diff --git a/cmd/cmsh/main.go b/cmd/cmsh/main.go
index 8793777..8296224 100644
--- a/cmd/cmsh/main.go
+++ b/cmd/cmsh/main.go
@@ -20,6 +20,8 @@ func main() {
 	instRepl := repl.New(
 		ucl.WithModule(builtins.OS()),
 		ucl.WithModule(builtins.FS(nil)),
+		ucl.WithModule(builtins.Log(nil)),
+		ucl.WithModule(builtins.Strs()),
 	)
 	ctx := context.Background()
 
diff --git a/cmd/playwasm/jsiter.go b/cmd/playwasm/jsiter.go
index 1e62e6d..e653f4b 100644
--- a/cmd/playwasm/jsiter.go
+++ b/cmd/playwasm/jsiter.go
@@ -21,9 +21,12 @@ func invokeUCLCallback(name string, args ...any) {
 func initJS(ctx context.Context) {
 	uclObj := make(map[string]any)
 
-	replInst := repl.New(ucl.WithOut(ucl.LineHandler(func(line string) {
-		invokeUCLCallback("onOutLine", line)
-	})))
+	replInst := repl.New(
+		ucl.WithModule(builtins.Log(nil)),
+		ucl.WithModule(builtins.Strs()),
+		ucl.WithOut(ucl.LineHandler(func(line string) {
+			invokeUCLCallback("onOutLine", line)
+		})))
 
 	uclObj["eval"] = js.FuncOf(func(this js.Value, args []js.Value) any {
 		if len(args) != 2 {
diff --git a/ucl/builtins/log.go b/ucl/builtins/log.go
new file mode 100644
index 0000000..c336833
--- /dev/null
+++ b/ucl/builtins/log.go
@@ -0,0 +1,44 @@
+package builtins
+
+import (
+	"context"
+	"log"
+	"strings"
+	"ucl.lmika.dev/ucl"
+)
+
+type LogMessageHandler func(s string) error
+
+func Log(handler LogMessageHandler) ucl.Module {
+	if handler == nil {
+		handler = func(s string) error {
+			log.Println(s)
+			return nil
+		}
+	}
+
+	lh := logHandler{handler: handler}
+
+	return ucl.Module{
+		Name: "log",
+		Builtins: map[string]ucl.BuiltinHandler{
+			"puts": lh.logMessage,
+		},
+	}
+}
+
+type logHandler struct {
+	handler LogMessageHandler
+}
+
+func (lh logHandler) logMessage(ctx context.Context, args ucl.CallArgs) (any, error) {
+	var sb strings.Builder
+	var s ucl.Object
+
+	for args.NArgs() > 0 {
+		if err := args.Bind(&s); err == nil {
+			sb.WriteString(s.String())
+		}
+	}
+	return nil, lh.handler(sb.String())
+}
diff --git a/ucl/builtins/log_test.go b/ucl/builtins/log_test.go
new file mode 100644
index 0000000..9071c02
--- /dev/null
+++ b/ucl/builtins/log_test.go
@@ -0,0 +1,47 @@
+package builtins_test
+
+import (
+	"bytes"
+	"context"
+	"github.com/stretchr/testify/assert"
+	"testing"
+	"ucl.lmika.dev/ucl"
+	"ucl.lmika.dev/ucl/builtins"
+)
+
+func TestLog_Puts(t *testing.T) {
+	tests := []struct {
+		desc    string
+		eval    string
+		want    any
+		wantErr bool
+		wantLog string
+	}{
+		{desc: "log 1", eval: `log:puts "bad stuff here"`, wantLog: "bad stuff here\n"},
+		{desc: "log 2", eval: `log:puts "bad " 123 " here " [1 2 3]`, wantLog: "bad 123 here [1 2 3]\n"},
+		{desc: "log 3", eval: `log:puts`, wantLog: "\n"},
+	}
+
+	for _, tt := range tests {
+		t.Run(tt.desc, func(t *testing.T) {
+			var logMessage bytes.Buffer
+
+			inst := ucl.New(
+				ucl.WithModule(builtins.Log(func(s string) error {
+					logMessage.WriteString(s)
+					logMessage.WriteByte('\n')
+					return nil
+				})),
+			)
+
+			res, err := inst.Eval(context.Background(), tt.eval)
+			if tt.wantErr {
+				assert.Error(t, err)
+			} else {
+				assert.NoError(t, err)
+				assert.Equal(t, tt.want, res)
+			}
+			assert.Equal(t, tt.wantLog, logMessage.String())
+		})
+	}
+}
diff --git a/ucl/userbuiltin.go b/ucl/userbuiltin.go
index 04021c3..78f3d47 100644
--- a/ucl/userbuiltin.go
+++ b/ucl/userbuiltin.go
@@ -96,8 +96,12 @@ func (u userBuiltin) invoke(ctx context.Context, args invocationArgs) (Object, e
 
 func (ca CallArgs) bindArg(v interface{}, arg Object) error {
 	switch t := v.(type) {
+	case *Object:
+		*t = arg
+		return nil
 	case *interface{}:
 		*t, _ = toGoValue(arg)
+		return nil
 	case *Invokable:
 		i, ok := arg.(invokable)
 		if !ok {
@@ -123,12 +127,14 @@ func (ca CallArgs) bindArg(v interface{}, arg Object) error {
 		} else {
 			*t = ""
 		}
+		return nil
 	case *int:
 		if iArg, ok := arg.(intObject); ok {
 			*t = int(iArg)
 		} else {
 			return errors.New("invalid arg")
 		}
+		return nil
 	}
 
 	switch t := arg.(type) {