Custom Types

In addition to all of the standard Thrift types, Spindle codegen makes available a few extras.

Enhanced Types

Enhanced types are types tagged with an annotation so as to produce either a custom protocol format or a custom Scala type. Enhanced types are defined with the enhanced_types annotation on a Thrift typedef, along with custom logic in the code generator to deal with the enhanced type. For example:

// A BSON ObjectId, stored as a binary blob.
typedef binary (enhanced_types="bson:ObjectId") ObjectId

// A UTC datetime, stored as millis since the epoch.
typedef i64 (enhanced_types="bson:DateTime") DateTime

Without the enhanced types mechanism, an ObjectId would be serialized as binary over the wire and represented as a ByteBuffer or an Array[Byte] in Scala code. With the enhanced types mechanism, this will be serialized as binary in the binary protocols (TBinaryProtocol, TCompactProtocol) but receive special handling in TBSONObjectProtocol (using the native ObjectId type) and in TReadableJSONProtocol (using a custom ObjectId encoding). In the Scala code, it will be represented as an instance of ObjectId.


In order to use space more efficiently, sometimes you want to store several boolean values into a single integer (i32) or long (i64) value. Spindle calls these fields “bitfields”. Bitfields come in two variants, with and without “set” bits. (With “set” bits, a single boolean value will take two bits, one to determine whether or not the boolean is set, and another for the boolean value itself.)

Spindle has some features to make working with bitfields more convenient. You can associate a i32 or i64 field with a “bitfield struct”. A bitfield struct is a Thrift struct with only boolean fields. If a field foo is marked with a bitfield_struct or bitfield_struct_no_setbits annotation, then an additional fooStruct method will be generated which returns a populated boolean struct.

Bitfield annotations are applied directly to a class field (not through a typedef), as follows:

struct Checkin {
  1: optional i32 flags (bitfield_struct="CheckinFlags")

struct CheckinFlags {
  1: optional bool sendToTwitter
  2: optional bool sendToFacebook
  3: optional bool geoCheat

In this example, the Checkin struct will have all the normal methods for dealing with flags as an i32, as well as a flagsStruct method that returns an instance of CheckinFlags.

Type-safe IDs

You often use BSON ObjectIds as primary keys in Mongo collections. This means common collectons like Checkin and Venue would use the same type as their primary key. In order to avoid errors (such as passing a List[ObjectId] of checkin IDs to a method expecting a List[ObjectId] of venue IDs), Spindle includes a mechanism for tagging common types so they have distinct type-safe version.

This behavior is triggered by using the new_type="true" annotation on a typedef. In Scala code, this will turn the type alias into a tagged type, which means it’s a new subtype of the aliased type. For example, CheckinId is a tagged ObjectId (unique subtype of ObjectId). Because it’s a subtype, you can use CheckinId anywhere you would expect an ObjectId. In order to use an ObjectId where a CheckinId is required, you will need to cast it. A convenience method (with the same name as the type) will be generated to perform this cast. For example: CheckinId(new ObjectId()).

Sample usage, in Thrift (ids.thrift):

package com.foursquare.types.gen

typedef ObjectId CheckinId (new_type="true")
typedef ObjectId VenueId (new_type="true")

struct Checkin {
  1: optional CheckinId id (wire_name="_id")

struct Venue {
  1: optional VenueId id (wire_name="_id")

And in Scala:

import com.foursquare.types.gen.IdsTypedefs.{CheckinId, VenueId}

// cast a regular ObjectId to CheckinId
val checkinId: CheckinId = CheckinId(new ObjectId())

// cast a regular ObjectId to VenueId
val venueId: VenueId = VenueId(new ObjectId())

// compile error, this won't work because VenueId and CheckinId are different subtypes of ObjectId
val checkinId2: CheckinId = venueId

// works, because CheckinId is a subtype of ObjectId and can be automatically upcast
val unsafeCheckinId: ObjectId = checkinId

Note that the tagged types live in an object with the same name as the thrift file with Typedefs appended to the end.