helper/schema: Defaults [GH-245]

helper/schema/schema.go

@@ -62,6 +62,10 @@ type Schema struct {
 	Optional bool
 	Required bool
 
+	// If this is non-nil, then this will be a default value that is used
+	// when this item is not set in the configuration/state.
+	Default interface{}
+
 	// The fields below relate to diffs.
 	//
 	// If Computed is true, then the result of this value is computed
@@ -280,6 +284,10 @@ func (m schemaMap) InternalValidate() error {
 			return fmt.Errorf("%s: One of optional, required, or computed must be set", k)
 		}
 
+		if v.Computed && v.Default != nil {
+			return fmt.Errorf("%s: Default must be nil if computed", k)
+		}
+
 		if len(v.ComputedWhen) > 0 && !v.Computed {
 			return fmt.Errorf("%s: ComputedWhen can only be set with Computed", k)
 		}
@@ -289,6 +297,10 @@ func (m schemaMap) InternalValidate() error {
 				return fmt.Errorf("%s: Elem must be set for lists", k)
 			}
 
+			if v.Default != nil {
+				return fmt.Errorf("%s: Default is not valid for lists or sets", k)
+			}
+
 			if v.Type == TypeList && v.Set != nil {
 				return fmt.Errorf("%s: Set can only be set for TypeSet", k)
 			} else if v.Type == TypeSet && v.Set == nil {
@@ -492,6 +504,9 @@ func (m schemaMap) diffString(
 	var originalN interface{}
 	var os, ns string
 	o, n, _ := d.diffChange(k)
+	if n == nil {
+		n = schema.Default
+	}
 	if schema.StateFunc != nil {
 		originalN = n
 		n = schema.StateFunc(n)

helper/schema/schema_test.go

@@ -98,6 +98,32 @@ func TestSchemaMap_Diff(t *testing.T) {
 			Err: false,
 		},
 
+		// Default
+		{
+			Schema: map[string]*Schema{
+				"availability_zone": &Schema{
+					Type:     TypeString,
+					Optional: true,
+					Default:  "foo",
+				},
+			},
+
+			State: nil,
+
+			Config: nil,
+
+			Diff: &terraform.ResourceDiff{
+				Attributes: map[string]*terraform.ResourceAttrDiff{
+					"availability_zone": &terraform.ResourceAttrDiff{
+						Old: "",
+						New: "foo",
+					},
+				},
+			},
+
+			Err: false,
+		},
+
 		// String with StateFunc
 		{
 			Schema: map[string]*Schema{
@@ -1016,6 +1042,19 @@ func TestSchemaMap_InternalValidate(t *testing.T) {
 			false,
 		},
 
+		// Computed but has default
+		{
+			map[string]*Schema{
+				"foo": &Schema{
+					Type:     TypeInt,
+					Optional: true,
+					Computed: true,
+					Default:  "foo",
+				},
+			},
+			true,
+		},
+
 		// List element not set
 		{
 			map[string]*Schema{
@@ -1026,6 +1065,18 @@ func TestSchemaMap_InternalValidate(t *testing.T) {
 			true,
 		},
 
+		// List default
+		{
+			map[string]*Schema{
+				"foo": &Schema{
+					Type:    TypeList,
+					Elem:    &Schema{Type: TypeInt},
+					Default: "foo",
+				},
+			},
+			true,
+		},
+
 		// List element computed
 		{
 			map[string]*Schema{