelCause(parent) then cancel(err)
• context.Cause: context.Cause(ctx) to get the error that caused cancellation

Go 1.21+

Built-ins:

• min/max: max(a, b) instead of if/else comparisons
• clear: clear(m) to delete all map entries, clear(s) to zero slice elements

slices package:

• slices.Contains: slices.Contains(items, x) instead of manual loops
• slices.Index: slices.Index(items, x) returns index (-1 if not found)
• slices.IndexFunc: slices.IndexFunc(items, func(item T) bool { return item.ID == id })
• slices.SortFunc: slices.SortFunc(items, func(a, b T) int { return cmp.Compare(a.X, b.X) })
• slices.Sort: slices.Sort(items) for ordered types
• slices.Max/slices.Min: slices.Max(items) instead of manual loop
• slices.Reverse: slices.Reverse(items) instead of manual swap loop
• slices.Compact: slices.Compact(items) removes consecutive duplicates in-place
• slices.Clip: slices.Clip(s) removes unused capacity
• slices.Clone: slices.Clone(s) creates a copy

maps package:

• maps.Clone: maps.Clone(m) instead of manual map iteration
• maps.Copy: maps.Copy(dst, src) copies entries from src to dst
• maps.DeleteFunc: maps.DeleteFunc(m, func(k K, v V) bool { return condition })

sync package:

• sync.OnceFunc: f := sync.OnceFunc(func() { ... }) instead of sync.Once + wrapper
• sync.OnceValue: getter := sync.OnceValue(func() T { return computeValue() })

context package:

• context.AfterFunc: stop := context.AfterFunc(ctx, cleanup) runs cleanup on cancellation
• context.WithTimeoutCause: ctx, cancel := context.WithTimeoutCause(parent, d, err)
• context.WithDeadlineCause: Similar with deadline instead of duration

Go 1.22+

Loops:

• for i := range n: for i := range len(items) instead of for i := 0; i < len(items); i++
• Loop variables are now safe to capture in goroutines (each iteration has its own copy)

cmp package:

• cmp.Or: cmp.Or(flag, env, config, "default") returns first non-zero value

// Instead of:
name := os.Getenv("NAME")
if name == "" {
    name = "default"
}
// Use:
name := cmp.Or(os.Getenv("NAME"), "default")

reflect package:

• reflect.TypeFor: reflect.TypeFor[T]() instead of reflect.TypeOf((*T)(nil)).Elem()

net/http:

• Enhanced http.ServeMux patterns: mux.HandleFunc("GET /api/{id}", handler) with method and path params
• r.PathValue("id") to get path parameters

Go 1.23+

• maps.Keys(m) / maps.Values(m) return iterators
• slices.Collect(iter) not manual loop to build slice from iterator
• slices.Sorted(iter) to collect and sort in one step

keys := slices.Collect(maps.Keys(m))       // not: for k := range m { keys = append(keys, k) }
sortedKeys := slices.Sorted(maps.Keys(m))  // collect + sort
for k := range maps.Keys(m) { process(k) } // iterate directly

time package

• time.Tick: As of Go 1.23, the garbage collector can recover unreferenced tickers even if they haven't been stopped, so time.Tick is safe for tickers that run until process shutdown. However, use time.NewTicker when you need explicit lifecycle control — call Stop() for early termination, cleanup, or deterministic shutdown in long-running servers or managed goroutines.

Go 1.24+

• t.Context(): ALWAYS use t.Context() when a test function needs a context.

func TestFoo(t *testing.T) {
    ctx := t.Context()
    result := doSomething(ctx)
}

• omitzero: ALWAYS use omitzero (not omitempty) for time.Duration, time.Time, structs, slices, maps.

type Config struct {
    Timeout time.Duration `json:"timeout,omitzero"`
}

• b.Loop(): ALWAYS use b.Loop() for the main loop in benchmark functions.

func BenchmarkFoo(b *testing.B) {
    for b.Loop() {
        doWork()
    }
}

• strings.SplitSeq / strings.FieldsSeq: ALWAYS use when iterating split results in a for-range loop. Also: bytes.SplitSeq, bytes.FieldsSeq.

for part := range strings.SplitSeq(s, ",") {
    process(part)
}

Go 1.25+

• wg.Go(fn): ALWAYS use wg.Go() when spawning goroutines with sync.WaitGroup.

var wg sync.WaitGroup
for _, item := range items {
    wg.Go(func() {
        process(item)
    })
}
wg.Wait()

Go 1.26+ (DO NOT USE — listed for future reference only)

These features are NOT available in the current project. Do not use them until the go.mod version is upgraded to 1.26+.

• new(val) — new() extended to accept expressions, not just types. Returns pointer to any value. new(0) → *int, new("s") → *string, new(T{}) → *T.

cfg := Config{
    Timeout: new(30),   // *int
    Debug:   new(true), // *bool
}

• errors.AsType[T](err) — generic alternative to errors.As(err, &target).

if pathErr, ok := errors.AsType[*os.PathError](err); ok {
    handle(pathErr)
}