diff --git a/cue/types.go b/cue/types.go
index e2e426b62..c9cd38725 100644
--- a/cue/types.go
+++ b/cue/types.go
@@ -39,6 +39,7 @@ import (
 	"cuelang.org/go/internal/core/runtime"
 	"cuelang.org/go/internal/core/subsume"
 	internaljson "cuelang.org/go/internal/encoding/json"
+	"cuelang.org/go/internal/iterutil"
 	"cuelang.org/go/internal/types"
 )
 
@@ -1125,7 +1126,7 @@ func (v Value) Len() Value {
 		case *adt.Vertex:
 			if x.IsList() {
 				n := &adt.Num{K: adt.IntKind}
-				n.X.SetInt64(int64(len(x.Elems())))
+				n.X.SetInt64(int64(iterutil.Count(x.Elems())))
 				if x.IsClosedList() {
 					return remakeFinal(v, n)
 				}
@@ -1177,7 +1178,7 @@ func (v Value) List() (Iterator, error) {
 // mustList is like [Value.List], but reusing ctx and leaving it to the caller
 // to apply defaults and check the kind.
 func (v Value) mustList(ctx *adt.OpContext) Iterator {
-	return Iterator{idx: v.idx, ctx: ctx, val: v, arcs: v.v.Elems()}
+	return Iterator{idx: v.idx, ctx: ctx, val: v, arcs: slices.Collect(v.v.Elems())}
 }
 
 // Null reports an error if v is not null.
diff --git a/internal/core/adt/composite.go b/internal/core/adt/composite.go
index 6f5977cbd..d7fa791eb 100644
--- a/internal/core/adt/composite.go
+++ b/internal/core/adt/composite.go
@@ -16,11 +16,13 @@ package adt
 
 import (
 	"fmt"
+	"iter"
 	"slices"
 
 	"cuelang.org/go/cue/ast"
 	"cuelang.org/go/cue/errors"
 	"cuelang.org/go/cue/token"
+	"cuelang.org/go/internal/iterutil"
 )
 
 // TODO: unanswered questions about structural cycles:
@@ -1162,7 +1164,7 @@ func (v *Vertex) Accept(ctx *OpContext, f Feature) bool {
 		switch v.BaseValue.(type) {
 		case *ListMarker:
 			// TODO(perf): use precomputed length.
-			if f.Index() < len(v.Elems()) {
+			if f.Index() < iterutil.Count(v.Elems()) {
 				return true
 			}
 			return !v.IsClosedList()
@@ -1262,15 +1264,17 @@ func (v *Vertex) LookupRaw(f Feature) *Vertex {
 }
 
 // Elems returns the regular elements of a list.
-func (v *Vertex) Elems() []*Vertex {
-	// TODO: add bookkeeping for where list arcs start and end.
-	a := make([]*Vertex, 0, len(v.Arcs))
-	for _, x := range v.Arcs {
-		if x.Label.IsInt() {
-			a = append(a, x)
+func (v *Vertex) Elems() iter.Seq[*Vertex] {
+	return func(yield func(*Vertex) bool) {
+		// TODO: add bookkeeping for where list arcs start and end.
+		for _, x := range v.Arcs {
+			if x.Label.IsInt() {
+				if !yield(x) {
+					break
+				}
+			}
 		}
 	}
-	return a
 }
 
 func (v *Vertex) Init(c *OpContext) {
diff --git a/internal/core/adt/context.go b/internal/core/adt/context.go
index 76a9f36eb..dc5fb08c5 100644
--- a/internal/core/adt/context.go
+++ b/internal/core/adt/context.go
@@ -16,6 +16,7 @@ package adt
 
 import (
 	"fmt"
+	"iter"
 	"reflect"
 	"regexp"
 	"sync/atomic"
@@ -1042,14 +1043,14 @@ func (c *OpContext) node(orig Node, x Expr, scalar bool, state Flags) *Vertex {
 }
 
 // Elems returns the evaluated elements of a list.
-func (c *OpContext) Elems(v Value) []*Vertex {
+func (c *OpContext) Elems(v Value) iter.Seq[*Vertex] {
 	list := c.list(v)
 	list.Finalize(c)
 	return list.Elems()
 }
 
 // RawElems returns the elements of the list without evaluating them.
-func (c *OpContext) RawElems(v Value) []*Vertex {
+func (c *OpContext) RawElems(v Value) iter.Seq[*Vertex] {
 	list := c.list(v)
 	return list.Elems()
 }
diff --git a/internal/core/adt/errors.go b/internal/core/adt/errors.go
index 94aa7274a..3c9034607 100644
--- a/internal/core/adt/errors.go
+++ b/internal/core/adt/errors.go
@@ -36,6 +36,7 @@ import (
 	"cuelang.org/go/cue/errors"
 	cueformat "cuelang.org/go/cue/format"
 	"cuelang.org/go/cue/token"
+	"cuelang.org/go/internal/iterutil"
 )
 
 // ErrorCode indicates the type of error. The type of error may influence
@@ -287,7 +288,7 @@ func (v *Vertex) reportFieldError(c *OpContext, pos token.Pos, f Feature, intMsg
 
 	var err errors.Error
 	if f.IsInt() {
-		err = c.NewPosf(pos, intMsg, f.Index(), len(v.Elems()))
+		err = c.NewPosf(pos, intMsg, f.Index(), iterutil.Count(v.Elems()))
 	} else {
 		err = c.NewPosf(pos, stringMsg, label)
 	}
diff --git a/internal/core/adt/tasks.go b/internal/core/adt/tasks.go
index d4ac100eb..399b15d8e 100644
--- a/internal/core/adt/tasks.go
+++ b/internal/core/adt/tasks.go
@@ -16,6 +16,7 @@ package adt
 
 import (
 	"fmt"
+	"slices"
 
 	"cuelang.org/go/cue/ast"
 	"cuelang.org/go/cue/token"
@@ -330,7 +331,7 @@ func processListVertex(c *OpContext, t *task, mode runMode) {
 
 	l := t.x.(*Vertex)
 
-	elems := l.Elems()
+	elems := slices.Collect(l.Elems())
 	isClosed := l.IsClosedList()
 
 	// TODO: Share with code above.
diff --git a/internal/core/compile/builtin.go b/internal/core/compile/builtin.go
index 81b36df3a..e098242b3 100644
--- a/internal/core/compile/builtin.go
+++ b/internal/core/compile/builtin.go
@@ -19,6 +19,7 @@ import (
 
 	"cuelang.org/go/cue/errors"
 	"cuelang.org/go/internal/core/adt"
+	"cuelang.org/go/internal/iterutil"
 )
 
 // This file contains predeclared builtins.
@@ -96,7 +97,7 @@ var lenBuiltin = &adt.Builtin{
 				// This should not happen, but be defensive.
 				return c.NewErrf("unevaluated vertex")
 			case *adt.ListMarker:
-				return c.NewInt64(int64(len(x.Elems())), v)
+				return c.NewInt64(int64(iterutil.Count(x.Elems())), v)
 
 			case *adt.StructMarker:
 				n := 0
@@ -192,14 +193,14 @@ var andBuiltin = &adt.Builtin{
 		c := call.OpContext()
 		arg := call.Arg(0)
 
-		list := c.RawElems(arg)
-		if len(list) == 0 {
-			return &adt.Top{}
-		}
+		seq := c.RawElems(arg)
 		a := []adt.Value{}
-		for _, c := range list {
+		for c := range seq {
 			a = append(a, c)
 		}
+		if len(a) == 0 {
+			return &adt.Top{}
+		}
 		return &adt.Conjunction{Values: a}
 	},
 }
@@ -214,7 +215,7 @@ var orBuiltin = &adt.Builtin{
 		args := call.Args()
 
 		d := []adt.Disjunct{}
-		for _, c := range c.RawElems(args[0]) {
+		for c := range c.RawElems(args[0]) {
 			d = append(d, adt.Disjunct{Val: c, Default: false})
 		}
 		if len(d) == 0 {
diff --git a/internal/core/compile/validator.go b/internal/core/compile/validator.go
index dbfa6b6fe..f417623fc 100644
--- a/internal/core/compile/validator.go
+++ b/internal/core/compile/validator.go
@@ -43,11 +43,8 @@ var matchNBuiltin = &adt.Builtin{
 		}
 
 		var errs []*adt.Bottom
-
-		constraints := c.Elems(args[2])
-
 		var count, possibleCount int64
-		for _, check := range constraints {
+		for check := range c.Elems(args[2]) {
 			v := adt.Unify(c, self, check)
 			if err := adt.Validate(c, v, finalCfg); err == nil {
 				// TODO: is it always true that the lack of an error signifies
diff --git a/internal/core/subsume/vertex.go b/internal/core/subsume/vertex.go
index fa7e49648..c5ed81b98 100644
--- a/internal/core/subsume/vertex.go
+++ b/internal/core/subsume/vertex.go
@@ -16,6 +16,7 @@ package subsume
 
 import (
 	"fmt"
+	"slices"
 
 	"cuelang.org/go/internal/core/adt"
 )
@@ -276,8 +277,8 @@ func (s *subsumer) listVertices(x, y *adt.Vertex) bool {
 		return false
 	}
 
-	xElems := x.Elems()
-	yElems := y.Elems()
+	xElems := slices.Collect(x.Elems())
+	yElems := slices.Collect(y.Elems())
 
 	switch {
 	case len(xElems) == len(yElems):
diff --git a/internal/iterutil/iter.go b/internal/iterutil/iter.go
new file mode 100644
index 000000000..991678b89
--- /dev/null
+++ b/internal/iterutil/iter.go
@@ -0,0 +1,25 @@
+// Copyright 2025 CUE Authors
+//
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+//
+//     http://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+
+package iterutil
+
+import "iter"
+
+func Count[E any](seq iter.Seq[E]) int {
+	n := 0
+	for range seq {
+		n++
+	}
+	return n
+}
diff --git a/internal/pkg/context.go b/internal/pkg/context.go
index f5e8d21ca..991767d52 100644
--- a/internal/pkg/context.go
+++ b/internal/pkg/context.go
@@ -311,7 +311,8 @@ func (c *CallCtxt) DecimalList(i int) (a []*apd.Decimal) {
 		return nil
 	}
 
-	for j, w := range v.Elems() {
+	j := 0
+	for w := range v.Elems() {
 		w.Finalize(c.ctx) // defensive
 		switch x := adt.Unwrap(adt.Default(w.Value())).(type) {
 		case *adt.Num:
@@ -339,6 +340,7 @@ func (c *CallCtxt) DecimalList(i int) (a []*apd.Decimal) {
 			c.Err = &callError{err}
 			return nil
 		}
+		j++
 	}
 	return a
 }
@@ -349,7 +351,8 @@ func (c *CallCtxt) StringList(i int) (a []string) {
 		return nil
 	}
 
-	for j, w := range v.Elems() {
+	j := 0
+	for w := range v.Elems() {
 		w.Finalize(c.ctx) // defensive
 		switch x := adt.Unwrap(adt.Default(w.Value())).(type) {
 		case *adt.String:
@@ -377,6 +380,7 @@ func (c *CallCtxt) StringList(i int) (a []string) {
 			c.Err = &callError{err}
 			return nil
 		}
+		j++
 	}
 	return a
 }
diff --git a/internal/pkg/types.go b/internal/pkg/types.go
index ef2bc5c94..63fc40923 100644
--- a/internal/pkg/types.go
+++ b/internal/pkg/types.go
@@ -15,6 +15,8 @@
 package pkg
 
 import (
+	"iter"
+
 	"cuelang.org/go/cue"
 	"cuelang.org/go/internal/core/adt"
 )
@@ -31,7 +33,7 @@ type List struct {
 }
 
 // Elems returns the elements of a list.
-func (l *List) Elems() []*adt.Vertex {
+func (l *List) Elems() iter.Seq[*adt.Vertex] {
 	return l.node.Elems()
 }
 
diff --git a/pkg/list/list.go b/pkg/list/list.go
index 46ca0789b..08a96fdb0 100644
--- a/pkg/list/list.go
+++ b/pkg/list/list.go
@@ -24,6 +24,7 @@ import (
 	"cuelang.org/go/cue/token"
 	"cuelang.org/go/internal/core/adt"
 	"cuelang.org/go/internal/core/eval"
+	"cuelang.org/go/internal/iterutil"
 	"cuelang.org/go/internal/pkg"
 	"cuelang.org/go/internal/types"
 	"cuelang.org/go/internal/value"
@@ -230,7 +231,7 @@ func Reverse(x []cue.Value) []cue.Value {
 
 // MinItems reports whether a has at least n items.
 func MinItems(list pkg.List, n int) (bool, error) {
-	count := len(list.Elems())
+	count := iterutil.Count(list.Elems())
 	if count >= n {
 		return true, nil
 	}
@@ -246,7 +247,7 @@ func MinItems(list pkg.List, n int) (bool, error) {
 
 // MaxItems reports whether a has at most n items.
 func MaxItems(list pkg.List, n int) (bool, error) {
-	count := len(list.Elems())
+	count := iterutil.Count(list.Elems())
 	if count > n {
 		return false, pkg.ValidationError{B: &adt.Bottom{
 			Code: adt.EvalError,