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	After
time.Now().Sub(start)	time.Since(start)

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

Go 1.8+ — time.Until

Before	After
deadline.Sub(time.Now())	time.Until(deadline)

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

Go 1.10+ — strings.Builder (loop concatenation)

Before	After
s += item in a loop	var b strings.Builder; b.WriteString(item)

// 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	After
err == io.EOF	errors.Is(err, io.EOF)

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

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

Before	After
// +build linux + //go:build linux (both present)	keep only //go:build linux

// 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	After
unsafe.Pointer(uintptr(ptr) + uintptr(n))	unsafe.Add(ptr, n)
(*[n]T)(unsafe.Pointer(p))[:] slice construction	unsafe.Slice(p, n)

// 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	After
interface{}	any

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

Go 1.18+ — strings.Cut

Before	After
i := strings.Index(s, sep); ... s[:i], s[i+len(sep):]	key, val, found := strings.Cut(s, sep)

// 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	After
i := bytes.Index(b, sep); ... b[:i], b[i+len(sep):]	before, after, found := bytes.Cut(b, sep)

// 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	After
buf = append(buf, fmt.Sprintf(...)...)	buf = fmt.Appendf(buf, ...)

// 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	After
atomic.StoreInt32(&v, 1) / atomic.LoadInt32(&v)	var v atomic.Int32; v.Store(1); v.Load()
atomic.AddInt64(&v, 1)	var v atomic.Int64; v.Add(1)
atomic.Value + type assertion	atomic.Pointer[T]

// 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	After
string([]byte(s))	strings.Clone(s)

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

Go 1.20+ — bytes.Clone

Before	After
make([]byte, len(src)); copy(dst, src)	bytes.Clone(src)

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

Go 1.20+ — strings.CutPrefix / strings.CutSuffix

Before	After
if strings.HasPrefix(s, p) { s = s[len(p):] }	if rest, ok := strings.CutPrefix(s, p); ok { s = rest }
if strings.HasSuffix(s, sf) { s = s[:len(s)-len(sf)] }	if rest, ok := strings.CutSuffix(s, sf); ok { s = rest }

// 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	After
fmt.Errorf("...: %w: %w", err1, err2)	errors.Join(err1, err2)

// 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	After
ctx, cancel := context.WithCancel(parent) + bare cancel()	ctx, cancel := context.WithCancelCause(parent) + cancel(err)

// 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	After
if a < b { v = a } else { v = b }	v = min(a, b)
if a > b { v = a } else { v = b }	v = max(a, b)
if x < lo { x = lo }; if x > hi { x = hi }	x = min(max(x, lo), hi)

// 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	After
for k := range m { delete(m, k) }	clear(m)
for i := range s { s[i] = zero }	clear(s)

// 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
Manual loop to find element	slices.Contains(items, target)
Loop returning index or -1	slices.Index(items, target)
sort.Slice(items, func(i,j int) bool { return items[i] < items[j] })	slices.SortFunc(items, cmp.Compare)
Max/min finding loop	slices.Max(items) / slices.Min(items)
Reverse swap loop	slices.Reverse(s)
Remove consecutive duplicates loop	slices.Compact(s)
s[:len(s):len(s)]	slices.Clip(s)
make([]T, len(src)); copy(dst, src)	slices.Clone(src)

// 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	After
append(s[:i], s[i+1:]...)	slices.Delete(s, i, i+1)
append(s[:i:i], append([]T{x}, s[i:]...)...)	slices.Insert(s, i, x)

// 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	After
reflect.DeepEqual(a, b) for comparable slices	slices.Equal(a, b)
reflect.DeepEqual(m1, m2) for comparable maps	maps.Equal(m1, m2)

// 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
Manual loop to copy a map	maps.Clone(m)
for k, v := range src { dst[k] = v }	maps.Copy(dst, src)
Loop + conditional delete	maps.DeleteFunc(m, predicate)

// 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	After
append(append([]T(nil), s1...), s2...)	slices.Concat(s1, s2)
append(slices.Clip(s1), s2...) for a fresh result	slices.Concat(s1, s2)

// 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	After
var once sync.Once; once.Do(func() { ... })	f := sync.OnceFunc(func() { ... }); f()
sync.Once + stored result variable	sync.OnceValue(func() T { return val })

// 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	After
go func() { <-ctx.Done(); cleanup() }()	stop := context.AfterFunc(ctx, cleanup)

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

Go 1.21+ — context.WithTimeoutCause / WithDeadlineCause

Before	After
context.WithTimeout(parent, d)	context.WithTimeoutCause(parent, d, err)

// 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	After
for i := 0; i < n; i++ { ... }	for i := range n { ... }
for i := 0; i < n; i++ { ... } (i unused)	for range n { ... }

// 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	After
for _, x := range items { x := x; ... }	for _, x := range items { ... }

// 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	After
Chain of if v == "" { v = fallback }	v := cmp.Or(val, fallback1, fallback2, ...)

// 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	After
reflect.TypeOf((*T)(nil)).Elem()	reflect.TypeFor[T]()

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

Go 1.22+ — Enhanced http.ServeMux

Before	After
mux.HandleFunc("/api/", h) + manual path parsing	mux.HandleFunc("GET /api/{id}", h) + r.PathValue("id")

// 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	After
rand.Intn(n)	rand.IntN(n)
rand.Int63n(n) / rand.Int31n(n)	rand.Int64N(n) / rand.Int32N(n)
rand.Seed(...) + global funcs	drop Seed; use auto-seeded top-level funcs or rand.N[T]

// 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	After
Custom func Walk(yield func(T) bool) callback traversal	func Walk() iter.Seq[T] returning an iterator

// 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	After
var keys []K; for k := range m { keys = append(keys, k) }	slices.Collect(maps.Keys(m))
var vals []V; for _, v := range m { vals = append(vals, v) }	slices.Collect(maps.Values(m))

// 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	After
for _, part := range strings.Split(s, sep)	for part := range strings.SplitSeq(s, sep)
for _, field := range strings.Fields(s)	for field := range strings.FieldsSeq(s)

// 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	After
for _, part := range bytes.Split(b, sep)	for part := range bytes.SplitSeq(b, sep)

// 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	After
for i := len(s) - 1; i >= 0; i-- { use(s[i]) }	for _, v := range slices.Backward(s) { use(v) }
reverse loop needing the index too	for i, v := range slices.Backward(s) { ... }

// 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	After
ctx, cancel := context.WithCancel(context.Background()); defer cancel()	ctx := t.Context()

// 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	After
bufio.Scanner line loop over a string/buffer	for line := range strings.Lines(s)

// 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	After
manual filepath.Clean + prefix check to block traversal	root, _ := os.OpenRoot(dir); root.Open(name)

// 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	After
json:"field,omitempty" (for time.Time, time.Duration, structs, slices, maps)	json:"field,omitzero"

// 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	After
for i := 0; i < b.N; i++ { ... }	for b.Loop() { ... }

// 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	After
wg.Add(1); go func() { defer wg.Done(); fn() }()	wg.Go(fn)

// 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	After
v := val; &v	new(val)
Helper func ptr[T any](v T) *T { return &v }	new(val) directly

// 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	After
var t *T; errors.As(err, &t)	t, ok := errors.AsType[*T](err)

// 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)

Before	After
T{U: U{x: 1}} (redundant embedded-type specifier)	T{x: 1}