diff --git a/d2ir/apply.go b/d2ir/apply.go
index 893bda82a..9fd025578 100644
--- a/d2ir/apply.go
+++ b/d2ir/apply.go
@@ -24,7 +24,10 @@ func (c *compiler) errorf(n d2ast.Node, f string, v ...interface{}) {
 func Apply(dst *Map, ast *d2ast.Map) error {
 	var c compiler
 	c.apply(dst, ast)
-	return c.err
+	if !c.err.Empty() {
+		return c.err
+	}
+	return nil
 }
 
 func (c *compiler) apply(dst *Map, ast *d2ast.Map) {
diff --git a/d2ir/apply_test.go b/d2ir/apply_test.go
index 5787e882f..32ac9281d 100644
--- a/d2ir/apply_test.go
+++ b/d2ir/apply_test.go
@@ -7,20 +7,17 @@ import (
 	"strings"
 	"testing"
 
+	"oss.terrastruct.com/util-go/assert"
 	"oss.terrastruct.com/util-go/diff"
 
 	"oss.terrastruct.com/d2/d2ast"
 	"oss.terrastruct.com/d2/d2ir"
 	"oss.terrastruct.com/d2/d2parser"
-	"oss.terrastruct.com/d2/internal/assert"
 )
 
 type testCase struct {
 	name string
-	text string
-	base *d2ir.Map
-
-	exp func(testing.TB, *d2ir.Map, error)
+	run  func(testing.TB, *d2ir.Map)
 }
 
 func TestApply(t *testing.T) {
@@ -30,12 +27,13 @@ func TestApply(t *testing.T) {
 }
 
 func testApplySimple(t *testing.T) {
-	tcs := []testCase{
+	t.Parallel()
+
+	tca := []testCase{
 		{
 			name: "one",
-			text: `x`,
-
-			exp: func(t testing.TB, m *d2ir.Map, err error) {
+			run: func(t testing.TB, m *d2ir.Map) {
+				err := parse(t, m, `x`)
 				assert.Success(t, err)
 				assertField(t, m, 1, 0, nil)
 
@@ -44,9 +42,8 @@ func testApplySimple(t *testing.T) {
 		},
 		{
 			name: "nested",
-			text: `x.y -> z.p`,
-
-			exp: func(t testing.TB, m *d2ir.Map, err error) {
+			run: func(t testing.TB, m *d2ir.Map) {
+				err := parse(t, m, `x.y -> z.p`)
 				assert.Success(t, err)
 				assertField(t, m, 4, 1, nil)
 
@@ -65,9 +62,8 @@ func testApplySimple(t *testing.T) {
 		},
 		{
 			name: "underscore_parent",
-			text: `x._ -> z`,
-
-			exp: func(t testing.TB, m *d2ir.Map, err error) {
+			run: func(t testing.TB, m *d2ir.Map) {
+				err := parse(t, m, `x._ -> z`)
 				assert.Success(t, err)
 				assertField(t, m, 2, 1, nil)
 
@@ -83,63 +79,76 @@ func testApplySimple(t *testing.T) {
 		},
 	}
 
-	runa(t, tcs)
+	runa(t, tca)
 }
 
-func runa(t *testing.T, tcs []testCase) {
-	for _, tc := range tcs {
+func runa(t *testing.T, tca []testCase) {
+	for _, tc := range tca {
 		tc := tc
 		t.Run(tc.name, func(t *testing.T) {
 			t.Parallel()
-
-			run(t, tc)
+			m := &d2ir.Map{}
+			tc.run(t, m)
 		})
 	}
 }
 
-func run(t testing.TB, tc testCase) {
+func parse(t testing.TB, dst *d2ir.Map, text string) error {
+	t.Helper()
+
 	d2Path := fmt.Sprintf("d2/testdata/d2ir/%v.d2", t.Name())
-	ast, err := d2parser.Parse(d2Path, strings.NewReader(tc.text), nil)
-	if err != nil {
-		tc.exp(t, nil, err)
-		t.FailNow()
-		return
-	}
+	ast, err := d2parser.Parse(d2Path, strings.NewReader(text), nil)
+	assert.Success(t, err)
 
-	dst := tc.base.Copy(nil).(*d2ir.Map)
 	err = d2ir.Apply(dst, ast)
-	tc.exp(t, dst, err)
-
-	err = diff.Testdata(filepath.Join("..", "testdata", "d2ir", t.Name()), dst)
 	if err != nil {
-		tc.exp(t, nil, err)
-		t.FailNow()
-		return
+		return err
 	}
+
+	err = diff.TestdataJSON(filepath.Join("..", "testdata", "d2ir", t.Name()), dst)
+	return err
 }
 
 func assertField(t testing.TB, n d2ir.Node, nfields, nedges int, primary interface{}, ida ...string) *d2ir.Field {
 	t.Helper()
 
-	m := d2ir.NodeToMap(n)
-	p := d2ir.NodeToPrimary(n)
+	var m *d2ir.Map
+	p := &d2ir.Scalar{
+		Value: &d2ast.Null{},
+	}
+	switch n := n.(type) {
+	case *d2ir.Field:
+		mm, ok := n.Composite.(*d2ir.Map)
+		if ok {
+			m = mm
+		} else {
+			t.Fatalf("unexpected d2ir.Field.Composite %T", n.Composite)
+		}
+		p = n.Primary
+	case *d2ir.Map:
+		m = n
+		p.Value = &d2ast.Null{}
+	default:
+		t.Fatalf("unexpected d2ir.Node %T", n)
+	}
 
 	var f *d2ir.Field
+	var ok bool
 	if len(ida) > 0 {
-		f = m.Get(ida)
-		if f == nil {
-			t.Fatalf("expected field %#v in map %#v but not found", ida, m)
+		f, ok = m.Get(ida)
+		if !ok {
+			t.Fatalf("expected field %v in map %s", ida, m)
 		}
 		p = f.Primary
-		m = d2ir.NodeToMap(f)
+		if f_m, ok := f.Composite.(*d2ir.Map); ok {
+			m = f_m
+		} else {
+			m = &d2ir.Map{}
+		}
 	}
 
-	if m.FieldCount() != nfields {
-		t.Fatalf("expected %d fields but got %d", nfields, m.FieldCount())
-	}
-	if m.EdgeCount() != nedges {
-		t.Fatalf("expected %d edges but got %d", nedges, m.EdgeCount())
-	}
+	assert.Equal(t, nfields, m.FieldCount())
+	assert.Equal(t, nedges, m.EdgeCount())
 	if !p.Equal(makeScalar(primary)) {
 		t.Fatalf("expected primary %#v but %#v", primary, p)
 	}
@@ -150,19 +159,27 @@ func assertField(t testing.TB, n d2ir.Node, nfields, nedges int, primary interfa
 func assertEdge(t testing.TB, n d2ir.Node, nfields int, primary interface{}, eid *d2ir.EdgeID) *d2ir.Edge {
 	t.Helper()
 
-	m := d2ir.NodeToMap(n)
-
-	e := m.GetEdge(eid)
-	if e == nil {
-		t.Fatalf("expected edge %#v in map %#v but not found", eid, m)
+	var m *d2ir.Map
+	switch n := n.(type) {
+	case *d2ir.Field:
+		mm, ok := n.Composite.(*d2ir.Map)
+		if ok {
+			m = mm
+		} else {
+			t.Fatalf("unexpected d2ir.Field.Composite %T", n.Composite)
+		}
+	case *d2ir.Map:
+		m = n
+	default:
+		t.Fatalf("unexpected d2ir.Node %T", n)
 	}
 
-	if e.Map.FieldCount() != nfields {
-		t.Fatalf("expected %d fields but got %d", nfields, e.Map.FieldCount())
-	}
-	if e.Map.EdgeCount() != 0 {
-		t.Fatalf("expected %d edges but got %d", 0, e.Map.EdgeCount())
+	e, ok := m.GetEdge(eid)
+	if !ok {
+		t.Fatalf("expected edge %v in map %s but not found", eid, m)
 	}
+
+	assert.Equal(t, nfields, e.Map.FieldCount())
 	if !e.Primary.Equal(makeScalar(primary)) {
 		t.Fatalf("expected primary %#v but %#v", primary, e.Primary)
 	}
diff --git a/d2ir/d2ir.go b/d2ir/d2ir.go
index 20ecdd08e..8facce8b4 100644
--- a/d2ir/d2ir.go
+++ b/d2ir/d2ir.go
@@ -1,6 +1,8 @@
 package d2ir
 
 import (
+	"encoding/json"
+	"fmt"
 	"strings"
 
 	"oss.terrastruct.com/d2/d2ast"
@@ -51,6 +53,7 @@ func (n *Edge) node()   {}
 func (n *Array) node()  {}
 func (n *Map) node()    {}
 
+func (n *Scalar) Parent() Parent { return n.parent }
 func (n *Field) Parent() Parent { return n.parent }
 func (n *Edge) Parent() Parent  { return n.parent }
 func (n *Array) Parent() Parent { return n.parent }
@@ -77,6 +80,17 @@ func (s *Scalar) Copy(newp Parent) Node {
 }
 
 func (s *Scalar) Equal(s2 *Scalar) bool {
+	if s == nil {
+		if s2 == nil {
+			return true
+		}
+		_, ok := s2.Value.(*d2ast.Null)
+		return ok
+	}
+	if s2 == nil {
+		_, ok := s.Value.(*d2ast.Null)
+		return ok
+	}
 	return s.Value.ScalarString() == s2.Value.ScalarString() && s.Value.Type() == s2.Value.Type()
 }
 
@@ -113,10 +127,10 @@ type Field struct {
 
 	Name string `json:"name"`
 
-	Primary   *Scalar   `json:"primary"`
-	Composite Composite `json:"composite"`
+	Primary   *Scalar   `json:"primary,omitempty"`
+	Composite Composite `json:"composite,omitempty"`
 
-	Refs []KeyReference `json:"refs"`
+	Refs []KeyReference `json:"refs,omitempty"`
 }
 
 func (f *Field) Copy(newp Parent) Node {
@@ -203,10 +217,10 @@ type Edge struct {
 
 	ID *EdgeID `json:"edge_id"`
 
-	Primary *Scalar `json:"primary"`
-	Map     *Map    `json:"map"`
+	Primary *Scalar `json:"primary,omitempty"`
+	Map     *Map    `json:"map,omitempty"`
 
-	Refs []EdgeReference `json:"refs"`
+	Refs []EdgeReference `json:"refs,omitempty"`
 }
 
 func (e *Edge) Copy(newp Parent) Node {
@@ -384,3 +398,11 @@ func (m *Map) GetEdge(eid *EdgeID) (*Edge, bool) {
 	}
 	return nil, false
 }
+
+func (m *Map) String() string {
+	b, err := json.Marshal(m)
+	if err != nil {
+		panic(fmt.Sprintf("d2ir: failed to marshal d2ir.Map: %v", err))
+	}
+	return string(b)
+}
diff --git a/d2ir/d2ir_test.go b/d2ir/d2ir_test.go
index 9fab4f612..52cae5aad 100644
--- a/d2ir/d2ir_test.go
+++ b/d2ir/d2ir_test.go
@@ -3,9 +3,10 @@ package d2ir_test
 import (
 	"testing"
 
+	"oss.terrastruct.com/util-go/assert"
+
 	"oss.terrastruct.com/d2/d2ast"
 	"oss.terrastruct.com/d2/d2ir"
-	"oss.terrastruct.com/d2/internal/assert"
 )
 
 func TestCopy(t *testing.T) {
@@ -13,14 +14,11 @@ func TestCopy(t *testing.T) {
 
 	const scalStr = `Those who claim the dead never return to life haven't ever been around.`
 	s := &d2ir.Scalar{
-		parent: nil,
-		Value:  d2ast.FlatUnquotedString(scalStr),
+		Value: d2ast.FlatUnquotedString(scalStr),
 	}
 	a := &d2ir.Array{
-		Parent: nil,
 		Values: []d2ir.Value{
 			&d2ir.Scalar{
-				parent: nil,
 				Value: &d2ast.Boolean{
 					Value: true,
 				},
@@ -28,7 +26,6 @@ func TestCopy(t *testing.T) {
 		},
 	}
 	m2 := &d2ir.Map{
-		Parent: nil,
 		Fields: []*d2ir.Field{
 			{Primary: s},
 		},
@@ -36,20 +33,17 @@ func TestCopy(t *testing.T) {
 
 	const keyStr = `Absence makes the heart grow frantic.`
 	f := &d2ir.Field{
-		Parent: nil,
-		Name:   keyStr,
+		Name: keyStr,
 
 		Primary:   s,
 		Composite: a,
 	}
 	e := &d2ir.Edge{
-		Parent: nil,
 
 		Primary: s,
 		Map:     m2,
 	}
 	m := &d2ir.Map{
-		Parent: nil,
 
 		Fields: []*d2ir.Field{f},
 		Edges:  []*d2ir.Edge{e},
@@ -58,20 +52,20 @@ func TestCopy(t *testing.T) {
 	m = m.Copy(nil).(*d2ir.Map)
 	f.Name = `Many a wife thinks her husband is the world's greatest lover.`
 
-	assert.Equal(t, m, m.Fields[0].Parent)
+	assert.Equal(t, m, m.Fields[0].Parent())
 	assert.Equal(t, keyStr, m.Fields[0].Name)
-	assert.Equal(t, m.Fields[0], m.Fields[0].Primary.parent)
-	assert.Equal(t, m.Fields[0], m.Fields[0].Composite.(*d2ir.Array).Parent)
+	assert.Equal(t, m.Fields[0], m.Fields[0].Primary.Parent())
+	assert.Equal(t, m.Fields[0], m.Fields[0].Composite.(*d2ir.Array).Parent())
 
 	assert.Equal(t,
 		m.Fields[0].Composite,
-		m.Fields[0].Composite.(*d2ir.Array).Values[0].(*d2ir.Scalar).parent,
+		m.Fields[0].Composite.(*d2ir.Array).Values[0].(*d2ir.Scalar).Parent(),
 	)
 
-	assert.Equal(t, m, m.Edges[0].Parent)
-	assert.Equal(t, m.Edges[0], m.Edges[0].Primary.parent)
-	assert.Equal(t, m.Edges[0], m.Edges[0].Map.Parent)
+	assert.Equal(t, m, m.Edges[0].Parent())
+	assert.Equal(t, m.Edges[0], m.Edges[0].Primary.Parent())
+	assert.Equal(t, m.Edges[0], m.Edges[0].Map.Parent())
 
-	assert.Equal(t, m.Edges[0].Map, m.Edges[0].Map.Fields[0].Parent)
-	assert.Equal(t, m.Edges[0].Map.Fields[0], m.Edges[0].Map.Fields[0].Primary.parent)
+	assert.Equal(t, m.Edges[0].Map, m.Edges[0].Map.Fields[0].Parent())
+	assert.Equal(t, m.Edges[0].Map.Fields[0], m.Edges[0].Map.Fields[0].Primary.Parent())
 }
diff --git a/d2parser/parse.go b/d2parser/parse.go
index d8351dcbc..0e7e5eefc 100644
--- a/d2parser/parse.go
+++ b/d2parser/parse.go
@@ -45,7 +45,7 @@ func Parse(path string, r io.RuneReader, opts *ParseOptions) (*d2ast.Map, error)
 	}
 
 	m := p.parseMap(true)
-	if !p.err.empty() {
+	if !p.err.Empty() {
 		return m, p.err
 	}
 	return m, nil
@@ -57,7 +57,7 @@ func ParseKey(key string) (*d2ast.KeyPath, error) {
 	}
 
 	k := p.parseKey()
-	if !p.err.empty() {
+	if !p.err.Empty() {
 		return nil, fmt.Errorf("failed to parse key %q: %w", key, p.err)
 	}
 	if k == nil {
@@ -72,7 +72,7 @@ func ParseMapKey(mapKey string) (*d2ast.Key, error) {
 	}
 
 	mk := p.parseMapKey()
-	if !p.err.empty() {
+	if !p.err.Empty() {
 		return nil, fmt.Errorf("failed to parse map key %q: %w", mapKey, p.err)
 	}
 	if mk == nil {
@@ -87,7 +87,7 @@ func ParseValue(value string) (d2ast.Value, error) {
 	}
 
 	v := p.parseValue()
-	if !p.err.empty() {
+	if !p.err.Empty() {
 		return nil, fmt.Errorf("failed to parse value %q: %w", value, p.err)
 	}
 	if v.Unbox() == nil {
@@ -130,7 +130,7 @@ type ParseError struct {
 	Errors  []d2ast.Error `json:"errs"`
 }
 
-func (pe ParseError) empty() bool {
+func (pe ParseError) Empty() bool {
 	return pe.IOError == nil && len(pe.Errors) == 0
 }
 
diff --git a/testdata/d2ir/TestApply/simple/nested.exp.json b/testdata/d2ir/TestApply/simple/nested.exp.json
new file mode 100644
index 000000000..fbfc21849
--- /dev/null
+++ b/testdata/d2ir/TestApply/simple/nested.exp.json
@@ -0,0 +1,4 @@
+{
+  "fields": null,
+  "edges": null
+}
diff --git a/testdata/d2ir/TestApply/simple/one.exp.json b/testdata/d2ir/TestApply/simple/one.exp.json
new file mode 100644
index 000000000..2dcf9ab44
--- /dev/null
+++ b/testdata/d2ir/TestApply/simple/one.exp.json
@@ -0,0 +1,8 @@
+{
+  "fields": [
+    {
+      "name": "x"
+    }
+  ],
+  "edges": null
+}
diff --git a/testdata/d2ir/TestApply/simple/underscore_parent.exp.json b/testdata/d2ir/TestApply/simple/underscore_parent.exp.json
new file mode 100644
index 000000000..fbfc21849
--- /dev/null
+++ b/testdata/d2ir/TestApply/simple/underscore_parent.exp.json
@@ -0,0 +1,4 @@
+{
+  "fields": null,
+  "edges": null
+}