Making internal state functional

by Stephen Compall on May 10, 2016


This is the ninth of a series of articles on “Type Parameters and Type Members”.

Scala’s CanBuildFrom API is relatively well-founded and flexible; in combination with GADTs, it can provide that flexibility in a fully type-safe way, if users choose not to circumvent it with typecasting.

However, it is designed in a purely mutable way; you cannot write a useful CanBuildFrom that does not rely on mutation, and you cannot use the API externally in a functional way.

Let’s design an alternative to CanBuildFrom that makes sense in a purely functional context, allowing both implementers and users to avoid unsightly mutation.

Spoiler warning! Our first pass will have one glaring inelegance. We will use concepts from previous articles in Type Parameters and Type Members to “invert the abstraction”, which will greatly simplify the design. Once you’re comfortable with the “inversion”, you can skip the intermediate step and use this technique directly in your own designs.

Disallowing functional approaches

The pattern of use of CanBuildFrom is

  1. apply the CBF to produce a Builder.
  2. Call += and ++= methods to “fill up” the Builder.
  3. Call result to “finalize” or “commit” to the final structure.
import collection.generic.CanBuildFrom

val cbf = implicitly[CanBuildFrom[Nothing, Int, List[Int]]]
val b = cbf()
b += 3
b ++= Seq(4, 5)

res0: List[Int] = List(3, 4, 5)

Let’s set aside that this is only suited to eager collections, not lazy ones like Stream. You can tell the problem by types: += and ++= have the return type this.type. Effectively, this means that if their implementations are purely functional, all they can do is return this:

  def +=(elem: Elem) = this
  def ++=(elems: TraversableOnce[Elem]) = this

Aside from the informal contract of Builder, which suggests that calls to these methods perform a side effect, the types enforce that they must perform any useful work by means of side effects.

Returning this.type permits these methods to be called in a superficially functional style:

 .++=(Seq(4, 5))

res1: List[Int] = List(3, 4, 5)

This retouch is only skin-deep, and can’t repair the defect making CanBuildFrom unsuitable for functional programs, but it implies that a functional alternative lurks nearby. Let’s go looking for it.

Step 1: explicit Builder state

First, we need to take the essential mutation out of Builder. That means it needs to provide an initial state, and the other methods must use it as a parameter and return value.

  1. We’ll add a new method to return the initial state.
  2. += and ++= will take that state as an argument, returning the new state instead of this.type.
  3. result will take the final state as an argument, still producing the result collection.

While the intermediate state might be the same as the final state, we don’t want to require that. So Builder also gains a type parameter to represent the type of state, S.

trait FunBuilder[S, -Elem, +To] {
  /** Produce the initial state. */
  def init: S

  // note everywhere 'S' was added
  def +=(s: S, elem: Elem): S
  def ++=(s: S, elems: TraversableOnce[Elem]): S
  def result(s: S): To

A sample FunBuilder

We can incrementally build a Vector, but it may not be the most efficient way. Instead, let’s try to accumulate a List, then construct the Vector once we’re done.

class VectorBuilderList[A]
    extends FunBuilder[List[A], A, Vector[A]] {

  def init = List()
  def +=(s: List[A], elem: A) = elem :: s
  def ++=(s: List[A], elems: TraversableOnce[A]) =
    elems.toList reverse_::: s
  def result(s: List[A]) =

val vbl = new VectorBuilderList[Int]
vbl.result(vbl.++=(vbl.+=(vbl.init, 2), Seq(3, 4)))

res0: scala.collection.immutable.Vector[Int] = Vector(2, 3, 4)

(There’s a problem with CanBuildFrom now, but we’ll hold off fixing it.)

A slightly different Builder

Maybe it would be better to optimize for the ++= “bulk add” method, though.

class VectorBuilderListList[A]
    extends FunBuilder[List[Traversable[A]], A, Vector[A]] {
  def init = List()
  def +=(s: List[Traversable[A]], elem: A) =
    Traversable(elem) :: s
  def ++=(s: List[Traversable[A]], elems: TraversableOnce[A]) =
    elems.toTraversable :: s
  def result(s: List[Traversable[A]]) =
    s.foldLeft(Vector[A]()){(z, as) => as ++: z}

val vbll = new VectorBuilderListList[Int]
vbll.result(vbll.++=(vbll.+=(vbll.init, 2), Seq(3, 4)))

res0: scala.collection.immutable.Vector[Int] = Vector(2, 3, 4)

Hide your state

The type of these builders are different, even though their usage is the same. This design also exposes what was originally internal state as part of the API. Luckily, CanBuildFrom makes a point of this when we try to integrate FunBuilder into our own CBF version; there’s nowhere to put the S type parameter.

trait FunCanBuildFrom[-From, -Elem, +To] {
  def apply(): FunBuilder[S, Elem, To]

…/FCBF.scala:42: not found: type S
  def apply(): FunBuilder[S, Elem, To]

We can hide the state by forcing the caller to deal with the builder in a state-generic context. One way to do this is with a generic continuation.

trait BuilderCont[+Elem, -To, +Z] {
  def continue[S](builder: FunBuilder[S, Elem, To]): Z

// in FunCanBuildFrom...
  def apply[Z](cont: BuilderCont[Elem, To, Z]): Z

Now we can implement a FunCanBuildFrom that can use either of the FunBuilders we’ve defined.

class VectorCBF[A](bulkOptimized: Boolean)
    extends FunCanBuildFrom[Any, A, Vector[A]] {
  def apply[Z](cont: BuilderCont[A, Vector[A], Z]) =
    if (bulkOptimized)
      cont continue (new VectorBuilderListList)
      cont continue (new VectorBuilderList)

Take a look at the type flow. The caller of apply is the one who decides the Z type. But the apply implementation chooses the S to pass to continue, which cannot know any more about what that state type is. (It can even choose different types based on runtime decisions.) Information hiding is restored.

val cbf = new VectorCBF[Int](true)
cbf{new BuilderCont[Int, Vector[Int], Vector[Int]] {
  def continue[S](vbl: FunBuilder[S, Int, Vector[Int]]) =
    vbl.result(vbl.++=(vbl.+=(vbl.init, 2), Seq(3, 4)))

res1: Vector[Int] = Vector(2, 3, 4)

Now the code using the FunBuilder can’t fiddle with the FunBuilder’s state values; it can only rewind to previously seen states, a norm to be expected in functional programming with persistent state values.

Existential types are abstraction inversion

This is rather a lot of inconvenient ceremony, though. Instead of passing a continuation that receives the S type as an argument along with the FunBuilder, let’s just have apply return the type along with the FunBuilder. We have a tool for returning a pair of type and value using that type.

  def apply(): FunBuilder[_, Elem, To]

Remember that existential types are pairs.

Having collapsed callee-of-callee back to caller perspective, let’s apply the rule of thumb from the first post in this series.

A type parameter is usually more convenient and harder to screw up, but if you intend to use it existentially in most cases, changing it to a member is probably better.

The usual case will be from the perspective of a CBF user, so the usual use of the S parameter is existential. So let’s turn it into the equivalent type member.

// rewrite the heading of FunBuilder as
trait FunBuilder[-Elem, +To] {
  type S
// and FunCanBuildFrom#apply as
  def apply(): FunBuilder[Elem, To]

// and the parameter S moves to a member
// for all implementations so far;
// fix until compile or see appendix

And we can see the information stays hidden.

scala> val cbf = new VectorCBF[Int](true)
cbf: fcbf.VectorCBF[Int] = fcbf.VectorCBF@4363e2ba

scala> val vb = cbf()
vb: fcbf.FunBuilder[Int,Vector[Int]] = fcbf.VectorBuilderListList@527c222e

scala> val with1 = vb.+=(vb.init, 2)
with1: vb.S = List(List(2))

scala> val with2 = vb.++=(with1, Seq(2, 3))
with2: vb.S = List(List(2, 3), List(2))

scala> vb.result(with2)
res0: Vector[Int] = Vector(2, 2, 3)

As in “Values never change types”, vb.S is abstract, existential, irreducible.

Last minute adjustments

Builder had to be separate from CanBuildFrom because the latter had to be stateless, with Builder needing to be stateful. Now that both are stateless, the FunBuilder API can probably be collapsed into FunCanBuildFrom.

This leaves the question, what about the mutable-state Builders? They can mutate the S, returning the input state from += and ++=. You can’t use S values to rewind such a FunBuilder, but you couldn’t before, anyway.

In the next part, “Avoiding refinement with dependent method types”, we’ll look at the meaning of Scala’s “dependent method types” feature, using it to replace some more type parameters with type members in non-existential use cases.

This article was tested with Scala 2.11.8.

Appendix: final FunBuilder examples

The rewrite from S type parameter to member in the FunBuilder implementations is a boring, mechanical transform, but I’ve included it here for easy reference.

class VectorBuilderList[A]
    extends FunBuilder[A, Vector[A]] {
  type S = List[A]

  def init = List()
  def +=(s: S, elem: A) = elem :: s
  def ++=(s: S, elems: TraversableOnce[A]) =
    elems.toList reverse_::: s
  def result(s: S) =

class VectorBuilderListList[A]
    extends FunBuilder[A, Vector[A]] {
  type S = List[Traversable[A]]

  def init = List()
  def +=(s: S, elem: A) =
    Traversable(elem) :: s
  def ++=(s: S, elems: TraversableOnce[A]) =
    elems.toTraversable :: s
  def result(s: S) =
    s.foldLeft(Vector[A]()){(z, as) => as ++: z}

class VectorCBF[A](bulkOptimized: Boolean)
    extends FunCanBuildFrom[Any, A, Vector[A]] {
  def apply() =
    if (bulkOptimized)
      new VectorBuilderListList
      new VectorBuilderList