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docs(.claude/docs): add modern Go reference for AI agents (#22558)
Add a Go 1.18-1.26 reference document (`.claude/docs/GO.md`) to guide AI agents toward modern Go idioms.
This commit is contained in:
Mathias Fredriksson
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# Modern Go (1.18–1.26)
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Reference for writing idiomatic Go. Covers what changed, what it
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replaced, and what to reach for. Respect the project's `go.mod` `go`
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line: don't emit features from a version newer than what the module
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declares. Check `go.mod` before writing code.
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## How modern Go thinks differently
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**Generics** (1.18): Design reusable code with type parameters instead
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of `interface{}` casts, code generation, or the `sort.Interface`
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pattern. Use `any` for unconstrained types, `comparable` for map keys
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and equality, `cmp.Ordered` for sortable types. Type inference usually
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makes explicit type arguments unnecessary (improved in 1.21).
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**Per-iteration loop variables** (1.22): Each loop iteration gets its
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own variable copy. Closures inside loops capture the correct value. The
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`v := v` shadow trick is dead. Remove it when you see it.
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**Iterators** (1.23): `iter.Seq[V]` and `iter.Seq2[K,V]` are the
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standard iterator types. Containers expose `.All()` methods returning
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these. Combined with `slices.Collect`, `slices.Sorted`, `maps.Keys`,
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etc., they replace ad-hoc "loop and append" code with composable,
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lazy pipelines. When a sequence is consumed only once, prefer an
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iterator over materializing a slice.
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**Error trees** (1.20–1.26): Errors compose as trees, not chains.
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`errors.Join` aggregates multiple errors. `fmt.Errorf` accepts multiple
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`%w` verbs. `errors.Is`/`As` traverse the full tree. Custom error
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types that wrap multiple causes must implement `Unwrap() []error` (the
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slice form), not `Unwrap() error`, or tree traversal won't find the
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children. `errors.AsType[T]` (1.26) is the type-safe way to match
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error types. Propagate cancellation reasons with
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`context.WithCancelCause`.
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**Structured logging** (1.21): `log/slog` is the standard structured
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logger. This project uses `cdr.dev/slog/v3` instead, which has a
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different API. Do not use `log/slog` directly.
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## Replace these patterns
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The left column reflects common patterns from pre-1.22 Go. Write the
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right column instead. The "Since" column tells you the minimum `go`
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directive version required in `go.mod`.
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| Old pattern | Modern replacement | Since |
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|---|---|---|
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| `interface{}` | `any` | 1.18 |
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| `v := v` inside loops | remove it | 1.22 |
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| `for i := 0; i < n; i++` | `for i := range n` | 1.22 |
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| `for i := 0; i < b.N; i++` (benchmarks) | `for b.Loop()` (correct timing, future-proof) | 1.24 |
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| `sort.Slice(s, func(i,j int) bool{…})` | `slices.SortFunc(s, cmpFn)` | 1.21 |
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| `wg.Add(1); go func(){ defer wg.Done(); … }()` | `wg.Go(func(){…})` | 1.25 |
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| `func ptr[T any](v T) *T { return &v }` | `new(expr)` e.g. `new(time.Now())` | 1.26 |
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| `var target *E; errors.As(err, &target)` | `t, ok := errors.AsType[*E](err)` | 1.26 |
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| Custom multi-error type | `errors.Join(err1, err2, …)` | 1.20 |
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| Single `%w` for multiple causes | `fmt.Errorf("…: %w, %w", e1, e2)` | 1.20 |
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| `rand.Seed(time.Now().UnixNano())` | delete it (auto-seeded); prefer `math/rand/v2` | 1.20/1.22 |
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| `sync.Once` + captured variable | `sync.OnceValue(func() T {…})` / `OnceValues` | 1.21 |
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| Custom `min`/`max` helpers | `min(a, b)` / `max(a, b)` builtins (any ordered type) | 1.21 |
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| `for k := range m { delete(m, k) }` | `clear(m)` (also zeroes slices) | 1.21 |
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| Index+slice or `SplitN(s, sep, 2)` | `strings.Cut(s, sep)` / `bytes.Cut` | 1.18 |
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| `TrimPrefix` + check if anything was trimmed | `strings.CutPrefix` / `CutSuffix` (returns ok bool) | 1.20 |
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| `strings.Split` + loop when no slice is needed | `strings.SplitSeq` / `Lines` / `FieldsSeq` (iterator, no alloc) | 1.24 |
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| `"2006-01-02"` / `"2006-01-02 15:04:05"` / `"15:04:05"` | `time.DateOnly` / `time.DateTime` / `time.TimeOnly` | 1.20 |
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| Manual `Before`/`After`/`Equal` chains for comparison | `time.Time.Compare` (returns -1/0/+1; works with `slices.SortFunc`) | 1.20 |
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| Loop collecting map keys into slice | `slices.Sorted(maps.Keys(m))` | 1.23 |
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| `fmt.Sprintf` + append to `[]byte` | `fmt.Appendf(buf, …)` (also `Append`, `Appendln`) | 1.18 |
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| `reflect.TypeOf((*T)(nil)).Elem()` | `reflect.TypeFor[T]()` | 1.22 |
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| `*(*[4]byte)(slice)` unsafe cast | `[4]byte(slice)` direct conversion | 1.20 |
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| `atomic.LoadInt64` / `StoreInt64` | `atomic.Int64` (also `Bool`, `Uint64`, `Pointer[T]`) | 1.19 |
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| `crypto/rand.Read(buf)` + hex/base64 encode | `crypto/rand.Text()` (one call) | 1.24 |
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| Checking `crypto/rand.Read` error | don't: return is always nil | 1.24 |
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| `time.Sleep` in tests | `testing/synctest` (deterministic fake clock) | 1.24/1.25 |
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| `json:",omitempty"` on zero-value structs like `time.Time{}` | `json:",omitzero"` (uses `IsZero()` method) | 1.24 |
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| `strings.Title` | `golang.org/x/text/cases` | 1.18 |
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| `net.IP` in new code | `net/netip.Addr` (immutable, comparable, lighter) | 1.18 |
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| `tools.go` with blank imports | `tool` directive in `go.mod` | 1.24 |
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| `runtime.SetFinalizer` | `runtime.AddCleanup` (multiple per object, no pointer cycles) | 1.24 |
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| `httputil.ReverseProxy.Director` | `.Rewrite` hook + `ProxyRequest` (Director deprecated in 1.26) | 1.20 |
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| `sql.NullString`, `sql.NullInt64`, etc. | `sql.Null[T]` | 1.22 |
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| Manual `ctx, cancel := context.WithCancel(…)` + `t.Cleanup(cancel)` | `t.Context()` (auto-canceled when test ends) | 1.24 |
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| `if d < 0 { d = -d }` on durations | `d.Abs()` (handles `math.MinInt64`) | 1.19 |
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| Implement only `TextMarshaler` | also implement `TextAppender` for alloc-free marshaling | 1.24 |
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| Custom `Unwrap() error` on multi-cause errors | `Unwrap() []error` (slice form; required for tree traversal) | 1.20 |
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## New capabilities
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These enable things that weren't practical before. Reach for them in the
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described situations.
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| What | Since | When to use it |
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|---|---|---|
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| `cmp.Or(a, b, c)` | 1.22 | Defaults/fallback chains: returns first non-zero value. Replaces verbose `if a != "" { return a }` cascades. |
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| `context.WithoutCancel(ctx)` | 1.21 | Background work that must outlive the request (e.g. async cleanup after HTTP response). Derived context keeps parent's values but ignores cancellation. |
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| `context.AfterFunc(ctx, fn)` | 1.21 | Register cleanup that fires on context cancellation without spawning a goroutine that blocks on `<-ctx.Done()`. |
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| `context.WithCancelCause` / `Cause` | 1.20 | When callers need to know WHY a context was canceled, not just that it was. Retrieve cause with `context.Cause(ctx)`. |
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| `context.WithDeadlineCause` / `WithTimeoutCause` | 1.21 | Attach a domain-specific error to deadline/timeout expiry (e.g. distinguish "DB query timed out" from "HTTP request timed out"). |
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| `errors.ErrUnsupported` | 1.21 | Standard sentinel for "not supported." Use instead of per-package custom sentinels. Check with `errors.Is`. |
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| `http.ResponseController` | 1.20 | Per-request flush, hijack, and deadline control without type-asserting `ResponseWriter` to `http.Flusher` or `http.Hijacker`. |
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| Enhanced `ServeMux` routing | 1.22 | `"GET /items/{id}"` patterns in `http.ServeMux`. Access with `r.PathValue("id")`. Wildcards: `{name}`, catch-all: `{path...}`, exact: `{$}`. Eliminates many third-party router dependencies. |
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| `os.Root` / `OpenRoot` | 1.24 | Confined directory access that prevents symlink escape. 1.25 adds `MkdirAll`, `ReadFile`, `WriteFile` for real use. |
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| `os.CopyFS` | 1.23 | Copy an entire `fs.FS` to local filesystem in one call. |
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| `os/signal.NotifyContext` with cause | 1.26 | Cancellation cause identifies which signal (SIGTERM vs SIGINT) triggered shutdown. |
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| `io/fs.SkipAll` / `filepath.SkipAll` | 1.20 | Return from `WalkDir` callback to stop walking entirely. Cleaner than a sentinel error. |
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| `GOMEMLIMIT` env / `debug.SetMemoryLimit` | 1.19 | Soft memory limit for GC. Use alongside or instead of `GOGC` in memory-constrained containers. |
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| `net/url.JoinPath` | 1.19 | Join URL path segments correctly. Replaces error-prone string concatenation. |
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| `go test -skip` | 1.20 | Skip tests matching a pattern. Useful when running a subset of a large test suite. |
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## Key packages
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### `slices` (1.21, iterators added 1.23)
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Replaces `sort.Slice`, manual search loops, and manual contains checks.
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Search: `Contains`, `ContainsFunc`, `Index`, `IndexFunc`,
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`BinarySearch`, `BinarySearchFunc`.
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Sort: `Sort`, `SortFunc`, `SortStableFunc`, `IsSorted`, `IsSortedFunc`,
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`Min`, `MinFunc`, `Max`, `MaxFunc`.
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Transform: `Clone`, `Compact`, `CompactFunc`, `Grow`, `Clip`,
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`Concat` (1.22), `Repeat` (1.23), `Reverse`, `Insert`, `Delete`,
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`Replace`.
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Compare: `Equal`, `EqualFunc`, `Compare`.
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Iterators (1.23): `All`, `Values`, `Backward`, `Collect`, `AppendSeq`,
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`Sorted`, `SortedFunc`, `SortedStableFunc`, `Chunk`.
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### `maps` (1.21, iterators added 1.23)
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Core: `Clone`, `Copy`, `Equal`, `EqualFunc`, `DeleteFunc`.
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Iterators (1.23): `All`, `Keys`, `Values`, `Insert`, `Collect`.
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### `cmp` (1.21, `Or` added 1.22)
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`Ordered` constraint for any ordered type. `Compare(a, b)` returns
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-1/0/+1. `Less(a, b)` returns bool. `Or(vals...)` returns first
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non-zero value.
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### `iter` (1.23)
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`Seq[V]` is `func(yield func(V) bool)`. `Seq2[K,V]` is
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`func(yield func(K, V) bool)`. Return these from your container's
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`.All()` methods. Consume with `for v := range seq` or pass to
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`slices.Collect`, `slices.Sorted`, `maps.Collect`, etc.
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### `math/rand/v2` (1.22)
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Replaces `math/rand`. `IntN` not `Intn`. Generic `N[T]()` for any
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integer type. Default source is `ChaCha8` (crypto-quality). No global
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`Seed`. Use `rand.New(source)` for reproducible sequences.
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### `log/slog` (1.21)
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`slog.Info`, `slog.Warn`, `slog.Error`, `slog.Debug` with key-value
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pairs. `slog.With(attrs...)` for logger with preset fields.
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`slog.GroupAttrs` (1.25) for clean group creation. Implement
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`slog.Handler` for custom backends.
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**Note:** This project uses `cdr.dev/slog/v3`, not `log/slog`. The
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API is different. Read existing code for usage patterns.
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## Pitfalls
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Things that are easy to get wrong, even when you know the modern API
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exists. Check your output against these.
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**Version misuse.** The replacement table has a "Since" column. If the
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project's `go.mod` says `go 1.22`, you cannot use `wg.Go` (1.25),
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`errors.AsType` (1.26), `new(expr)` (1.26), `b.Loop()` (1.24), or
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`testing/synctest` (1.24). Fall back to the older pattern. Always
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check before reaching for a replacement.
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**`slices.Sort` vs `slices.SortFunc`.** `slices.Sort` requires
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`cmp.Ordered` types (int, string, float64, etc.). For structs, custom
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types, or multi-field sorting, use `slices.SortFunc` with a comparator
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function. Using `slices.Sort` on a non-ordered type is a compile error.
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**`for range n` still binds the index.** `for range n` discards the
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index. If you need it, write `for i := range n`. Writing
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`for range n` and then trying to use `i` inside the loop is a compile
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error.
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**Don't hand-roll iterators when the stdlib returns one.** Functions
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like `maps.Keys`, `slices.Values`, `strings.SplitSeq`, and
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`strings.Lines` already return `iter.Seq` or `iter.Seq2`. Don't
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reimplement them. Compose with `slices.Collect`, `slices.Sorted`, etc.
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**Don't mix `math/rand` and `math/rand/v2`.** They have different
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function names (`Intn` vs `IntN`) and different default sources. Pick
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one per package. Prefer v2 for new code. The v1 global source is
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auto-seeded since 1.20, so delete `rand.Seed` calls either way.
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**Iterator protocol.** When implementing `iter.Seq`, you must respect
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the `yield` return value. If `yield` returns `false`, stop iteration
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immediately and return. Ignoring it violates the contract and causes
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panics when consumers break out of `for range` loops early.
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**`errors.Join` with nil.** `errors.Join` skips nil arguments. This is
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intentional and useful for aggregating optional errors, but don't
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assume the result is always non-nil. `errors.Join(nil, nil)` returns
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nil.
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**`cmp.Or` evaluates all arguments.** Unlike a chain of `if`
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statements, `cmp.Or(a(), b(), c())` calls all three functions. If any
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have side effects or are expensive, use `if`/`else` instead.
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**Timer channel semantics changed in 1.23.** Code that checks
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`len(timer.C)` to see if a value is pending no longer works (channel
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capacity is 0). Use a non-blocking `select` receive instead:
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`select { case <-timer.C: default: }`.
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**`context.WithoutCancel` still propagates values.** The derived
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context inherits all values from the parent. If any middleware stores
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request-scoped state (deadlines, trace IDs) via `context.WithValue`,
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the background work sees it. This is usually desired but can be
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surprising if the values hold references that should not outlive the
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request.
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## Behavioral changes that affect code
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- **Timers** (1.23): unstopped `Timer`/`Ticker` are GC'd immediately.
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Channels are unbuffered: no stale values after `Reset`/`Stop`. You no
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longer need `defer t.Stop()` to prevent leaks.
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- **Error tree traversal** (1.20): `errors.Is`/`As` follow
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`Unwrap() []error`, not just `Unwrap() error`. Multi-error types must
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expose the slice form for child errors to be found.
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- **`math/rand` auto-seeded** (1.20): the global RNG is auto-seeded.
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`rand.Seed` is a no-op in 1.24+. Don't call it.
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- **GODEBUG compat** (1.21): behavioral changes are gated by `go.mod`'s
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`go` line. Upgrading the version opts into new defaults.
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- **Build tags** (1.18): `//go:build` is the only syntax. `// +build`
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is gone.
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- **Tool install** (1.18): `go get` no longer builds. Use
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`go install pkg@version`.
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- **Doc comments** (1.19): support `[links]`, lists, and headings.
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- **`go test -skip`** (1.20): skip tests by name pattern from the
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command line.
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- **`go fix ./...` modernizers** (1.26): auto-rewrites code to use
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newer idioms. Run after Go version upgrades.
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## Transparent improvements (no code changes)
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Swiss Tables maps, Green Tea GC, PGO, faster `io.ReadAll`,
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stack-allocated slices, reduced cgo overhead, container-aware
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GOMAXPROCS. Free on upgrade.
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