---
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

!`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- Never use outdated patterns when a modern alternative is available

---

## 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`

```go
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

```go
// 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

```go
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:
```go
func TestFoo(t *testing.T) {
    ctx, cancel := context.WithCancel(context.Background())
    defer cancel()
    result := doSomething(ctx)
}
```
After:
```go
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:
```go
type Config struct {
    Timeout time.Duration `json:"timeout,omitempty"` // doesn't work for Duration!
}
```
After:
```go
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:
```go
func BenchmarkFoo(b *testing.B) {
    for i := 0; i < b.N; i++ {
        doWork()
    }
}
```
After:
```go
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:
```go
for _, part := range strings.Split(s, ",") {
    process(part)
}
```
After:
```go
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:
```go
var wg sync.WaitGroup
for _, item := range items {
    wg.Add(1)
    go func() {
        defer wg.Done()
        process(item)
    }()
}
wg.Wait()
```
After:
```go
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:
```go
timeout := 30
debug := true
cfg := Config{
    Timeout: &timeout,
    Debug:   &debug,
}
```
After:
```go
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:
```go
var pathErr *os.PathError
if errors.As(err, &pathErr) {
    handle(pathErr)
}
```
After:
```go
if pathErr, ok := errors.AsType[*os.PathError](err); ok {
    handle(pathErr)
}
```