gen/enum: Generate <Name>_Values function (#308)

This generates a `<Name>_Values` function for all enums which returns
all known values for that enum.

CHANGELOG.md

@@ -9,6 +9,8 @@ v1.3.0 (unreleased)
 -   Enums now implement the `encoding.TextUnmarshaler` interface. This allows
     retrieving enum values by name and integrates better with other encoding
     formats.
+-   Enums now have a `<EnumName>_Values()` function which returns all known
+    values for that enum.
 
 
 v1.2.0 (2017-04-17)

gen/enum.go

@@ -75,6 +75,10 @@ func enum(g Generator, spec *compile.EnumSpec) error {
 				<enumItemName $enumName .> <$enumName> = <.Value>
 			<end>
 			)
+
+		func <$enumName>_Values() []<$enumName> {
+			return []<$enumName>{<range .Spec.Items><enumItemName $enumName .>,<end>}
+		}
 		<end>
 
 		<$v := newVar "v">

gen/enum_test.go

@@ -310,6 +310,12 @@ func TestInvalidJSON(t *testing.T) {
 		})
 	}
 }
+
+func TestEnumValuesCanBeListed(t *testing.T) {
+	values := te.EnumDefault_Values()
+	assert.Equal(t, values, []te.EnumDefault{te.EnumDefaultFoo, te.EnumDefaultBar, te.EnumDefaultBaz})
+}
+
 func TestUnmarshalTextReturnsValue(t *testing.T) {
 	var v te.EnumDefault
 	err := v.UnmarshalText([]byte("Foo"))

gen/testdata/thrift/enums.thrift

@@ -35,3 +35,6 @@ enum RecordType {
 enum lowerCaseEnum {
     containing, lower_case, items
 }
+
+// collision with RecordType_Values() function.
+enum RecordType_Values { FOO, BAR }