Specification of the execution context of a program by choosing its inputs when convenient (e.g. run-time).

In other words, a program that, given a certain external dependency, is ready to execute.

val program =
  e: Employee => {
    println("Employee printer v1.0")
    println("Employee: " + e.name)
  }

To execute, just provide the dependency.

program(new Employee("Fred"))

This is otherwise known as passing arguments

When we write methods, we specify programs with dependencies in the form of arguments.

def logTheRepo(r: Repository, l: Logger) {
  l.log(r.getData())
}

trait Repository { def getData(): List[String] }
trait Logger     { def log(data: List[String]): Unit }

The logTheRepo method can be modularized based on its inputs.

We can rewrite our program as a bunch of nested single-argument functions.

def logTheRepo =
  val r: Repository => {
    val l: Logger => {
      l.log(r.getData())
    }
  }

logTheRepo is a function that, given a Repository, returns a function that takes a Logger and returns Unit.

val iNeedARepository = logTheRepo
val iNeedALogger     = iNeedARepository(someRepository)

iNeedALogger(someLogger)

This nested/callback style is hard to read.

It would be nice if we could do without all this explicit nesting, and write our code in a declarative style.

def logTheRepo =
  {
    val r: Repository = read[Repository]
    val l: Logger     = read[Logger]
    l.log(r.getData())
  }

How do we implement read?

We want read to convert our program into something akin to nested single-argument functions, as before.

def read[A](implicit m: Manifest[A]): A @cpsParam[Any, Program[A]] =
  shift {
    k: (A => Any) => Program(m.erasure.asInstanceOf[Class[A]], k)
  }

case class Program[A](c: Class[A], f: A => Any)

Program(
  c = classOf[Repository],
  f = Repository => Program(
                         c = classOf[Logger],
                         f = Logger => Unit
                       )
)

Now logTheRepo is a Program. How can we execute it?

def interpret(x: Any): Any =
  x match {
    case Program(c, f) if c.isAssignableFrom(classOf[Repository])
      => interpreter(f(FooRepository))
    case Program(c, f) if c.isAssignableFrom(classOf[Logger])
      => interpreter(f(FooLogger))
    case _
      => x
  }

object FooRepository extends Repository {
  def getData() = List("a", "b", "c")
}

object FooLogger extends Logger {
  def log(data: List[String]) = data.foreach(println(_))
}

How do we use this interpreter?

Use Scala's reset function to convert our @cpsParam-annotated function into something we can use directly.

interpret(reset(logTheRepo)) // prints "a", "b", and "c" to new lines

def logTheRepo =
  {
    val r: Repository = read[Repository]
    val l: Logger     = read[Logger]
    l.log(r.getData())
  }

def interpret(x: Any): Any =
  x match {
    case Program(c, f) if c.isAssignableFrom(classOf[Repository]) => interpreter(f(FooRepository))
    case Program(c, f) if c.isAssignableFrom(classOf[Logger])     => interpreter(f(FooLogger))
    case _                                                        => x
  }

Expand Program to two possible types that represent "more stuff to do" and "all done".

sealed trait Program
case class Return(a: Any) extends Program
case class With[A](c: Class[A], f: A => Program) extends Program

Clean up the pattern match.

def logTheRepo =
  program {
    val r: Repository = read[Repository]
    val l: Logger     = read[Logger]
    l.log(r.getData())
  }

def run(p: Program): Any =
  p match {
    case With(c, f) if c.isA[Repository] => run(f(FooRepository))
    case With(c, f) if c.isA[Logger]     => run(f(FooLogger))
    case Return(a)                       => a
  }

Java's type erasure == sadface.