Scala Spores, safe mobile closures: SIP-21

The Spores project is built and tested using sbt. It has two modules: spores-core and spores-pickling. The spores-core module contains the core type definitions and the spore macro. The spores-pickling module integrates Spores with scala/pickling by providing picklers for Spores.

To build the core Spores module:

> project spores-core
> compile

To run the test suite:

> test

The spores-core and spores-pickling modules for Scala 2.11 are available on Maven Central and Sonatype. You can use Spores in your sbt project by simply adding the following dependency to your build file:

libraryDependencies += "org.scala-lang.modules" %% "spores-core" % "0.2.4"

To enable integration with Pickling, add the following dependency:

libraryDependencies += "org.scala-lang.modules" %% "spores-pickling" % "0.2.4"

Or you can just directly download the jar files (spores-core, spores-pickling).

Many users expressed concern that spores would be both unusable with for- expression syntax, and would be incompatible (without a lot of boilerplate) with normal closures or higher-order functions which take normal functions as arguments.

To remedy the incompatibility with for-expressions, we propose a new capture syntax. Here is an exemplary use in the context of a hypothetical DCollection type:

def lookup(i: Int): DCollection[Int] = ...
val indices: DCollection[Int] = ...

for { i <- indices
      j <- lookup(i)
} yield j + capture(i)

trait DCollection[A] {
  def map[B](sp: Spore[A, B]): DCollection[B]
  def flatMap[B](sp: Spore[A, DCollection[B]): DCollection[B]
}

A stable path is an expression which only contains selections and identifiers (no applications, for example), and for which each selected entity is stable. In this context, stable means that the entity or object in question is introduced by object definitions or by value definitions of non-volatile types.

Adapted from the Scala Language specification (section 3.1), a path is defined to be one of the following:

• C.this, where C references a class. The path this is taken as a shorthand for C.this where C is the name of the class directly enclosing the reference.
• x where x is a package.
• p.x where p is a path and x is a stable member of p. Stable members are packages or members introduced by object definitions or by value definitions of non-volatile types. (Section 3.6 of the SLS.)
• C.super.x or C.super[M].x where C references a class and x references a stable member of the super class or designated parent class M of C. The prefix super is taken as a shorthand for C.super where C is the name of the class directly enclosing the reference.

A path refers to an object, that is, it ends with an identifier.

• Need to make it more convenient to create a nullary spore
• Should objects be allowed in paths? The reason is that they are initialized lazily, so if we don't allow lazy vals, then allowing objects (which could end up being initialized only when the spore is applied) doesn't make a lot of sense.