diff --git a/cmd/gen-go-sample/inittree.go b/cmd/gen-go-sample/inittree.go
new file mode 100644
index 00000000000..66071d79fef
--- /dev/null
+++ b/cmd/gen-go-sample/inittree.go
@@ -0,0 +1,158 @@
+// Copyright 2018 Google LLC
+//
+// 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
+//
+//     https://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 main
+
+import (
+	"bufio"
+	"io"
+	"strings"
+	"text/scanner"
+	"unicode"
+
+	"github.com/googleapis/gapic-generator-go/internal/errors"
+)
+
+// initTree represents a node in the initialization tree.
+type initTree struct {
+	// T is either a composite value (struct/array/map), where keys and vals are set,
+	// or a simple value, where leafVal is set.
+
+	// Use array representation; we need order, and we probably won't
+	// have many pairs anyway.
+	keys []string
+	vals []*initTree
+
+	// Text of the literal. If the literal is a string, it's already quoted.
+	leafVal string
+}
+
+func (t *initTree) get(k string) *initTree {
+	for i, key := range t.keys {
+		if k == key {
+			return t.vals[i]
+		}
+	}
+
+	v := new(initTree)
+	t.keys = append(t.keys, k)
+	t.vals = append(t.vals, v)
+	return v
+}
+
+func (t *initTree) Parse(txt string) error {
+	var sc scanner.Scanner
+	sc.Init(strings.NewReader(txt))
+	sc.Mode = scanner.ScanIdents | scanner.ScanInts | scanner.ScanFloats | scanner.ScanStrings
+
+	// Scanner reports error by calling sc.Error; we save the error so we always report
+	// the first.
+	var err error
+	report := func(e error) error {
+		if err == nil {
+			err = e
+		}
+		return err
+	}
+	sc.Error = func(_ *scanner.Scanner, msg string) {
+		e := errors.E(nil, msg)
+		e = errors.E(e, "while scaning: %q", txt)
+		report(e)
+	}
+
+	// Just do simple structs for now.
+	// TODO(pongad): allow map and array index.
+	// spec = ident { '.' ident } [ '=' value ] .
+
+	if sc.Scan() != scanner.Ident {
+		return report(errors.E(nil, "expected ident, found %q", sc.TokenText()))
+	}
+	t = t.get(sc.TokenText())
+
+	for {
+		switch sc.Scan() {
+		case '=':
+			goto equal
+		case scanner.EOF:
+			// TODO: set t.leafVal to zero value of type.
+			return report(nil)
+
+		case '.':
+			if sc.Scan() != scanner.Ident {
+				return report(errors.E(nil, "expected ident, found %q", sc.TokenText()))
+			}
+			t = t.get(sc.TokenText())
+
+		default:
+			return report(errors.E(nil, "unexpected %q", sc.TokenText()))
+		}
+	}
+
+	// TODO(pongad): check that value and type match
+	// TODO(pongad): check that we don't write two values onto the same node
+	// TODO(pongad): handle resource names
+	// TODO(pongad): properly validate and print enums
+equal:
+	switch r := sc.Scan(); r {
+	case scanner.Int, scanner.Float, scanner.String, scanner.Ident:
+		t.leafVal = sc.TokenText()
+	default:
+		return report(errors.E(nil, "expected value, found %q", sc.TokenText()))
+	}
+
+	if sc.Scan() != scanner.EOF {
+		return report(errors.E(nil, "expected EOF, found %q", sc.TokenText()))
+	}
+	return err
+}
+
+func (t *initTree) Print(w io.Writer) error {
+	bw := bufio.NewWriter(w)
+	t.print(bw, 1)
+	return bw.Flush()
+}
+
+func (t *initTree) print(w *bufio.Writer, ind int) {
+	if v := t.leafVal; v != "" {
+		w.WriteString(v)
+		return
+	}
+
+	// TODO(pongad): Figure out how to print type.
+	w.WriteString("TYPE{\n")
+	for i, k := range t.keys {
+		for i := 0; i < ind; i++ {
+			w.WriteByte('\t')
+		}
+		snakeToCapCamel(w, k)
+		w.WriteString(": ")
+		t.vals[i].print(w, ind+1)
+		w.WriteString(",\n")
+	}
+	w.WriteString("}")
+}
+
+func snakeToCapCamel(w *bufio.Writer, s string) {
+	cap := true
+	for _, r := range s {
+		if r == '_' {
+			cap = true
+		} else if cap {
+			cap = false
+			w.WriteRune(unicode.ToUpper(r))
+		} else {
+			w.WriteRune(r)
+		}
+	}
+}
diff --git a/cmd/gen-go-sample/inittree_test.go b/cmd/gen-go-sample/inittree_test.go
new file mode 100644
index 00000000000..cf186659ed8
--- /dev/null
+++ b/cmd/gen-go-sample/inittree_test.go
@@ -0,0 +1,65 @@
+// Copyright 2018 Google LLC
+//
+// 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
+//
+//     https://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 main
+
+import (
+	"strings"
+	"testing"
+)
+
+func TestTree(t *testing.T) {
+	specs := []string{
+		`a.b = 1`,
+		`a.c = "xyz"`,
+		`a.d = 2.718281828`,
+		`x = 123`,
+	}
+
+	var root initTree
+	for _, s := range specs {
+		if err := root.Parse(s); err != nil {
+			t.Fatal(err)
+		}
+	}
+
+	for _, tst := range []struct {
+		path []string
+		val  string
+	}{
+		{[]string{"a", "b"}, "1"},
+		{[]string{"a", "c"}, `"xyz"`},
+		{[]string{"a", "d"}, "2.718281828"},
+		{[]string{"x"}, "123"},
+	} {
+		node := &root
+		for _, p := range tst.path {
+			node = node.get(p)
+		}
+		if node.leafVal != tst.val {
+			t.Errorf("%s = %q, want %q", strings.Join(tst.path, "->"), node.leafVal, tst.val)
+		}
+	}
+
+	if t.Failed() {
+		t.SkipNow()
+	}
+
+	var buf strings.Builder
+	buf.WriteByte('\n')
+	if err := root.Print(&buf); err != nil {
+		t.Error(err)
+	}
+	t.Log(buf.String())
+}
diff --git a/cmd/gen-go-sample/main.go b/cmd/gen-go-sample/main.go
index e364b64df8f..7a75c7ad1f9 100644
--- a/cmd/gen-go-sample/main.go
+++ b/cmd/gen-go-sample/main.go
@@ -97,7 +97,7 @@ func main() {
 						log.Fatal(errors.E(err, "value set: %q", vs))
 					}
 					if err := gen.commit(!*nofmt); err != nil {
-						log.Fatal(errors.E(err, "value set: %q", vs))
+						log.Fatal(errors.E(err, "can't commit value set: %q", vs))
 					}
 				}
 			}
@@ -161,7 +161,7 @@ func (g *generator) commit(gofmt bool) error {
 	if gofmt {
 		b2, err := format.Source(b)
 		if err != nil {
-			return err
+			return errors.E(err, "syntax error, run with -nofmt to find out why?")
 		}
 		b = b2
 	}
@@ -214,11 +214,25 @@ func (g *generator) genSample(ifaceName, methName, regTag string, valSet SampleV
 		return errors.E(err, "can't import input type: %q", inType)
 	}
 
-	p("  req := %s.%s{", inSpec.Name, inType.GetName())
+	var itree initTree
 	for _, def := range valSet.Parameters.Defaults {
-		p("// %s", def)
+		if err := itree.Parse(def); err != nil {
+			return errors.E(err, "can't set default value: %q", def)
+		}
+	}
+	{
+		w := g.pt.Writer()
+
+		if _, err := w.Write([]byte("req := ")); err != nil {
+			return err
+		}
+		if err := itree.Print(g.pt.Writer()); err != nil {
+			return err
+		}
+		if _, err := w.Write([]byte{'\n'}); err != nil {
+			return err
+		}
 	}
-	p("  }")
 
 	// TODO(pongad): handle non-unary
 	p("  resp, err := c.%s(ctx, req)", methName)
diff --git a/cmd/gen-go-sample/main_test.go b/cmd/gen-go-sample/main_test.go
index 251c3faddf9..e67903b62fb 100644
--- a/cmd/gen-go-sample/main_test.go
+++ b/cmd/gen-go-sample/main_test.go
@@ -84,7 +84,11 @@ func TestSample(t *testing.T) {
 	vs := SampleValueSet{
 		ID: "my_value_set",
 		Parameters: SampleParameter{
-			Defaults: []string{"foo=bar"},
+			Defaults: []string{
+				`a.x = 42`,
+				`a.y = 3.14159`,
+				`b = "foobar"`,
+			},
 		},
 	}
 	if err := g.genSample("MyService", "MyMethod", "awesome_region", vs); err != nil {
diff --git a/cmd/gen-go-sample/test.bash b/cmd/gen-go-sample/test.bash
index 3bf722103a7..9104ac5c222 100755
--- a/cmd/gen-go-sample/test.bash
+++ b/cmd/gen-go-sample/test.bash
@@ -25,6 +25,6 @@ if [ -z $COMMON_PROTO ]; then
 	exit 1
 fi
 
-./gen-go-sample \
+./gen-go-sample $* \
   -gapic "$GOOGLEAPIS/google/cloud/language/v1/language_gapic.yaml" \
   -desc <(protoc -o /dev/stdout --include_imports -I "$COMMON_PROTO" -I "$GOOGLEAPIS" "$GOOGLEAPIS"/google/cloud/language/v1/*.proto)
diff --git a/cmd/gen-go-sample/testdata/sample.want b/cmd/gen-go-sample/testdata/sample.want
index 4ce6501ee2c..0e1ca8bcdb6 100644
--- a/cmd/gen-go-sample/testdata/sample.want
+++ b/cmd/gen-go-sample/testdata/sample.want
@@ -4,9 +4,13 @@ func sampleMyMethod() {
 	ctx := context.Background()
 	c := mypackagepb.NewClient(ctx)
 
-	req := mypackagepb.InputType{
-		// foo=bar
-	}
+req := TYPE{
+	A: TYPE{
+		X: 42,
+		Y: 3.14159,
+},
+	B: "foobar",
+}
 	resp, err := c.MyMethod(ctx, req)
 	if err != nil {
 		// TODO: Handle error.
diff --git a/internal/printer/printer.go b/internal/printer/printer.go
index 42dbee2a56d..9a452d16b2e 100644
--- a/internal/printer/printer.go
+++ b/internal/printer/printer.go
@@ -18,6 +18,7 @@ package printer
 import (
 	"bytes"
 	"fmt"
+	"io"
 	"strings"
 )
 
@@ -65,6 +66,11 @@ func (p *P) Printf(s string, args ...interface{}) {
 	}
 }
 
+// Writer returns a writer that writes to p's underlying buffer without performing indentation.
+func (p *P) Writer() io.Writer {
+	return &p.buf
+}
+
 // Bytes returns the bytes written by Printf. It is valid up to the next call to Printf or Reset.
 func (p *P) Bytes() []byte {
 	return p.buf.Bytes()