netvl / picopickle

A simple yet extensible shapeless-based serialization library


picopickle 0.3.2

picopickle is a serialization library for Scala. Its main features are:

  • Small and almost dependency-less (the core library depends only on shapeless).
  • Extensibility: you can define your own serializators for your types and you can create custom backends, that is, you can use the same library for the different serialization formats (collections, JSON, BSON, etc.); other parts of the serialization behavior like nulls handling can also be customized.
  • Flexibility and convenience: the default serialization format is fine for most uses, but it can be customized almost arbitrarily with support from a convenient converters DSL.
  • Static serialization without reflection: shapeless Generic macros are used to provide serializers for arbitrary types, which means that no reflection is used.


Getting started

The library is published to the Maven central, so you can just add the following line to your build.sbt file in order to use the core library:

libraryDependencies += "io.github.netvl.picopickle" %% "picopickle-core" % "0.3.2"

The library is compiled for Scala versions 2.10, 2.11, 2.12. If you use 2.10, however, you will need to add Macro Paradise compiler plugin because shapeless macros depend on it:

libraryDependencies += compilerPlugin("org.scalamacros" %% "paradise" % "2.0.1" cross CrossVersion.full)
// or
addCompilerPlugin("org.scalamacros" %% "paradise" % "2.0.1" cross CrossVersion.full)

Scala 2.11/12 users do not need this as all relevant macro support is already present in 2.11/12.

Backend dependencies

Picopickle supports different backends. A backend defines the target serialization format, for example, JSON, BSON or just regular collections. The core library provides collections backend, and an additional JSON backend based on Jawn parser is available as picopickle-backend-jawn:

libraryDependencies += "io.github.netvl.picopickle" %% "picopickle-backend-jawn" % "0.3.2"

Jawn backend uses Jawn parser (naturally!) to read JSON strings but it uses custom renderer to print JSON AST as a string in order to keep dependencies to the minimum. This renderer is very basic and does not support pretty-printing; this is something which is likely to be fixed in one of the future versions.

You can create your own backends to support your own data formats; more information on how to do it is available below. It is likely that more officially supported backends will be available later.

Serialization mechanism

picopickle uses the pretty standard typeclass approach where the way the type is serialized or deserialized is defined through implicit objects (called Reader[T] and Writer[T] in picopickle) in scope. The library defines corresponding instances for a lot of standard types:

  • primitives and other basic types: Byte, Short, Int, Long, Float, Double, Boolean, Char, String, Unit, Null, Symbol, Option[T], Either[A, B];
  • tuples (currently generated as a part of build process for lengths from 1 to 22);
  • most of standard Scala collections;
  • sealed trait hierarchies: case classes and case objects, possibly implementing some sealed trait, and the sealed trait itself.

Serializers for sealed trait hierarchies are derived automatically with the help of shapeless LabelledGeneric type class. The library defines several generic instances for the core shapeless types (HList and Coproduct), and shapeless does the hard work of inspecting case classes and sealed traits.

Since sealed trait hierarchies are equivalent to algebraic data types, their representation with the shapeless type is fairly natural: each case class/case object is represented by a HList of corresponding field types labelled with field names, and the whole hierarchy is represented by a Coproduct of the corresponding types which implement the sealed trait.

picopickle also supports recursive types, that is, when a case class eventually depends on itself or on the sealed trait it belongs to, for example:

sealed trait Root
case object A extends Root
case class B(x: Int, b: Option[B]) extends Root  // depends on itself
case class C(next: Root) extends Root  // depends on the sealed trait

picopickle also supports default values and variable arguments in case classes and renaming of fields or sealed trait descendants with a bit of custom macros.


Basic usage

picopickle is structured using the cake pattern, that is, it consists of several traits providing parts of the functionality which are then combined into a single object called a pickler. It provides everything necessary for the serialization via a glob import:

import some.package.SomePickler._
write("Hello") shouldEqual SomePicklerBackend.StringValue("Hello")

The core library and the Jawn backend library provide default picklers, so if you don't need any customization (e.g. you don't need to define custom serializers for your types) you can just import the internals of one of these picklers:

import io.github.netvl.picopickle.backends.collections.CollectionsPickler._
case class A(x: Int, y: String)
write(A(10, "hi")) shouldEqual Map("x" -> 10, "y" -> "hi")
read[A](Map("x" -> 10, "y" -> "hi")) shouldEqual A(10, "hi")

Jawn-based pickler also provides additional functions, readString()/writeString() and readAst()/writeAst(), which [de]serialize objects to strings and JSON AST to strings, respectively:

import io.github.netvl.picopickle.backends.jawn.JsonPickler._
case class A(x: Int, y: String)
writeString(A(10, "hi")) shouldEqual """{"x":10,"y":"hi"}"""
readString[A]("""{"x":10,"y":"hi"}""") shouldEqual A(10, "hi")

Currently the string JSON representation is not prettified (but prettification may be implemented in later versions).

Serializer objects

Sometimes you need to work with serialization and deserializaton in the same piece of code (e.g. writing and reading data from database). Then it would be convenient to have read and write methods fixed for some specific type, both for correctness sake and in order to instantiate corresponding readers and writers in one place (which potentially may speed up the compilation).

picopickle provides a special serializer class which can be constructed for any type which has Reader and Writer instances. This class provides read and write methods specified for the type which this serializer is created for:

import io.github.netvl.picopickle.backends.collections.CollectionsPickler._
case class A(x: Int, y: String)

val aSerializer = serializer[A]
aSerializer.write(A(10, "hi")) shouldEqual Map("x" -> 10, "y" -> "hi")
// aSerializer.write("whatever")  // won't compile - write() accepts values of type A only"x" -> 10, "y" -> "hi") shouldEqual A(10, "hi")
// val x: String ="whatever")  // won't compile - read() returns values of type A

Jawn-based pickler extends this class to provide readString and writeString methods:

import io.github.netvl.picopickle.backends.jawn.JsonPickler._

case class A(x: Int, y: String)

val aSerializer = serializer[A]

aSerializer.writeString(A(10, "hi")) shouldEqual """{"x":10,"y":"hi"}"""
aSerializer.readString("""{"x":10,"y":"hi"}""") shouldEqual A(10, "hi")

Custom picklers

It is possible that you would want to define custom serializers for some of your types. In that case you can define custom serializer instances in a trait which "depends" on BackendComponent and TypesComponent via a self type annotation:

import io.github.netvl.picopickle.{BackendComponent, TypesComponent}
case class DefinedByInt(x: Int, y: String)
trait CustomSerializers {
  this: BackendComponent with TypesComponent =>
  implicit val definedByIntWriter: Writer[DefinedByInt] = Writer {
    case DefinedByInt(x, _) => backend.makeNumber(x)
  implicit val definedByIntReader: Reader[DefinedByInt] = Reader {
    case backend.Extract.Number(x) => DefinedByInt(x.intValue(), x.intValue().toString)

Then this trait should be mixed into the corresponding pickler trait conveniently defined in the library in order to create the pickler object:

import io.github.netvl.picopickle.backends.jawn.JsonPickler
object CustomPickler extends JsonPickler with CustomSerializers

You can also define the serializers directly in the pickler object if they are not supposed to be reused or if you only have one pickler object in your program:

import io.github.netvl.picopickle.backends.jawn.JsonPickler
object CustomPickler extends JsonPickler {
  implicit val definedByIntWriter: Writer[DefinedByInt] = Writer {
    case DefinedByInt(x, _) => backend.makeNumber(x)
  implicit val definedByIntReader: Reader[DefinedByInt] = Reader {
    case backend.Extract.Number(x) => DefinedByInt(x.intValue(), x.intValue().toString)

Alternatively, you can import internals of some pickler and define serializers anywhere, but then you need to add an import for wherever these serializers are defined in:

object CustomPickler extends JsonPickler

object CustomSerializers {
  import CustomPickler._
  implicit val definedByIntWriter: Writer[DefinedByInt] = Writer {
    case DefinedByInt(x, _) => backend.makeNumber(x)
  implicit val definedByIntReader: Reader[DefinedByInt] = Reader {
    case backend.Extract.Number(x) => DefinedByInt(x.intValue(), x.intValue().toString)

import CustomSerializers._

CustomPickler.write(DefinedByInt(10, "10")) shouldEqual """10"""

This way also prohibits you from using the same serializers for different kinds of picklers in your program.

picopickle provides several utilities which help you writing custom serializers and deserializers; at first, however, we need to explain what backends are.


A backend in picopickle defines an intermediate AST called backend representation which is the media which values of other types can be serialized into. For example, for JSON it is JSON AST, that is, a set of classes which together can be used to form any correct JSON object tree. Additionally, a backend provides methods to construct these values generically from basic Scala types and collections and to deconstruct these values back into these basic types.

In general, a backend may be arbitrarily complex. It can consist of a lot of classes with various relationships between them and all the necessary methods to construct them. However, in order to provide the ability to serialize arbitrary types to arbitrary backend representations, some restrictions should be put on the structure of the backend representation, that is, there should be some minimal set of primitives which should be supported by all backends. picopickle requires that all backends support basic JSON-like tree AST, that is, objects keyed by strings, arrays indexed by integers, strings, numbers, booleans and null. Using these primitives, picopickle is able to provide serializers for basic primitive types and sealed trait hierarchies out of the box.

Backend trait is used to represent backends in Scala code. This trait contains abstract types which define the AST and a lot of methods to construct the AST from basic types. Each implementation of this trait should provide the following abstract types:

type BValue
type BObject <: BValue
type BArray <: BValue
type BString <: BValue
type BNumber <: BValue
type BBoolean <: BValue
type BNull <: BValue

Also each implementation must provide a set of methods for converting between these abstract types and basic Scala types. The mapping is as follows:

BObject  -> Map[String, BValue]
BArray   -> Vector[BValue]
BString  -> String
BNumber  -> Number
BBoolean -> Boolean
BNull    -> Null

That is, each backend should provide methods to convert from BValue to Map[String, BValue] and back etc. These methods can be divided into three groups:

  • those which convert Scala values to backend representation: prefixed with make;
  • those which convert backend representation to Scala values: prefixed with from;
  • those which extract concrete backend type (e.g. BObject, BString) from the abstract BValue: prefixed with get.

The last group of methods return Option[<corresponding type>] because they are partial in their nature.

There are also some convenience methods like makeEmptyObject or getArrayValueAt which can be defined via a conversion with the corresponding from method and then a query on the resulting Scala object, but these methods may query the underlying backend representation directly, saving on the intermediate objects construction.

In order to create a custom backend you need to implement Backend trait first:

object MyBackend extends Backend {
  type BValue = ...

Then you need to create a cake component for this backend; this component must implement BackendComponent trait:

trait MyBackendComponent extends BackendComponent {
  override val backend = MyBackend

And finally you should extend DefaultPickler, mixing it with your backend component:

trait MyPickler extends DefaultPickler with MyBackendComponent
object MyPickler extends MyPickler

Naturally, you can choose not to merge the DefaultPickler fully into your pickler if you don't want to, for example, if you don't need the automatic writers materialization for sealed trait hierarchies. In that case you can mix in only those traits you need. See DefaultPickler documentation to find out which components it consists of (TODO).

After this and MyPickler.write methods will work with your backend representation.

Extending backends

Sometimes the set of types and methods provided by the default Backend trait is simply not enough because the desired target representation supports more types. One example is BSON, which supports, aside from all the standard JSON types, also things like date and time, object ids, explicit 32-bit and 64-bit integers and byte arrays. Naturally, one would like to automatically serialize Scala types to the most efficient representation available.

picopickle is extensible in this regard too. Because backends are just implementations of one trait, nothing prevents you adding new concrete types into your backend implementation, adding new conversion functions and build your own serializers which make use of these functions:

// Define a new backend
object BsonBackend extends Backend {
  // implement required types
  override type BValue = BsonValue
  override type BObject = BsonDocument
  // add new types
  type BObjectId = BsonObjectId
  type BInt32 = BsonInt32
  type BInt64 = BsonInt64
  // add new conversion functions, possibly following the existing interface
  def fromObjectId(oid: BObjectId): ObjectId = ...
  def makeObjectId(oid: ObjectId): BObjectId = ...
  def getObjectId(value: BValue): BObjectId = ...
  def fromInt32(n: BInt32): Int = ...
  def fromInt64(n: BInt64): Long = ...

// define a backend component
trait BsonBackendComponent extends BackendComponent {
  override val backend = BsonBackend

// define a trait with custom serializers
trait BsonBackendSerializers {
  // it should depend on the concrete BsonBackendComponent, not on generic BackendComponent
  this: BsonBackendComponent with TypesComponent =>
  import backend._
  // and here we can use all custom functions defined in the custom backend
  implicit val objectIdReadWriter: ReadWriter[ObjectId] = 
    ReadWriter.writing(backend.makeObjectId).reading {
      case bv: BObjectId => backend.fromObjectId(bv)
  implicit val intReadWriter: ReadWriter[Int] =
    ReadWriter.writing(backend.makeInt32).reading {
      case bv: BInt32 => backend.fromInt32(bv)

// finally, define the pickler trait by mixing it all together
trait BsonPickler extends DefaultPickler with BsonBackendComponent with BsonBackendSerializers
object BsonPickler extends BsonPickler

Note that picklers defined in the custom trait will have a greater priority than picklers inherited from the DefaultPickler trait. Therefore, intReadWriter defined in the trait above will be used instead of the intReader/intWriter pair defined in PrimitiveReadWritersComponent which is inherited by DefaultPickler.

You can find an example of this technique in the official BSON backend implementation.

Creating custom serializers

picopickle defines Writer and Reader basic types in TypesComponent which are called serializers. They are responsible for converting arbitrary types to their backend representation and back, respectively. The most basic way to construct custom serializers is to use apply method on Reader and Writer companion objects, which take PartialFunction[backend.BValue, T] and T => backend.BValue, respectively (you can find examples of both above).

(Terminology note: Writer and Reader are called serializers, while typed serialization objects described above, that is, the ones returned by the call of serializer[T] method, are called serializer objects. While related, these are different things. Serializer objects are completely optional, you won't have to use them if you don't want; on the other hand, serializers are the key entities in picopickle and you can't do away with them.)

Any Writer, since it receives a total function, should be able to serialize any values of its corresponding type. Reader, however, can fail to match the backend representation. See below for more information on error handling in picopickle.

TypesComponent also defines a combined serializer called ReadWriter:

type ReadWriter[T] = Reader[T] with Writer[T]

Its companion object also provides convenient facilities to create its instances. The example above can be rewritten with ReadWriter like this:

implicit val definedByIntReadWriter: ReadWriter[DefinedByInt] = ReadWriter.reading {
  case backend.Extract.Number(x) => DefinedByInt(x.intValue(), x.intValue().toString)
}.writing {
  case DefinedByInt(x, _) => backend.makeNumber(x)

You can switch reading/writing branches order if you like.

Extractors and backend conversion implicits

Backend trait provides methods to create and deconstruct objects of backend representation: these are make*, from* and get* methods described above. To simplify writing custom serializers, however, picopickle provides a set of tools which help you writing conversions. The most basic of them are extractors and backend conversion implicits.

Backend object contains several singleton objects with unapply methods which can be used to pattern-match on backend.BValue and obtain the low-level values out of it, for example, to get a Map[String, backend.BValue] out of backend.BObject, if this particular backend.BValue which you're matching on indeed is a backend.BObject:

backend.makeObject(...) match {
  case backend.Extract.Object(m) =>  // m is of type Map[String, backend.BValue]

There are extractors for all of the main backend representation variants:

  • backend.Extract.Object
  • backend.Extract.Array
  • backend.Extract.String
  • backend.Extract.Number
  • backend.Extract.Boolean

Their unapply implementation simply calls corresponding get* and from* methods, like this:

object Extractors {
  object String {
    def unapply(value: BValue): Option[String] = getString(value).map(fromString)

The opposite conversion (from primitives to the backend representation) can be done with make* methods on the backend, but picopickle also provides a set of implicit decorators which provide toBackend method on all of the basic types. These decorators are defined in backend.conversionImplicits object:

import backend.conversionImplicits._
val s: backend.BString = "hello world".toBackend
// the above is equivalent to this:
val s: backend.BString = backend.makeString("hello world")

These implicit methods are somewhat more convenient than make* functions.


Low-level conversions, however, may be overly verbose to write. picopickle provides a declarative way of defining how the backend representation should be translated to the desired Scala objects and vice versa. This is done with converters.

A converter looks much like a ReadWriter; however, it is parameterized by two types, source and target:

trait Converter[-T, +U] {
  def toBackend(v: T): backend.BValue
  def isDefinedAt(bv: backend.BValue): Boolean
  def fromBackend(bv: backend.BValue): U

The converters library defines several implicit conversions which allow any converter to be used as the corresponding Reader, Writer or ReadWriter:

Converter[T, _] -> Writer[T]
Converter[_, U] -> Reader[U]
Converter[T, T] -> ReadWriter[T]

A converter which consumes and produces the same type is called an identity converter for that type. Naturally, only identity converters can be used as ReadWriters. Identity converters have a convenient type alias Converter.Id[T].

Converters library also defines several combinators on converters which allow combining them to obtain new converters, and it also provides built-in converters for basic primitive types and objects and arrays.

For example, here is how you can define a conversion for some case class manually:

case class A(a: Boolean, b: Double)
trait CustomSerializers extends JsonPickler {
  import shapeless._
  import converters._
  val aConverter: Converter.Id[A] = unlift(A.unapply) >>> obj {
    "a" -> bool ::
    "b" -> num.double ::
  } >>> A.apply _
  val aReadWriter: ReadWriter[A] = aConverter  // an implicit conversion is used here

Here obj.apply is used to define an identity converter for Boolean :: Double :: HNil, and >>> operations "prepend" and "append" a deconstructor and a constructor for class A:

A.unapply          : A => Option[(Boolean, Double)]
unlift(A.unapply)  : A => (Boolean, Double)
A.apply _          : (Boolean, Double) => A
obj {
  "a" -> bool ::
  "b" -> num.double ::
}                  : Converter.Id[Boolean :: Double :: HNil]

bool and num.double are identity converters for Boolean and Double, respectively.

>>> operations employ a little of shapeless magic to convert the functions like the ones above to functions which consume and produce HLists. There is also >> combinator which does not use shapeless and "prepends" and "appends" a function of corresponding type directly:

(A => B) >> Converter[B, C] >> (C => D)  ->  Converter[A, D]
// compare:
(A => (T1, T2, ..., Tn)) >>> Converter.Id[T1 :: T2 :: ... :: Tn :: HNil] >>> ((T1, T2, ..., Tn) => A)  ->  Converter.Id[A]

Note that this is very type-safe. For example, if you get the order or the types of fields in obj wrong, it won't compile.

picopickle additionally provides a convenient implicit alias for andThen on functions, also called >>. Together with >> on converters this allows writing chains of transformations easily. For example, suppose you have an object which can be represented as an array of bytes. Then you want to serialize this byte array as a string in Base64 encoding. This can be written as follows:

import java.util.Base64
import java.nio.charset.StandardCharsets
case class Data(s: String)
object Data {
  def asBytes(d: Data) = d.s.getBytes(StandardCharsets.UTF_8)
  def fromBytes(b: Array[Byte]) = Data(new String(b, StandardCharsets.UTF_8))
val dataReadWriter: ReadWriter[Data] =
  Data.asBytes _ >>
  Base64.getEncoder.encodeToString _ >>
  str >>
  Base64.getDecoder.decode _ >>
  Data.fromBytes _

The sequence of functions chained with >> naturally defines the transformation order in both directions.

Similar thing is also possible for arrays. For example, you can serialize your case class as an array of fields:

val aReadWriter: ReadWriter[A] = unlift(A.unapply) >>> arr(bool :: num.double :: HNil) >>> A.apply _

Naturally, there are converters for homogeneous arrays and objects too - they allow mapping to Scala collections:

val intListConv: Converter.Id[List[Int]] =[List].of(
val vecTreeMapConv: Converter.Id[TreeMap[String, Vector[Double]]] =[TreeMap].to([Vector].of(num.double))

There is also a converter which delegates to Reader and Writer if corresponding implicit instances are available:

val optionStringConv: Converter.Id[Option[String]] = value[Option[String]]

You can find more on converters in their Scaladoc section (TODO).

Supported types

By default picopickle provides a lot of serializers for various types which do their best to represent their respective types in the serialized form as close as possible. These serializers are then mixed into a single pickler.

The serializers are defined in a couple of traits:

io.github.netvl.picopickle.{CollectionReaders, CollectionWriters, CollectionReaderWritersComponent}
io.github.netvl.picopickle.{ShapelessReaders, ShapelessWriters, ShapelessReaderWritersComponent}
io.github.netvl.picopickle.{PrimitiveReaders, PrimitiveWriters, PrimitiveReaderWritersComponent}
io.github.netvl.picopickle.{TupleReaders, TupleWriters, TupleReaderWritersComponent}  // generated automatically

Every serializer is an overloadable def or val, so you can easily customize serialization format by overriding the corresponding implicit definition with your own one.

Examples below use JsonPickler, so it is implicitly assumed that something like

import io.github.netvl.picopickle.backends.jawn.JsonPickler._

is present in the code.

Primitives and basic types

picopickle natively supports serialization of all primitive and basic types:

writeString(1: Int)       shouldEqual "1"
writeString(2L: Long)     shouldEqual "2"
writeString(12.2: Double) shouldEqual "3"
writeString('a')          shouldEqual "\"a\""
writeString("hello")      shouldEqual "\"hello\""
writeString(true)         shouldEqual "true"
writeString(false)        shouldEqual "false"
writeString(null)         shouldEqual "null"
writeString('symbol)      shouldEqual "\"symbol\""

By default characters are serialized as strings, but, for example, collections backend redefines this behavior.

picopickle also can serialize Option[T] and Either[L, R] as long as there are serializers for their type parameters:

writeString(Some(1)) shouldEqual "[1]"
writeString(None)    shouldEqual "1"

writeString(Left("hello"))   shouldEqual """[0,"hello"]"""
writeString(Right('goodbye)) shouldEqual """[1,"goodbye"]"""

Optional values are also handled specially when they are a part of case class definition; see below for more explanation.

Please note that Either[L, R] serialization format is not final and can change in future versions.

Numbers and accuracy

Most JSON libraries represent numbers as 64-bit floats, i.e. Doubles, but some numerical values do not fit into Double, and rounding occurs:

80000000000000000.0 shouldEqual 80000000000000008.0   // does not throw

In order to represent numbers as accurately as possible picopickle by default serializes all Longs which cannot be represented as Double precisely as strings:

writeString(80000000000000000L)      shouldEqual "\"80000000000000000\""
writeString(Double.PositiveInfinity) shouldEqual "Infinity"

The same mechanism will probably be used when BigInt/BigDecimal handlers will be added.

In some backends, however, this behavior can be overridden, as is done, for example, in the official BSON backend.


Tuples are serialized as arrays:

writeString((1, true, "a"))  shouldEqual "[1,true,\"a\"]"

The only exception is a tuple of zero items, usually called Unit. It is serialized as an empty object:

writeString(())           shouldEqual "{}"

Naturally, all elements of tuples must be serializable as well.

Tuple serializer instances are generated as a part of build process, and currently only tuples with the length up to and including 22 are supported.


Most of Scala collections library classes are supported, including all of the abstract ones below the Iterable, as well as arrays:

writeString(Iterable(1, 2, 3, 4))    shouldEqual "[1,2,3,4]"
writeString(Seq(1, 2, 3, 4))         shouldEqual "[1,2,3,4]"
writeString(Set(1, 2, 3, 4))         shouldEqual "[1,2,3,4]"
writeString(Map(1 -> 2, 3 -> 4))     shouldEqual "[[1,2],[3,4]]"

writeString(1 :: 2 :: 3 :: Nil)      shouldEqual "[1,2,3]"
writeString(Vector(1, 2, 3))         shouldEqual "[1,2,3]"
wrtieString(TreeMap(1 -> 2, 3 -> 4)) shouldEqual "[[1,2],[3,4]]"

writeString(Array(1, 2, 3))          shouldEqual "[1,2,3]"

Mutable collections can be [de]serialized as well.

Maps are serialized like iterables of two-element tuples, that is, into arrays of two-element arrays. However, if the map has string keys (which is determined statically), it will be serialized as an object:

writeString(Map("a" -> 1, "b" -> 2)) shouldEqual """{"a":1,"b":2}"""

The above behavior of serializing maps with string keys is the default, but it can be extended. See below.

If you're using abstract collection types like Seq, Set or Map, picopickle will work flawlessly. If you use concrete collection types, however, there could be problems. picopickle has a lot of instances for most of the main concrete implementations, but not for all of them. If you need something which is not present in the library, feel free to file an issue.

Map serialization with non-string keys

JSON-like languages usually don't allow using non-string values as object keys, and picopickle enforces this restriction by its BObject representation which requires string keys. However, this is sometimes overly restrictive, especially in a richly typed language like Scala and because of common patterns which follow from this.

It is not unusual for Scala projects to have a newtype or several for String, for example, for different kind of identifiers:

case class PostId(id: String)
case class UserId(id: String)

Alternatively, it is possible to have such simple value class which does not wrap a String but which can easily be converted to and from a string:

case class EntityPath(elems: Vector[String]) {
  override def toString = elems.mkString("/")
object EntityPath {
  def fromString(s: String) = EntityPath(s.split("/").toVector)

It is sometimes desirable to have these classes as keys in maps:

type UserMap = Map[UserId, User]
type EntityLocations = Map[EntityPath, Entity]

One would naturally want for these maps to have an object-based representation (instead of an array of arrays) because keys are easily converted to and from strings. In picopickle, however, only maps of type Map[String, T] can be directly serialized as objects.

To allow this kind of pattern, picopickle provides a way to define custom converters for map keys. When a map with keys of type T is serialized or deserialized, and if there is an instance of type ObjectKeyReader[T]/ObjectKeyWriter[T]/ObjectKeyReadWriter[T] in scope, then it will be used to obtain a String from T (or vice versa) which will then be used as an object key:

implicit val userIdObjectKeyReadWriter = ObjectKeyReadWriter(, UserId)

// below a `_.toString` conversion is implicitly used
implicit val entityPathObjectKeyReadWriter = ObjectKeyReadWriter(EntityPath.fromString)

write[UserMap](Map(UserId("u1") -> ..., UserId("u2") -> ...)) shouldEqual 
  Map("u1" -> ..., "u2" -> ...)
write[EntityLocations](Map(EntityPath(Vector("a", "b")) -> ..., EntityPath(Vector("a", "c")) -> ...)) shouldEqual
  Map("a/b" -> ..., "a/c" -> ...)

// reading works just as well

However, with this flexibility in large codebases where one pickler is shared by lots of different classes it is easy to accidentally add a conversion which would break serialization format in some other part of the project. To allow controlling this picopickle supports disabling of automatic map serialization for unknown key types. You would need then to define an object key serializer for this particular type or explicitly allow maps with this type as a key to be serialized as an array of arrays. You need to create a custom pickler and mix MapPicklingDisabledByDefault trait into it:

object CustomPickler extends CollectionsPickler with MapPicklingDisabledByDefault

// won't compile because there is no ObjectKeyWriter[Int] in scope and serialization of maps
// with Int keys is not allowed
write(Map(1 -> "a", 2 -> "b"))  

// ---

object CustomPickler extends CollectionsPickler with MapPicklingDisabledByDefault {
  implicit val intObjectKeyReadWriter = ObjectKeyReadWriter(_.toInt)

// works because we have defined an object key serializer for Int
write(Map(1 -> "a", 2 -> "b")) shouldEqual Map("1" -> "a", "2" -> "b")

// ---

object CustomPickler extends CollectionsPickler with MapPicklingDisabledByDefault {
  implicit val intObjectKeyAllowed = allowMapPicklingWithKeyOfType[Int]

// works because we explicitly allowed maps of type Map[Int, T] to be serialized as an array of arrays
write(Map(1 -> "a", 2 -> "b")) shouldEqual Vector(Vector(1, "a"), Vector(1, "b"))

Note that currently even if map pickling is allowed like in the above piece of code, putting an object key serializer for the corresponding type will force picopickle to use it, allowing potential unexpected changes of serialization format like described above. However: first, this will be fixed in future versions; second, it still disallows one to accidentally serialize maps as arrays of arrays and then have broken format by deliberate introduction of keys serializer, which looks like the most likely possibility of introducing such breaking changes.

Sealed trait hierarchies

picopickle supports automatic serialization of sealed trait hierarchies (STH), that is, case classes, probably inheriting a sealed trait. In other words, picopickle can serialize algebraic data types.

The most trivial examples of STH are standalone case objects and case classes:

case object A
case class B(x: Int, y: A)

writeString(A)        shouldEqual "{}"
writeString(B(10, A)) shouldEqual """{"x":10,"y":{}}"""

By default picopickle serializes case classes as objects with keys being the names of the fields. Case objects are serialized as empty objects.

Case classes and objects can have a sealed trait as their parent:

sealed trait Root
case object A extends Root
case class B(x: Int, y: Boolean)
case class C(name: String, y: Root) extends Root

When you explicitly set the serialized type to Root (or pass a value of type Root but not of some concrete subclass), it will be serialized as an object with a discriminator key:

writeString[Root](A)           shouldEqual """{"$variant":"A"}"""
writeString[Root](B(10, true)) shouldEqual """{"$variant":"B","x":10,"y":true}"""
writeString[Root](C("me", A))  shouldEqual """{"$variant":"C","name":"me","y":{"$variant":"A"}}"""

If you don't request Root explicitly, the classes will be serialized as if they were not a part of an STH:

writeString(B(10, true)) shouldEqual """{"x":10,"y":true}"""

Usually this is not a problem, however, because if you are working with a sealed trait, you usually have variables of its type, not of its subtypes.

Sealed trait hierarchies serialization is implemented using shapeless LabelledGeneric implicitly materialized instances and a bit of custom macros which handle field renaming and default values (both are not supported by shapeless natively).

Changing the discriminator key

You can customize the discriminator key used by shapeless serializers by overriding discriminatorKey field defined in io.github.netvl.picopickle.SealedTraitDiscriminator trait (its default value is "$variant"):

object CustomPickler extends JsonPickler {
  override val discriminatorKey = "$type"

// STH is from the example above
CustomPickler.writeString[Root](A) shouldEqual """{"$type":"A"}"""

Of course, you can extract it into a separate trait and mix it into different picklers if you want.

Alternatively, since 0.2.0 you can specify the discriminator key for the specific sealed trait hierarchy by putting a @discriminator annotation on the sealed trait:

import io.github.netvl.picopickle.discriminator

@discriminator("status") sealed trait Root
case object Stopped extends Root
case class Running(name: String) extends Root

writeString[Root](Stopped)       shouldEqual """{status:"Stopped"}"""
writeString[Root](Running("me")) shouldEqual """{status:"Running","name":"me"}"""

If @discriminator annotation is present, then its value will be used as discriminator key; otherwise, the default value from discriminatorKey pickler field will be used.

Serialization of optional fields

If a case class has a field of type Option[T], then this field is serialized in a different way than a regular option: if the value of the field is None, then the corresponding key will be absent from the serialized data, and if it is Some(x), then the key will be present and its value will be just x, without an additional layer of an array:

case class A(name: String, x: Option[Long])

writeString(A("absent"))            shouldEqual """{"name":"absent"}"""
writeString(A("present", Some(42L)) shouldEqual """{"name":"present","x":42}"""

This allows easy evolution of your data structures - you can always add an Optional field and the data serialized before this update will still be deserialized correctly, putting a None into the new field.

If an optional field again contains an option:

case class A(x: Option[Option[Long]])

then the "outer" option is serialized as described in the above paragraph while the "inner" option is serialized as a possibly empty array, just like options are serialized in other contexts:

writeString(A(None))            shouldEqual """{}"""
writeString(A(Some(None)))      shouldEqual """{"x":[]}"""
writeString(A(Some(Some(10L)))) shouldEqual """{"x":[10]}"""

Renaming fields and sealed trait variants

picopickle also provides an ability to rename fields and STH variant labels. This can be done by annotating fields with @key annotation:

import io.github.netvl.picopickle.key

sealed trait Root
@key("0") case object A
@key("1") case class B(@key("a") x: Int, @key("b") y: Boolean)

writeString[Root](A)            shouldEqual """{"$variant":"0"}"""
writeString[Root](B(10, false)) shouldEqual """{"$variant":"1","a":10,"b":false}"""

Keys always are strings, though.

Default values of case class fields

picopickle also respects default values defined in a case class, which simplifies changes in your data classes even more. If a field has a default value and the serialized object does not contain the corresponding field, the default value will be used:

case class A(n: Int = 11)

readString[A]("""{"n":22}""") shouldEqual A(22)
readString[A]("""{}""")       shouldEqual A()

As you can see, this mechanism naturally interferes with the optional fields handling. picopickle resolves this conflict in the following way: if no value is present at the corresponding key and a default value is set for the field, then it takes precedence over option handling. This affects a rather rare case when there is an optional field with a default value other than None:

case class A(n: Option[Int] = Some(10))

readString[A]("{}") shouldEqual A(Some(10))  // not A(None)

This is what usually expected in such situation.


Since version 0.2.0 picopickle supports reading and writing case classes with variable arguments. All of the arguments passed to such case class will be serialized as an array:

case class A(x: Int*)

writeString(A(1, 2, 3)) shouldEqual """{"x":[1,2,3]}"""

Naturally, all elements of this array are serialized with their respective serializers.


null value, as is widely known, tends to cause problems, and it is discouraged in idiomatic Scala code. Unfortunately, sometimes you need to interact with external systems which do use nulls. JSON has null value as well. Because of this picopickle supports nulls (it even has BNull as one of the fundamental backend types) but it also provides means to control how nulls should be handled.

Reader and Writer traits do not contain any special logic to handle nulls. Instances of Reader and Writer created through their companion objects, however, do have such logic: they delegate null handling to a NullHandler instance provided by NullHandlerComponent. NullHandler is a trait of the following structure:

trait NullHandler {
  def handlesNull: Boolean
  def toBackend[T](value: T, cont: T => backend.BValue): backend.BValue
  def fromBackend[T](value: backend.BValue, cont: backend.BValue => T): T

That is, it is a kind of a preprocessor which inspects the passed value for nulls and can [de]serialize them specially or prohibit the [de]serialization at all.

By default picopickle allows nulls everywhere (DefaultPickler includes DefaultNullHandlerComponent). That is, if a null is serialized, it will be represented unconditionally with backend.BNull, and backend.BNull will be deserialized (again, unconditionally) as a null.

There is another NullHandlerComponent implementation, namely ProhibitiveNullHandlerComponent, which disallows serialization of nulls, throwing an exception if it encounters a null value either in Scala object or in a backend object. If you don't need to keep compatibility with some external system which uses null values then it may be sensible to extend the desired pickler, overriding the default null handler:

trait MyJsonPickler extends JsonPickler with ProhibitiveNullHandlerComponent

As long as you use Reader/Writer companion objects or converters to create your custom serializers, the null handling behavior will be consistent for all types handled by your pickler.

Accurate numbers serialization

Some backends do not allow serializing some numbers accurately. For example, most JSON implementations represent all numbers with 64-bit floating point numbers, i.e. Doubles. Scala Long, for example, can't be represented accurately with Double in its entirety. This is even more true for big integers and decimals.

picopickle backends provide means to serialize arbitrary numbers as accurately as possible with these methods:

def makeNumberAccurately(n: Number): BValue
def fromNumberAccurately(value: BValue): Number

You can see that these methods take and return BValue instead of BNumber. Backend implementations can take advantage of this and serialize long numbers as strings or in some other format in order to keep the precision. Built-in serializers for numbers use these methods by default.

picopickle also provides a special trait, DoubleOrStringNumberRepr, which provides methods to store a number as a BNumber if it can be represented precisely in Double as a BString otherwise. This trait is useful e.g. when writing a JSON-based backend.

Value classes

With picopickle, you can opt-in to serialize value classes (i.e. the ones extending the AnyVal class) directly as values, bypassing the usual map representation of objects. To enable this behavior, extend your pickler with ValueClassReaderWritersComponent:

trait MyJsonPickler extends JsonPickler with ValueClassReaderWritersComponent
import MyJsonPickler._

class A(val x: Int) extends AnyVal
writeString(A(10)) shouldEqual "10"  // not """{"x":10}"""

Official backends

Collections pickler

picopickle has several "official" backends. One of them, provided by picopickle-core library, allows serialization into a tree of collections. This backend is available immediately with only the core dependency:

libraryDependencies += "io.github.netvl.picopickle" %% "picopickle-core" % "0.3.2"

In this backend the following AST mapping holds:

BValue   -> Any
BObject  -> Map[String, Any]
BArray   -> Vector[Any]
BString  -> String
BNumber  -> Number
BBoolean -> Boolean
BNull    -> Null

In this backend the backend representation coincide with the target media, so no conversion methods except the basic read/write are necessary.

This backend also tweaks the default Char serializer to write and read characters as Chars, not as Strings (which is the default behavior).

Note that everything else, even other collections, are still serialized as usual, so, for example, tuples are represented as vectors and maps are represented as vectors of vectors:

write((2: Int, "abcde": String))  ->  Vector(2, "abcde")
write(Map(1 -> 2, 3 -> 4))        ->  Vector(Vector(1, 2), Vector(3, 4))

Collections pickler also do not use accurate number serialization because its backend representation is already as accurate as possible.

JSON pickler

Another official backend is used for conversion to and from JSON. JSON parsing is done with jawn library; JSON rendering, however, is custom. This backend is available in picopickle-backend-jawn:

libraryDependencies += "io.github.netvl.picopickle" %% "picopickle-backend-jawn" % "0.3.2"

This backend's AST is defined in io.github.netvl.picopickle.backends.jawn.JsonAst and consists of several basic case classes corresponding to JSON basic types. No additional utilities for JSON manipulation are provided; you should use another library if you want this.

JSON backend additionally provides two sets of methods: readAst/writeAst, which convert JSON AST from and to the JSON rendered as a string, and readString/writeString, which [de]serialize directly from and to a string. Usually the last pair of methods is what you want to use when you want to work with JSON serialization.

No support for streaming serialization is available and is not likely to appear in the future because of the abstract nature of backends (not every backend support streaming, for example, collections backend doesn't) and because it would require a completely different architecture.

BSON pickler

Another official backend is used for conversion to and from BSON AST, as defined by MongoDB BSON library.

libraryDependencies += "io.github.netvl.picopickle" %% "picopickle-backend-mongodb-bson" % "0.3.2"

In this backend the following AST mapping holds:

BValue   -> BsonValue
BObject  -> BsonDocument
BArray   -> BsonArray
BString  -> BsonString
BNumber  -> BsonNumber
BBoolean -> BsonBoolean
BNull    -> BsonNull

BSON backend also defines additional types as follows:

BObjectId -> BsonObjectId
BInt32    -> BsonInt32
BInt64    -> BsonInt64
BDouble   -> BsonDouble
BDateTime -> BsonDateTime
BBinary   -> BsonBinary
BSymbol   -> BsonSymbol

Additional functions for conversion from Scala core types to these types are available in the backend:

  def fromBinary(bin: BBinary): Array[Byte]
  def makeBinary(arr: Array[Byte]): BBinary
  def getBinary(value: BValue): Option[BBinary]
  def fromObjectId(oid: BObjectId): ObjectId
  def makeObjectId(oid: ObjectId): BObjectId
  def getObjectId(value: BValue): Option[BObjectId]
  def fromDateTime(dt: BDateTime): Long
  def makeDateTime(n: Long): BDateTime
  def getDateTime(value: BValue): Option[BDateTime]

  def fromSymbol(sym: BSymbol): Symbol
  def makeSymbol(sym: Symbol): BSymbol
  def getSymbol(value: BValue): Option[BSymbol]

  def fromInt32(n: BInt32): Int
  def makeInt32(n: Int): BInt32
  def getInt32(value: BValue): Option[BsonInt32]

  def fromInt64(n: BInt64): Long
  def makeInt64(n: Long): BInt64
  def getInt64(value: BValue): Option[BsonInt64]

  def fromDouble(n: BDouble): Double
  def makeDouble(n: Double): BDouble
  def getDouble(value: BValue): Option[BsonDouble]

Corresponding extractors are available in backend.BsonExtract object, and backend conversion implicits are defined in backend.bsonConversionImplicits:

  Reader {
    case backend.BsonExtract.ObjectId(oid) =>
      // oid: backend.BObjectId == org.bson.BsonObjectId
  import backend.bsonConversionImplicits._
  val bin: backend.BBinary = Array[Byte](1, 2, 3).toBackend

This backend overrides numerical readers and writers to serialize Scala numbers to the smallest type possible, i.e. Byte, Short and Int are serialized as BInt32, Long is serialized as BInt64, and Float and Double are serialized as BDouble. You can see that in this backend there is no need to use additional measures to serialize numbers accurately.

This backend also provides serializers for Array[Byte], Symbol, Date and ObjectId types which are serialized as BBinary, BSymbol, BDateTime and BObjectId, respectively.

And finally, this backend provides identity serializers for all BValue children types, that is, it serializes BValue as BValue, BString as BString, BInt64 as BInt64 and so on.

Error handling

While serialization is straightforward and should never fail (if it does, it is most likely a bug in the library or in some Writer implementation), deserialization is prone to errors because the serialized representation usually has free-form structure and is not statically mapped on its Scala representation.

picopickle has a special exception type which is thrown upon deserialization errors. This exception is defined in ExceptionsComponent like this:

  case class ReadException(message: String, data: backend.BValue, cause: Throwable = null)
    extends BaseException(message, cause)

  object ReadException {
    def apply(reading: String, expected: String, got: backend.BValue): ReadException =
      ReadException(s"reading $reading, expected $expected, got $got", data = got)

When deserialization of some type is attempted over a backend representation which is incompatible with the requested type, for most of the built-in deserializers the exception will contain the message about what was being read, what was expected and what was actually provided to the deserializer:

io.github.netvl.picopickle.ExceptionsComponent$ReadException: reading number, expected number or string containing a number, got JsonObject(Map(some -> JsonString(thing)))

You can participate in this exception handling with your own deserializers very easily. Reader and ReadWriter has certain methods to create deserializers which allow you to use custom messages for errors:

case class A(name: String)

// Pre-defined message format, like above
Reader.reading[A] {
  case backend.Extract.String(s) => A(s)
}.orThrowing(whenReading = "A", expected = "string")

// Arbitrary custom message
Reader.reading[A] {
  case backend.Extract.String(s) => A(s)
}.orThrowing(v => s"Got $v instead of string when reading A")

// ReadWriters also can be customized
  .reading { case backend.Extract.String(s) => A(s) }
  .orThrowing(whenReading = "A", expected = "string")
// Works in any order
ReadWriter.reading[A] { case backend.Extract.String(s) => A(s) }
  .orThrowing(whenReading = "A", expected = "string")

In readers constructed in the above form the error will be thrown when the partial function used for reading is not defined on the incoming value. That is, the following reader won't ever throw a ReadException:

Reader.reading[A] {
  case value => A(backend.fromString(value.asInstanceOf[BString]))
}.orThrowing(whenReading = "A", expected = "string")

It will throw a ClassCastException instead if something which is not a string is supplied.

If you still need to use a catch-all partial function for a reader, you can always throw a ReadException yourself:

Reader[A] {
  case value => if (value.isInstanceOf[String]) A(backend.fromString(value.asInstanceOf[BString])
                else throw ReadException(reading = "A", expected = "string", got = value)

While the example above is absolutely contrived, there are legitimate use cases for it.

Additional backend implementations may inherit ExceptionsComponent.BaseException to implement custom errors. For example, this is done in JSON backend to wrap a Jawn parse exception.

Finally, Pickler trait provides tryRead() method which returns Try[T] instead of T returned by read(). This method never throws any exceptions and instead returns them as a Failure variant of Try[T]. Serializer objects also have such methods, as well as official backends with custom serialization methods, like Jawn's tryReadString().


picopickle does not support serializing Any in any form because it relies on the static knowledge of types being serialized. However, its design, as far as I can tell, in principle does not disallow writing a serializer for Any which would use reflection. This is not even in plans, however.

It also seems that trying to serialize sealed trait hierarchies where the sealed trait itself has a type parameter causes the compiler to die horribly. Regular parameterized case classes work fine, however.

Object graphs with circular loops are not supported and will cause stack overflows. This is not usually a problem because it is only possible to construct such graphs when at least a part of them is mutable (e.g. a var field or a mutable collection) which is discouraged in idiomatic Scala code.

Due to limitations of how Scala reflection/macros work, it is better not to re-define serializers in the same place as the serialized classes if these classes form a sealed trait hierarchy. For example, something like this won't work:

object Serializers {
  import SomePickler._
  sealed trait Root
  case class A(x: Int) extends Root
  case object B extends Root
  implicit val rootReadWriter = ReadWriter[Root]

This won't compile because it is impossible to inspect the sealed trait hierarchy of Root at the point where a LabelledGeneric is materialized here (in the implicit parameters of ReadWriter[Root] call). If you want to pre-generate serializers for your classes, write them in another object:

object Classes {
  sealed trait Root
  case class A(x: Int) extends Root
  case object B extends Root

object Serializers {
  import SomePicker._
  import Classes._
  implicit val rootReadWriter = ReadWriter[Root]


  • Consider adding support for more types
  • Consider adding more converters (e.g. for tuples)
  • Add proper support for error handling in conversions
  • Add more tests
  • Add more documentation



  • Updated scala to 2.12.3


  • Updated scala to 2.11.8
  • Added support for serializing value classes as values


  • Updated shapeless to 2.3.0, macroparadise to 2.1.0, jawn to 0.8.4, bson to 3.2.2, scala to 2.10.6
  • Switched to macro-compat instead of hand-written macro API for 2.10 and 2.11


  • Updated shapeless to 2.2.3, jawn to 0.8.8, scala to 2.11.7
  • Fixed support for varargs (consequence of shapeless update)
  • Improved reader interface (added readOrElse method and changed existing code to depend on it)
  • Added proper error handling (#2)
  • Added new BSON-based backend (#6)
  • Added support for changing STH discriminator key on per-STH basis (#7)


  • Added serializer object feature (#5)
  • Added support for serializing arbitrary types as map keys provided there is a converter (#4)


  • Updated Scala 2.10 minor version (4 -> 5)


  • Fixed handling of classes with overloaded apply method in companions (#1)


  • More serializer instances
  • Added generic handling for accurate numbers serialization
  • Added collections backend
  • Support for recursive types
  • Added converters
  • Improved API for custom serializers
  • Added support for renaming fields and sealed trait variants
  • Added support for default values in case classes
  • Added proper support for nulls
  • Added test generators
  • Started adding tests


  • Added more instances for primitive types
  • Improved API


  • Initial release