chore(internal): support default value struct tag

Author: stainless-app[bot]

internal/apiform/tag.go

@@ -9,13 +9,15 @@ const apiStructTag = "api"
 const jsonStructTag = "json"
 const formStructTag = "form"
 const formatStructTag = "format"
+const defaultStructTag = "default"
 
 type parsedStructTag struct {
-	name     string
-	required bool
-	extras   bool
-	metadata bool
-	omitzero bool
+	name         string
+	required     bool
+	extras       bool
+	metadata     bool
+	omitzero     bool
+	defaultValue any
 }
 
 func parseFormStructTag(field reflect.StructField) (tag parsedStructTag, ok bool) {
@@ -45,9 +47,23 @@ func parseFormStructTag(field reflect.StructField) (tag parsedStructTag, ok bool
 	}
 
 	parseApiStructTag(field, &tag)
+	parseDefaultStructTag(field, &tag)
 	return tag, ok
 }
 
+func parseDefaultStructTag(field reflect.StructField, tag *parsedStructTag) {
+	if field.Type.Kind() != reflect.String {
+		// Only strings are currently supported
+		return
+	}
+
+	raw, ok := field.Tag.Lookup(defaultStructTag)
+	if !ok {
+		return
+	}
+	tag.defaultValue = raw
+}
+
 func parseApiStructTag(field reflect.StructField, tag *parsedStructTag) {
 	raw, ok := field.Tag.Lookup(apiStructTag)
 	if !ok {

internal/apijson/encoder.go

@@ -12,6 +12,8 @@ import (
 	"time"
 
 	"github.com/tidwall/sjson"
+
+	shimjson "github.com/beeper/desktop-api-go/internal/encoding/json"
 )
 
 var encoders sync.Map // map[encoderEntry]encoderFunc
@@ -271,6 +273,12 @@ func (e *encoder) newStructTypeEncoder(t reflect.Type) encoderFunc {
 			if err != nil {
 				return nil, err
 			}
+			if ef.tag.defaultValue != nil && (!field.IsValid() || field.IsZero()) {
+				encoded, err = shimjson.Marshal(ef.tag.defaultValue)
+				if err != nil {
+					return nil, err
+				}
+			}
 			if encoded == nil {
 				continue
 			}

internal/apijson/json_test.go

@@ -614,3 +614,20 @@ func TestEncode(t *testing.T) {
 		})
 	}
 }
+
+type StructWithDefault struct {
+	Type string `json:"type" default:"foo"`
+}
+
+func TestDefault(t *testing.T) {
+	value := StructWithDefault{}
+	expected := `{"type":"foo"}`
+
+	raw, err := Marshal(value)
+	if err != nil {
+		t.Fatalf("serialization of %v failed with error %v", value, err)
+	}
+	if string(raw) != expected {
+		t.Fatalf("expected %+#v to serialize to %s but got %s", value, expected, string(raw))
+	}
+}

internal/apijson/tag.go

@@ -8,13 +8,15 @@ import (
 const apiStructTag = "api"
 const jsonStructTag = "json"
 const formatStructTag = "format"
+const defaultStructTag = "default"
 
 type parsedStructTag struct {
-	name     string
-	required bool
-	extras   bool
-	metadata bool
-	inline   bool
+	name         string
+	required     bool
+	extras       bool
+	metadata     bool
+	inline       bool
+	defaultValue any
 }
 
 func parseJSONStructTag(field reflect.StructField) (tag parsedStructTag, ok bool) {
@@ -42,9 +44,23 @@ func parseJSONStructTag(field reflect.StructField) (tag parsedStructTag, ok bool
 
 	// the `api` struct tag is only used alongside `json` for custom behaviour
 	parseApiStructTag(field, &tag)
+	parseDefaultStructTag(field, &tag)
 	return tag, ok
 }
 
+func parseDefaultStructTag(field reflect.StructField, tag *parsedStructTag) {
+	if field.Type.Kind() != reflect.String {
+		// Only strings are currently supported
+		return
+	}
+
+	raw, ok := field.Tag.Lookup(defaultStructTag)
+	if !ok {
+		return
+	}
+	tag.defaultValue = raw
+}
+
 func parseApiStructTag(field reflect.StructField, tag *parsedStructTag) {
 	raw, ok := field.Tag.Lookup(apiStructTag)
 	if !ok {