value-wrapper

A micro-library to provide a convenient way of wrapping and unwrapping scala value classes

A value class is a case class, of type W, that wraps a single value, of type V and optionally extends AnyVal. Value classes are a really convenient and lightweight way of strongly typing primitive values to avoid passing them to functions incorrectly.

For example:

case class PersonId(id: Long) extends AnyVal
case class AccountId(id: Long) extends AnyVal

case class Person(id: PersonId, name: String)
case class Account(id: AccountId, name: String)

def getPerson(id: PersonId): Person
def getAccount(id: Account): Account

Although both Persons and Accounts are identified by Longs, by wrapping those Long values in different value classes we can be sure that we can't accidentally pass an identifier to a Person to the getAccount function without the compiler telling us we've done the wrong thing.

Value classes can cause some unwanted friction

As useful as value classes are, there are a lot of situations where wrapping and unwrapping the values can cause enough friction in your code that you wonder if they're worth the effort. For example, if you render the Person class defined above to JSON using Play Framework's very handy macro-generated Writes instances you end up with an undesirable extra level of structure in the output, e.g.

    {
        "id": {
            "id": 1
        },
        "name": "Fred"
    }

when we'd rather just have

    {
        "id": 1,
        "name": "Fred"
    }

To get around this you end up having to write your own Writes, and associated Reads implementation to deal with the extra structure. This quickly adds up to a lot of annoying boilerplate. The purpose of the value-wrapper library is to eliminate that boilerplate. By providing instances of ValueWrapper for a given wrapper and wrapped type we can build generic versions of things like Reads and Writes that work for all value classes.

Going fully generic with Shapeless

But building instances of ValueWrapper for our value classes still leaves us with boilerplate like this:

implicit val personIdWrapper = new ValueWrapper[PersonId, Long]  {
  def wrap(l: Long) : Either[String, PersonId] = Right(PersonId(l))  // or could do validation here
  def unwrap(p: PersonId): Long = p.id
}

This is less boilerplate than before and the typeclass instance can be used to construct multiple different kinds of other typeclasses, e.g. Play's QueryStringBindables and PathBindables, but given that the implementation is pretty much the same for all single-value case classes, wouldn't it be great if we could eliminate it altogether? Turns out we can, with a small dash of Shapeless.

The ValueWrapperGen trait provided in this library provides an implicit function that tells the compiler how to generate a ValueWrapper instance for any single-value case class (and, in fact, any class that Shapeless considers to look enough like a single-value case class that it can build an appropriate Generic instance for). Extend ValueWrapperGen or import ValueWrapperGen._ to bring that implicit function into scope and any other function you have that defines an implicit parameter of type ValueWrapper[W, V] will get provided with a compiler-generated instance with no additional code!

So now you can write something like this:

trait ValueClassWrites extends ValueWrapperGen {
  implicit def genericWrites[W, V](implicit vw: ValueWrapper[W, V],
                                   wv: Writes[V]): Writes[W] =
    new Writes[W] {
      override def writes(w: W): JsValue = wv.writes(vw.unwrap(w))
    }
}

and use that implicit function to generate a Writes instance for any value class. In fact, this is exactly what the play-bindings library provides, as well as generic instance constructors for Reads, PathBindable and QueryStringBindable.

Validation and Normalisation of values

Sometimes you want to validate the value before allowing the value class to be created. value-wrapper provides a type class called Validator[W, V] that will let you do this. For example, we might want to ensure that the Long being used to construct a UserId must be non-negative. We could provide an instance of Validator[W, V] that looks like this:

implicit val vl = new Validator[UserId, Long] {
  override def validate(l: Long): Either[String, Long] = if (l >= 0) Right(l) else Left(s"Id must be non-negative ($l)")
}

If this validator is in implicit scope at the point where the compiler is instantiating a ValueWrapper for UserId then it will get picked up and used as part of the code that constructs the instance of W.

Looking at the declaration of validate there are a couple of things to note:

• If validation is successful then it returns a Right[V], not a Right[W]. The purpose of validate is to validate the value of type V in the context of the type W, not to construct the instance of W. This allows us to define different validations applied to a primitive type, say Long depending on the type it is going to be wrapped in.
• The value wrapped in the Right[V] is the value that will be used by the binders to construct the instance of W. This gives us the chance to change the value as part of the validation. For example, we might want to normalise strings by stripping whitespace like this:

case class Foo(s:String)

implicit val vl = new Validator[Foo, String] {
  override def validate(s:String): Either[String, String] = Right(s.trim)
}