helper/backend: add default values

helper/backend/backend.go

@@ -44,7 +44,18 @@ type Path struct {
 
 // FieldSchema is a basic schema to describe the format of a path field.
 type FieldSchema struct {
-	Type FieldType
+	Type    FieldType
+	Default interface{}
+}
+
+// DefaultOrZero returns the default value if it is set, or otherwise
+// the zero value of the type.
+func (s *FieldSchema) DefaultOrZero() interface{} {
+	if s.Default != nil {
+		return s.Default
+	}
+
+	return s.Type.Zero()
 }
 
 func (t FieldType) Zero() interface{} {

helper/backend/backend_test.go

@@ -0,0 +1,31 @@
+package backend
+
+import (
+	"reflect"
+	"testing"
+)
+
+func TestFieldSchemaDefaultOrZero(t *testing.T) {
+	cases := map[string]struct {
+		Schema *FieldSchema
+		Value  interface{}
+	}{
+		"default set": {
+			&FieldSchema{Type: TypeString, Default: "foo"},
+			"foo",
+		},
+
+		"default not set": {
+			&FieldSchema{Type: TypeString},
+			"",
+		},
+	}
+
+	for name, tc := range cases {
+		actual := tc.Schema.DefaultOrZero()
+		if !reflect.DeepEqual(actual, tc.Value) {
+			t.Fatalf("bad: %s\n\nExpected: %#v\nGot: %#v",
+				name, tc.Value, actual)
+		}
+	}
+}

helper/backend/field_data.go

@@ -27,7 +27,7 @@ func (d *FieldData) Get(k string) interface{} {
 
 	value, ok := d.GetOk(k)
 	if !ok {
-		value = schema.Type.Zero()
+		value = schema.DefaultOrZero()
 	}
 
 	return value
@@ -47,7 +47,7 @@ func (d *FieldData) GetOk(k string) (interface{}, bool) {
 	}
 
 	if ok && result == nil {
-		result = schema.Type.Zero()
+		result = schema.DefaultOrZero()
 	}
 
 	return result, ok

helper/backend/field_data_test.go

@@ -43,6 +43,18 @@ func TestFieldDataGet(t *testing.T) {
 			"",
 		},
 
+		"string type, unset value with default": {
+			map[string]*FieldSchema{
+				"foo": &FieldSchema{
+					Type:    TypeString,
+					Default: "bar",
+				},
+			},
+			map[string]interface{}{},
+			"foo",
+			"bar",
+		},
+
 		"int type, int value": {
 			map[string]*FieldSchema{
 				"foo": &FieldSchema{Type: TypeInt},