modern-go

Modernize Go source code by applying version-appropriate idioms, APIs, and language features. Works like go fix plus additional transformations curated from the Go team's modernize analysis passes and community best practices.

Usage

Invoke this skill when the user asks to modernize Go code. By default, modernize the entire project; the user may specify a file or directory instead.

When invoked:

1. Detect the project's Go version from go.mod (the go directive).
2. Find all .go files in the target scope (excluding vendor/, .git/, testdata/).
3. For each file, apply all transformations for versions ≤ the project's Go version, starting from the oldest to the newest.
4. After all transformations, print a summary of what was changed and what was skipped.

If the user specifies a file or directory, limit the scope to that path.

Transformation Catalog

Each transformation includes a Go version gate—only apply when the project's go.mod version ≥ that version. Never apply a transformation that requires a version higher than the project declares.

Go 1.0+ — time.Since

// before
elapsed := time.Now().Sub(start)
// after
elapsed := time.Since(start)

Go 1.8+ — time.Until

// before
remaining := deadline.Sub(time.Now())
// after
remaining := time.Until(deadline)

Go 1.10+ — strings.Builder (loop concatenation)

// before
s := ""
for _, item := range items {
    s += item
}
// after
var b strings.Builder
for _, item := range items {
    b.WriteString(item)
}
s := b.String()

Only when += concatenation happens inside a loop.

Go 1.13+ — errors.Is

// before
if err == io.EOF {
    return
}
// after
if errors.Is(err, io.EOF) {
    return
}

Go 1.17+ — //go:build constraints (plusbuild)

// before
//go:build linux && amd64
// +build linux,amd64

package foo
// after
//go:build linux && amd64

package foo

The plusbuild modernizer removes obsolete // +build constraint lines once the equivalent //go:build line is present (the //go:build syntax landed in Go 1.17). Only strip the old line when a matching //go:build already exists — never drop the sole constraint.

Go 1.17+ — unsafe.Add / unsafe.Slice (unsafefuncs)

// before — pointer arithmetic via uintptr
p2 := unsafe.Pointer(uintptr(ptr) + uintptr(offset))
// after
p2 := unsafe.Add(ptr, offset)

// before — building a slice from a base pointer
s := (*[1 << 30]byte)(unsafe.Pointer(p))[:n:n]
// after
s := unsafe.Slice(p, n)

The unsafefuncs modernizer (gopls v0.22.0) rewrites error-prone uintptr pointer math into unsafe.Add / unsafe.Slice, which the compiler and go vet understand as GC-safe.

Go 1.18+ — any

// before
func decode(v interface{}) error { ... }
// after
func decode(v any) error { ... }

Go 1.18+ — strings.Cut

// before
if i := strings.Index(s, "="); i >= 0 {
    key, val := s[:i], s[i+1:]
}
// after
if key, val, found := strings.Cut(s, "="); found {
    ...
}

Go 1.18+ — bytes.Cut

// before
if i := bytes.Index(b, sep); i >= 0 {
    before, after := b[:i], b[i+len(sep):]
}
// after
before, after, found := bytes.Cut(b, sep)

Go 1.19+ — fmt.Appendf

// before
buf = append(buf, fmt.Sprintf("x=%d", x)...)
// after
buf = fmt.Appendf(buf, "x=%d", x)

Go 1.19+ — Type-safe atomics (atomictypes)

// before
var ready int32
atomic.StoreInt32(&ready, 1)
if atomic.LoadInt32(&ready) == 1 { ... }

// after
var ready atomic.Int32
ready.Store(1)
if ready.Load() == 1 { ... }

// before
var cache atomic.Value
cache.Store(&Config{})
cfg := cache.Load().(*Config)

// after
var cache atomic.Pointer[Config]
cache.Store(&Config{})
cfg := cache.Load()

The atomictypes modernizer (gopls v0.22.0, AtomicTypesAnalyzer) rewrites both the variable declaration and every call site. Typed wrappers (atomic.Int32/Int64/Uint32/Uint64/Bool/Pointer[T]) have identical performance but prevent accidental non-atomic access and fix 64-bit alignment crashes on 32-bit architectures.

Go 1.20+ — strings.Clone

// before
s2 := string([]byte(s)) // force copy
// after
s2 := strings.Clone(s)

Go 1.20+ — bytes.Clone

// before
dst := make([]byte, len(src))
copy(dst, src)
// after
dst := bytes.Clone(src)

Go 1.20+ — strings.CutPrefix / strings.CutSuffix

// before
if strings.HasPrefix(s, "pre_") {
    s = s[len("pre_"):]
}
// after
if rest, ok := strings.CutPrefix(s, "pre_"); ok {
    s = rest
}

// before
if strings.HasSuffix(s, ".txt") {
    s = s[:len(s)-len(".txt")]
}
// after
if rest, ok := strings.CutSuffix(s, ".txt"); ok {
    s = rest
}

Go 1.20+ — errors.Join

// before
return fmt.Errorf("load config: %w: %w", err1, err2)
// after
return errors.Join(fmt.Errorf("load config"), err1, err2)

Go 1.20+ — context.WithCancelCause

// before
ctx, cancel := context.WithCancel(parent)
// ... somewhere ...
cancel()

// after
ctx, cancel := context.WithCancelCause(parent)
cancel(ErrShutdown)
// caller: context.Cause(ctx) → ErrShutdown

Go 1.21+ — min / max

// before
lo := a
if b < lo {
    lo = b
}
// after
lo := min(a, b)

// before
if x < 0 {
    x = 0
}
if x > 100 {
    x = 100
}
// after
x = min(max(x, 0), 100)

Go 1.21+ — clear

// before
for k := range m {
    delete(m, k)
}
// after
clear(m)

// before
for i := range s {
    s[i] = 0
}
// after
clear(s)

Go 1.21+ — slices package

// before → after: slices.Contains(items, target)
found := false
for _, x := range items {
    if x == target {
        found = true
        break
    }
}

// before → after: slices.Index(items, target)
for i, x := range items {
    if x == target {
        return i
    }
}
return -1

// before → after: slices.SortFunc(items, cmp.Compare)
sort.Slice(items, func(i, j int) bool { return items[i] < items[j] })

// before → after: slices.Max(items) / slices.Min(items)
max := items[0]
for _, v := range items[1:] {
    if v > max {
        max = v
    }
}

// before → after: slices.Reverse(s)
for i, j := 0, len(s)-1; i < j; i, j = i+1, j-1 {
    s[i], s[j] = s[j], s[i]
}

// before → after: slices.Compact(s)
i := 0
for j := 1; j < len(s); j++ {
    if s[j] != s[i] {
        i++
        s[i] = s[j]
    }
}
s = s[:i+1]

// before → after: slices.Clip(s)
s = s[:len(s):len(s)]

// before → after: slices.Clone(src)
dst := make([]T, len(src))
copy(dst, src)

Requires importing "slices" and "cmp" (for SortFunc).

Go 1.21+ — slices.Delete / slices.Insert

// before — element removal (classic aliasing/leak footgun)
s = append(s[:i], s[i+1:]...)
// after
s = slices.Delete(s, i, i+1)

// before — insert at index i
s = append(s[:i], append([]T{v}, s[i:]...)...)
// after
s = slices.Insert(s, i, v)

slices.Delete zeroes the tail elements to avoid retaining pointers (the manual append form leaks). Requires importing "slices".

Go 1.21+ — slices.Equal / maps.Equal

// before
if reflect.DeepEqual(got, want) { ... }   // got, want are []string
// after
if slices.Equal(got, want) { ... }

Faster and type-safe, with no reflection. Only for element types that are directly comparable (use slices.EqualFunc / maps.EqualFunc otherwise). Requires importing "slices" or "maps".

Go 1.21+ — maps package

// before → after: maps.Clone(m)
dst := make(map[K]V)
for k, v := range src {
    dst[k] = v
}

// before → after: maps.Copy(dst, src)
for k, v := range src {
    dst[k] = v
}

// before → after: maps.DeleteFunc(m, func(k K, v V) bool { return v == 0 })
for k, v := range m {
    if v == 0 {
        delete(m, k)
    }
}

Requires importing "maps".

Go 1.22+ — slices.Concat (appendclipped)

// before
all := append(append([]int(nil), a...), b...)
// after
all := slices.Concat(a, b)

// before — three-way concat
merged := append(append(append([]string(nil), x...), y...), z...)
// after
merged := slices.Concat(x, y, z)

The appendclipped modernizer replaces nested append concatenation of multiple slices with slices.Concat, which allocates a fresh, correctly-sized result. slices.Concat was added in Go 1.22. Requires importing "slices". Only apply when the pattern builds a new slice (starts from []T(nil) or a clipped base) — not when it appends in place to an existing slice.

Go 1.21+ — sync.OnceFunc / sync.OnceValue

// before
var once sync.Once
func init() { once.Do(func() { setup() }) }

// after
var initOnce = sync.OnceFunc(func() { setup() })

// before
var once sync.Once
var cfg *Config
func getConfig() *Config {
    once.Do(func() { cfg = loadConfig() })
    return cfg
}
// after
var getConfig = sync.OnceValue(func() *Config { return loadConfig() })

Go 1.21+ — context.AfterFunc

// before
go func() {
    <-ctx.Done()
    conn.Close()
}()
// after
stop := context.AfterFunc(ctx, func() { conn.Close() })

Go 1.21+ — context.WithTimeoutCause / WithDeadlineCause

// before
ctx, cancel := context.WithTimeout(parent, 5*time.Second)
// after
ctx, cancel := context.WithTimeoutCause(parent, 5*time.Second, ErrTimeout)

Only apply when a meaningful cause error is available.

Go 1.22+ — Range over integer

// before
for i := 0; i < len(items); i++ {
    process(i, items[i])
}
// after
for i := range len(items) {
    process(i, items[i])
}

// before
for i := 0; i < n; i++ {
    doWork()
}
// after
for range n {
    doWork()
}

Go 1.22+ — Loop variable shadowing removal

// before
for _, x := range items {
    x := x          // capture for goroutine
    go func() { use(x) }()
}
// after
for _, x := range items {
    go func() { use(x) }()
}

The x := x capture idiom is redundant since Go 1.22.

Go 1.22+ — cmp.Or

// before
name := os.Getenv("NAME")
if name == "" {
    name = os.Getenv("USER")
}
if name == "" {
    name = "anonymous"
}
// after
name := cmp.Or(os.Getenv("NAME"), os.Getenv("USER"), "anonymous")

Requires importing "cmp".

Go 1.22+ — reflect.TypeFor

// before
t := reflect.TypeOf((*MyType)(nil)).Elem()
// after
t := reflect.TypeFor[MyType]()

Go 1.22+ — Enhanced http.ServeMux

// before
mux.HandleFunc("/api/", func(w http.ResponseWriter, r *http.Request) {
    id := strings.TrimPrefix(r.URL.Path, "/api/")
    ...
})
// after
mux.HandleFunc("GET /api/{id}", func(w http.ResponseWriter, r *http.Request) {
    id := r.PathValue("id")
    ...
})

Go 1.22+ — math/rand/v2

// before
import "math/rand"
n := rand.Intn(100)
// after
import "math/rand/v2"
n := rand.IntN(100)

// before — type-specific bound
d := time.Duration(rand.Int63n(int64(max)))
// after — generic N works for any integer type
d := rand.N(max)

math/rand/v2 (Go 1.22) drops the deprecated global Seed (top-level funcs are auto-seeded) and adds generic rand.N[T]. Semantic migration: the random stream differs from math/rand, so do not apply where reproducibility from a fixed seed matters. Flag as a suggestion, not auto-apply.

Go 1.23+ — Range over function (iterators)

// before — callback-based iteration
func (t *Tree) Each(fn func(v int)) {
    for _, v := range t.values {
        fn(v)
    }
}
// caller: t.Each(func(v int) { use(v) })

// after — standard iterator, usable with range
func (t *Tree) All() iter.Seq[int] {
    return func(yield func(int) bool) {
        for _, v := range t.values {
            if !yield(v) { return }
        }
    }
}
// caller: for v := range t.All() { use(v) }

Adopt the iter.Seq[T] / iter.Seq2[K,V] protocol (Go 1.23) so custom containers compose with range, slices.Collect, maps.Keys, etc. Requires importing "iter". Flag as a suggestion — it reshapes the API surface.

Go 1.23+ — Iterator helpers

// before
var keys []string
for k := range m {
    keys = append(keys, k)
}
// after
keys := slices.Collect(maps.Keys(m))

// before
var vals []int
for _, v := range m {
    vals = append(vals, v)
}
// after
vals := slices.Collect(maps.Values(m))

Requires importing "slices" and "maps".

Go 1.23+ — strings.SplitSeq / strings.FieldsSeq

// before
for _, part := range strings.Split(line, ",") {
    process(part)
}
// after
for part := range strings.SplitSeq(line, ",") {
    process(part)
}

Only when the loop body does not need the index or the full slice.

Go 1.23+ — bytes.SplitSeq / bytes.FieldsSeq

// before
for _, part := range bytes.Split(data, sep) {
    process(part)
}
// after
for part := range bytes.SplitSeq(data, sep) {
    process(part)
}

Go 1.23+ — slices.Backward (slicesbackward)

// before
for i := len(items) - 1; i >= 0; i-- {
    process(items[i])
}
// after
for _, v := range slices.Backward(items) {
    process(v)
}

// before — index still needed
for i := len(items) - 1; i >= 0; i-- {
    fmt.Println(i, items[i])
}
// after
for i, v := range slices.Backward(items) {
    fmt.Println(i, v)
}

The slicesbackward modernizer (gopls v0.22.0) replaces manual descending-index loops with the slices.Backward iterator. Requires importing "slices". Caveat: the rewrite preserves exact semantics in normal cases, but do not apply it when the loop body mutates the slice length or the index is used for out-of-band arithmetic — those edge cases can become unsound.

Go 1.24+ — t.Context() in tests

// before
func TestFetch(t *testing.T) {
    ctx, cancel := context.WithCancel(context.Background())
    defer cancel()
    result := fetch(ctx)
}
// after
func TestFetch(t *testing.T) {
    result := fetch(t.Context())
}

Go 1.24+ — strings.Lines / bytes.Lines

// before
sc := bufio.NewScanner(strings.NewReader(s))
for sc.Scan() {
    process(sc.Text())
}
// after
for line := range strings.Lines(s) {
    process(strings.TrimSuffix(line, "\n"))
}

strings.Lines / bytes.Lines (Go 1.24) return line iterators — no Scanner setup, no default 64KB token-size limit. Note the yielded line retains its trailing \n, unlike Scanner.Text(); trim it if the old code relied on stripped lines. Only apply for in-memory strings/buffers, not streaming io.Readers.

Go 1.24+ — os.Root (directory-scoped filesystem access)

// before — hand-rolled path-traversal guard
p := filepath.Join(base, name)
if !strings.HasPrefix(filepath.Clean(p), filepath.Clean(base)+string(os.PathSeparator)) {
    return errUnsafePath
}
f, err := os.Open(p)
// after — the OS enforces the boundary
root, err := os.OpenRoot(base)
if err != nil { return err }
defer root.Close()
f, err := root.Open(name) // symlinks/".." escaping base are rejected

os.Root (Go 1.24) confines all operations to a directory tree, rejecting .. and symlink escapes at the syscall layer — far more robust than string prefix checks. Security hardening: flag as a strong suggestion wherever user-controlled paths are joined to a base directory.

Go 1.24+ — omitzero struct tag

// before
type Config struct {
    Timeout time.Duration `json:"timeout,omitempty"`
    Labels  []string      `json:"labels,omitempty"`
}
// after
type Config struct {
    Timeout time.Duration `json:"timeout,omitzero"`
    Labels  []string      `json:"labels,omitzero"`
}

Only for types where omitempty fails: time.Time, time.Duration, structs, slices, maps. Flag as suggestion, not auto-apply.

Go 1.24+ — b.Loop() in benchmarks

// before
func BenchmarkHash(b *testing.B) {
    for i := 0; i < b.N; i++ {
        hash(input)
    }
}
// after
func BenchmarkHash(b *testing.B) {
    for b.Loop() {
        hash(input)
    }
}

Go 1.25+ — sync.WaitGroup.Go

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

Go 1.26+ — new with expressions

// before
timeout := 30
debug := true
cfg := Config{
    Timeout: &timeout,
    Debug:   &debug,
}
// after
cfg := Config{
    Timeout: new(30),
    Debug:   new(true),
}

// before
func ptr[T any](v T) *T { return &v }
cfg := Config{Count: ptr(10)}

// after
cfg := Config{Count: new(10)}

Go 1.26+ — errors.AsType

// before
var pathErr *os.PathError
if errors.As(err, &pathErr) {
    log.Println(pathErr.Path)
}
// after
if pathErr, ok := errors.AsType[*os.PathError](err); ok {
    log.Println(pathErr.Path)
}

Go 1.27+ — Embedded field literals (embedlit)

type Base struct {
    ID   int
    Name string
}
type User struct {
    Base
    Age int
}

// before
u := User{
    Base: Base{ID: 1, Name: "alice"},
    Age:  30,
}
// after
u := User{
    ID:   1,
    Name: "alice",
    Age:  30,
}

The embedlit modernizer (gopls v0.22.0, EmbedLitAnalyzer) strips redundant embedded-struct field-type specifiers from composite literals. Go 1.27 lets you initialize promoted fields directly without the nested literal. Only apply when the promoted field names don't collide with the outer struct's own fields.

Operation Phases

Phase 1: Detect

Read go.mod to extract the Go version (go 1.xx line). If no go.mod is found, default to go 1.21.

Phase 2: Gather files

Find all .go files in the target scope (project root, or user-specified file/directory). Exclude vendor/, .git/, and testdata/ directories.

Phase 3: Apply transformations

For each .go file, apply all transformations for versions ≤ the detected Go version. Process files sequentially. For each file:

1. Read the file content.
2. Identify applicable transformations by scanning for the "Before" patterns.
3. Apply each transformation using the Edit tool.
4. Run goimports -w (or gofmt -w) on the file after all edits.

Never apply a transformation that requires a version higher than the project's Go version.

Phase 4: Report

Print a summary table showing:

• File: path relative to project root
• Transformations applied: list of transformation names per file
• Total files modified and total transformations applied
• Skipped transformations (available but not applicable due to version constraints) and their required Go version

Example Summary Output

## Modernization Summary

| File | Transformations |
|---|---|
| main.go | any, strings.Cut, min/max (2 occurrences) |
| pkg/handler.go | range over int (3), slices.Contains, t.Context() |
| pkg/util.go | new(expr) (1), errors.AsType → errors.Is |

**3 files modified, 10 transformations applied**

Skipped (requires higher Go version):
- new(expr): requires go 1.26 (project is go 1.24)
- WaitGroup.Go: requires go 1.25 (project is go 1.24)

Safety Rules

• Never apply transformations that change semantics in edge cases without the user's awareness.
• Do not apply omitzero blindly—it changes JSON serialization behavior; flag it as a suggestion instead.
• Treat semantic migrations as suggestions, not auto-applies: math/rand/v2 (changes the random stream), iter.Seq iterators (reshapes the API), os.Root (behavior/error-path change). Point them out and let the user opt in.
• When replacing a bufio.Scanner loop with strings.Lines/bytes.Lines, remember the yielded line keeps its trailing \n; add a TrimSuffix if the old code relied on Scanner.Text() semantics.
• Do not apply strings.SplitSeq or bytes.SplitSeq when the loop body references the index or the full slice elsewhere.
• Do not apply strings.Builder if the concatenation happens outside a loop (single += is fine).
• When a transformation requires a new import, ensure the import is added to the file.
• After all edits, run goimports -w on each modified file to clean up imports.
• If goimports is not available, fall back to gofmt -w.
• If the project has no go.mod, ask the user for the target Go version before proceeding.

Automated tooling (Go 1.27+)

Many of these transformations are now shipped as official modernizers in gopls/go fix. On Go 1.27+ the whole set can be applied across a module with:

go fix ./...

gopls v0.22.0 added four notable passes covered above:

Other modernizers in the same suite that this catalog covers: minmax, efaceany, fmtappendf, stringscut, stringsseq, sortslice/slicescontains, mapsloop, stditerators, forvar, rangeint, testingcontext, bloop, waitgroup, newexpr, errorsastype, appendclipped (→ slices.Concat), and plusbuild (→ drop obsolete // +build).

To disable an over-eager pass (e.g. slicesbackward rewriting loops that mutate the slice), scope the run with -fixes or exclude that analyzer in your editor's gopls settings.