diff --git a/cmdlang/builtins.go b/cmdlang/builtins.go
index 6b137fc..8b8e6fa 100644
--- a/cmdlang/builtins.go
+++ b/cmdlang/builtins.go
@@ -113,6 +113,23 @@ func callBuiltin(ctx context.Context, args invocationArgs) (object, error) {
 	return inv.invoke(ctx, args.shift(1))
 }
 
+func lenBuiltin(ctx context.Context, args invocationArgs) (object, error) {
+	if err := args.expectArgn(1); err != nil {
+		return nil, err
+	}
+
+	switch v := args.args[0].(type) {
+	case strObject:
+		return intObject(len(string(v))), nil
+	case listable:
+		return intObject(v.Len()), nil
+	case hashable:
+		return intObject(v.Len()), nil
+	}
+
+	return intObject(0), nil
+}
+
 func indexBuiltin(ctx context.Context, args invocationArgs) (object, error) {
 	if err := args.expectArgn(1); err != nil {
 		return nil, err
diff --git a/cmdlang/inst.go b/cmdlang/inst.go
index 97f2332..53e35e2 100644
--- a/cmdlang/inst.go
+++ b/cmdlang/inst.go
@@ -31,6 +31,7 @@ func New(opts ...InstOption) *Inst {
 	rootEC.addCmd("set", invokableFunc(setBuiltin))
 	rootEC.addCmd("toUpper", invokableFunc(toUpperBuiltin))
 	//rootEC.addCmd("cat", invokableFunc(catBuiltin))
+	rootEC.addCmd("len", invokableFunc(lenBuiltin))
 	rootEC.addCmd("index", invokableFunc(indexBuiltin))
 	rootEC.addCmd("call", invokableFunc(callBuiltin))
 
diff --git a/cmdlang/objs.go b/cmdlang/objs.go
index 33a3d4d..2f5d010 100644
--- a/cmdlang/objs.go
+++ b/cmdlang/objs.go
@@ -4,6 +4,7 @@ import (
 	"context"
 	"errors"
 	"fmt"
+	"github.com/lmika/gopkgs/fp/slices"
 	"reflect"
 	"strconv"
 )
@@ -160,7 +161,7 @@ func fromGoValue(v any) (object, error) {
 	case reflect.Slice:
 		return listableProxyObject{resVal}, nil
 	case reflect.Struct:
-		return structProxyObject{resVal}, nil
+		return newStructProxyObject(resVal), nil
 	}
 
 	return proxyObject{v}, nil
@@ -392,7 +393,15 @@ func (p listableProxyObject) Index(i int) object {
 }
 
 type structProxyObject struct {
-	v reflect.Value
+	v  reflect.Value
+	vf []reflect.StructField
+}
+
+func newStructProxyObject(v reflect.Value) structProxyObject {
+	return structProxyObject{
+		v:  v,
+		vf: slices.Filter(reflect.VisibleFields(v.Type()), func(t reflect.StructField) bool { return t.IsExported() }),
+	}
 }
 
 func (s structProxyObject) String() string {
@@ -404,7 +413,7 @@ func (s structProxyObject) Truthy() bool {
 }
 
 func (s structProxyObject) Len() int {
-	return s.v.Type().NumField()
+	return len(s.vf)
 }
 
 func (s structProxyObject) Value(k string) object {
@@ -416,14 +425,13 @@ func (s structProxyObject) Value(k string) object {
 }
 
 func (s structProxyObject) Each(fn func(k string, v object) error) error {
-	for i := 0; i < s.v.Type().NumField(); i++ {
-		f := s.v.Type().Field(i).Name
-		v, err := fromGoValue(s.v.Field(i).Interface())
+	for _, f := range s.vf {
+		v, err := fromGoValue(s.v.FieldByName(f.Name).Interface())
 		if err != nil {
 			v = nil
 		}
 
-		if err := fn(f, v); err != nil {
+		if err := fn(f.Name, v); err != nil {
 			return err
 		}
 	}
diff --git a/cmdlang/testbuiltins_test.go b/cmdlang/testbuiltins_test.go
index fca1ff5..6b5fe84 100644
--- a/cmdlang/testbuiltins_test.go
+++ b/cmdlang/testbuiltins_test.go
@@ -386,3 +386,62 @@ func TestBuiltins_Index(t *testing.T) {
 		})
 	}
 }
+
+func TestBuiltins_Len(t *testing.T) {
+	tests := []struct {
+		desc string
+		expr string
+		want string
+	}{
+		{desc: "len of list 1", expr: `len ["alpha" "beta" "gamma"]`, want: "3\n"},
+		{desc: "len of list 2", expr: `len ["alpha"]`, want: "1\n"},
+		{desc: "len of list 3", expr: `len []`, want: "0\n"},
+
+		{desc: "len of hash 1", expr: `len ["first":"alpha" "second":"beta" "third":"gamma"]`, want: "3\n"},
+		{desc: "len of hash 2", expr: `len ["first":"alpha" "second":"beta"]`, want: "2\n"},
+		{desc: "len of hash 3", expr: `len ["first":"alpha"]`, want: "1\n"},
+		{desc: "len of hash 4", expr: `len [:]`, want: "0\n"},
+
+		{desc: "len of string 1", expr: `len "Hello, world"`, want: "12\n"},
+		{desc: "len of string 2", expr: `len "chair"`, want: "5\n"},
+		{desc: "len of string 3", expr: `len ""`, want: "0\n"},
+
+		{desc: "len of int", expr: `len 1232`, want: "0\n"},
+		{desc: "len of nil", expr: `len ()`, want: "0\n"},
+
+		{desc: "go list 1", expr: `goInt | len`, want: "3\n"},
+		{desc: "go struct 1", expr: `goStruct | len`, want: "3\n"},
+		{desc: "go struct 2", expr: `index (goStruct) Gamma | len`, want: "2\n"},
+	}
+
+	for _, tt := range tests {
+		t.Run(tt.desc, func(t *testing.T) {
+			ctx := context.Background()
+			outW := bytes.NewBuffer(nil)
+
+			inst := New(WithOut(outW), WithTestBuiltin())
+			inst.SetBuiltin("goInt", func(ctx context.Context, args CallArgs) (any, error) {
+				return []int{6, 5, 4}, nil
+			})
+			inst.SetBuiltin("goStruct", func(ctx context.Context, args CallArgs) (any, error) {
+				return struct {
+					Alpha   string
+					Beta    string
+					Gamma   []int
+					hidden  string
+					missing string
+				}{
+					Alpha:   "foo",
+					Beta:    "bar",
+					Gamma:   []int{22, 33},
+					hidden:  "hidden",
+					missing: "missing",
+				}, nil
+			})
+			err := inst.EvalAndDisplay(ctx, tt.expr)
+
+			assert.NoError(t, err)
+			assert.Equal(t, tt.want, outW.String())
+		})
+	}
+}