reflect: add iterative related methods

Fixes #66056

Change-Id: I1e24636e43e68cd57576c39b014e0826fb6c322c
GitHub-Last-Rev: 319ad8e
GitHub-Pull-Request: #66824
Reviewed-on: https://go-review.googlesource.com/c/go/+/578815
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Reviewed-by: Ian Lance Taylor <iant@google.com>
Auto-Submit: Ian Lance Taylor <iant@google.com>
Reviewed-by: Cherry Mui <cherryyz@google.com>

api/next/66056.txt

@@ -0,0 +1,4 @@
+pkg reflect, method (Value) Seq() iter.Seq[Value] #66056
+pkg reflect, method (Value) Seq2() iter.Seq2[Value, Value] #66056
+pkg reflect, type Type interface, CanSeq() bool #66056
+pkg reflect, type Type interface, CanSeq2() bool #66056

doc/next/6-stdlib/99-minor/reflect/66056.md

@@ -0,0 +1,4 @@
+The new methods [Value.Seq] and [Value.Seq2] return sequences that iterate over the value
+as though it were used in a for/range loop.
+The new methods [Type.CanSeq] and [Type.CanSeq2] report whether calling
+[Value.Seq] and [Value.Seq2], respectively, will succeed without panicking.

src/reflect/iter.go

@@ -0,0 +1,140 @@
+// Copyright 2024 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+package reflect
+
+import "iter"
+
+// Seq returns an iter.Seq[reflect.Value] that loops over the elements of v.
+// If v's kind is Func, it must be a function that has no results and
+// that takes a single argument of type func(T) bool for some type T.
+// If v's kind is Pointer, the pointer element type must have kind Array.
+// Otherwise v's kind must be Int, Int8, Int16, Int32, Int64, Uint, Uint8, Uint16, Uint32, Uint64, Uintptr,
+// Array, Chan, Map, Slice, or String.
+func (v Value) Seq() iter.Seq[Value] {
+	if canRangeFunc(v.typ()) {
+		return func(yield func(Value) bool) {
+			rf := MakeFunc(v.Type().In(0), func(in []Value) []Value {
+				return []Value{ValueOf(yield(in[0]))}
+			})
+			v.Call([]Value{rf})
+		}
+	}
+	switch v.Kind() {
+	case Int, Int8, Int16, Int32, Int64:
+		return func(yield func(Value) bool) {
+			for i := range v.Int() {
+				if !yield(ValueOf(i)) {
+					return
+				}
+			}
+		}
+	case Uint, Uint8, Uint16, Uint32, Uint64, Uintptr:
+		return func(yield func(Value) bool) {
+			for i := range v.Uint() {
+				if !yield(ValueOf(i)) {
+					return
+				}
+			}
+		}
+	case Pointer:
+		if v.Elem().kind() != Array {
+			break
+		}
+		return func(yield func(Value) bool) {
+			v = v.Elem()
+			for i := range v.Len() {
+				if !yield(ValueOf(i)) {
+					return
+				}
+			}
+		}
+	case Array, Slice:
+		return func(yield func(Value) bool) {
+			for i := range v.Len() {
+				if !yield(ValueOf(i)) {
+					return
+				}
+			}
+		}
+	case String:
+		return func(yield func(Value) bool) {
+			for i := range v.String() {
+				if !yield(ValueOf(i)) {
+					return
+				}
+			}
+		}
+	case Map:
+		return func(yield func(Value) bool) {
+			i := v.MapRange()
+			for i.Next() {
+				if !yield(i.Key()) {
+					return
+				}
+			}
+		}
+	case Chan:
+		return func(yield func(Value) bool) {
+			for value, ok := v.Recv(); ok; value, ok = v.Recv() {
+				if !yield(value) {
+					return
+				}
+			}
+		}
+	}
+	panic("reflect: " + v.Type().String() + " cannot produce iter.Seq[Value]")
+}
+
+// Seq2 is like Seq but for two values.
+func (v Value) Seq2() iter.Seq2[Value, Value] {
+	if canRangeFunc2(v.typ()) {
+		return func(yield func(Value, Value) bool) {
+			rf := MakeFunc(v.Type().In(0), func(in []Value) []Value {
+				return []Value{ValueOf(yield(in[0], in[1]))}
+			})
+			v.Call([]Value{rf})
+		}
+	}
+	switch v.Kind() {
+	case Pointer:
+		if v.Elem().kind() != Array {
+			break
+		}
+		return func(yield func(Value, Value) bool) {
+			v = v.Elem()
+			for i := range v.Len() {
+				if !yield(ValueOf(i), v.Index(i)) {
+					return
+				}
+			}
+		}
+	case Array, Slice:
+		return func(yield func(Value, Value) bool) {
+			for i := range v.Len() {
+				if !yield(ValueOf(i), v.Index(i)) {
+					return
+				}
+			}
+		}
+	case String:
+		return func(yield func(Value, Value) bool) {
+			for i, v := range v.String() {
+				if !yield(ValueOf(i), ValueOf(v)) {
+					return
+				}
+			}
+		}
+	case Map:
+		return func(yield func(Value, Value) bool) {
+			i := v.MapRange()
+			for i.Next() {
+				if !yield(i.Key(), i.Value()) {
+					return
+				}
+			}
+		}
+	}
+	panic("reflect: " + v.Type().String() + " cannot produce iter.Seq2[Value, Value]")
+}