use-modern-go

name	use-modern-go
description	Apply modern Go syntax guidelines based on the project's Go version. Use whenever writing, editing, or reviewing Go (.go) code — not only when explicitly asked for guidelines.

Modern Go Guidelines

Detected Go Version

!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.

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• Never use outdated patterns when a modern alternative is available

When modernizing existing Go files, if the installed toolchain is Go 1.26 or newer, run go fix on the target package(s) before reading any file.

go version                            # gate on the installed toolchain

go fix -diff ./path/to/pkg/...        # preview (the unflagged form writes files in place)
go fix ./path/to/pkg/...              # apply
go fix ./path/to/pkg/...              # run a second pass; one fix often enables another

go build ./path/to/pkg/...            # verify nothing broke
go test ./path/to/pkg/...

Then read the post-fix files and apply manual modernizations from the Features list for anything go fix did not catch. Do not Read a Go file you plan to modernize until go fix has run on its package, otherwise you will end up doing work the tool would have done for free.

Caveats:

• go fix only applies a fix when the file's or module's declared Go version permits the target feature (go 1.X in go.mod or //go:build go1.X in the file).
• Generated files (per //go:generate) are skipped. Fix the generator instead.
• Each run analyzes one build configuration. For code with GOOS/GOARCH build tags, re-run per combo (e.g. GOOS=linux GOARCH=amd64 go fix ./..., then GOOS=darwin GOARCH=arm64 go fix ./...).
• The tool resolves syntactic conflicts but can leave semantic ones (e.g. now-unused variables). Compilation errors after go fix are normal and require manual resolution.
• go tool fix help lists the available fixers; -name=false disables a specific one (e.g. go fix -minmax=false ./...).

If the installed toolchain is older than Go 1.26, skip go fix (its older versions only contain a handful of long-deprecated stdlib rewrites that are not useful here) and go straight to reading files and applying the Features list manually.

---

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: Use time.Tick freely — as of Go 1.23, the garbage collector can recover unreferenced tickers, even if they haven't been stopped. The Stop method is no longer necessary to help the garbage collector. There is no longer any reason to prefer NewTicker when Tick will do.

Go 1.24+

• t.Context() not context.WithCancel(context.Background()) in tests. ALWAYS use t.Context() when a test function needs a context.

Before:

func TestFoo(t *testing.T) {
    ctx, cancel := context.WithCancel(context.Background())
    defer cancel()
    result := doSomething(ctx)
}

After:

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

• omitzero not omitempty in JSON struct tags. ALWAYS use omitzero for time.Duration, time.Time, structs, slices, maps.

Before:

type Config struct {
    Timeout time.Duration `json:"timeout,omitempty"` // doesn't work for Duration!
}

After:

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

• b.Loop() not for i := 0; i < b.N; i++ in benchmarks. ALWAYS use b.Loop() for the main loop in benchmark functions.

Before:

func BenchmarkFoo(b *testing.B) {
    for i := 0; i < b.N; i++ {
        doWork()
    }
}

After:

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

• strings.SplitSeq not strings.Split when iterating. ALWAYS use SplitSeq/FieldsSeq when iterating over split results in a for-range loop.

Before:

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

After:

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

Also: strings.FieldsSeq, bytes.SplitSeq, bytes.FieldsSeq.

Go 1.25+

• wg.Go(fn) not wg.Add(1) + go func() { defer wg.Done(); ... }(). ALWAYS use wg.Go() when spawning goroutines with sync.WaitGroup.

Before:

var wg sync.WaitGroup
for _, item := range items {
    wg.Add(1)
    go func() {
        defer wg.Done()
        process(item)
    }()
}
wg.Wait()

After:

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

Go 1.26+

• new(val) not x := val; &x — returns pointer to any value. Go 1.26 extends new() to accept expressions, not just types. Type is inferred: new(0) → *int, new("s") → *string, new(T{}) → *T. DO NOT use x := val; &x pattern — always use new(val) directly. DO NOT use redundant casts like new(int(0)) — just write new(0). Common use case: struct fields with pointer types.

Before:

timeout := 30
debug := true
cfg := Config{
    Timeout: &timeout,
    Debug:   &debug,
}

After:

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

• errors.AsType[T](err) not errors.As(err, &target). ALWAYS use errors.AsType when checking if error matches a specific type.

Before:

var pathErr *os.PathError
if errors.As(err, &pathErr) {
    handle(pathErr)
}

After:

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