Scala: custom control structures with several code blocks

Question

Is it possible to create a custom control structure with several code blocks, in the fashion of before { block1 } then { block2 } finally { block3 }? The question is about the sugar part only - I know the functionality can be easily achieved by passing the three blocks to a method, like doInSequence(block1, block2, block3).

A real life example. For my testing utilities I'd like to create a structure like this:

getTime(1000) {
  // Stuff I want to repeat 1000 times.
} after { (n, t) => 
  println("Average time: " + t / n)
}

EDIT:

Finally I came up with this solution:

object MyTimer {
  def getTime(count: Int)(action : => Unit): MyTimer = {
    val start = System.currentTimeMillis()
    for(i <- 1 to count) { action }
    val time = System.currentTimeMillis() - start
    new MyTimer(count, time)
  }
}

class MyTimer(val count: Int, val time: Long) {
  def after(action: (Int, Long) => Unit) = {
    action(count, time)
  }
}

// Test
import MyTimer._

var i = 1
getTime(100) {
  println(i)
  i += 1
  Thread.sleep(10)
} after { (n, t) => 
  println("Average time: " + t.toDouble / n)
}

The output is:

1
2
3
...
99
100
Average time: 10.23

It is mostly based on the answer by Thomas Lockney, I just added the companion object to be able to import MyTimer._

Thank you all, guys.

Virgel

Answer 1

For your given example, the key would be to have the return type of getTime have the after method on it. Depending on the context, you could use a single class or trait which wraps up both methods. Here's a very simplified example of how you might approach it:

class Example() {
  def getTime(x: Int)(f : => Unit): Example = {
    for(i <- 0 to x) {
      // do some stuff
      f
      // do some more stuff
    }
    // calculate your average
    this
  }
  def after(f: (Int, Double) => Unit) = {
    // do more stuff
  }
}

Size

Answer 2

General principle. You can of course have f take parameters as well. (Note that the name of the methods have no meaning in this example.)

scala> class Foo {
     | def before(f: => Unit) = { f; this }
     | def then(f: => Unit) = { f; this }
     | def after(f: => Unit) = { f; this }
     | }
defined class Foo

scala> object Foo { def apply() = new Foo }
defined module Foo

scala> Foo() before { println("before...") } then {
     | println("then...") } after {
     | println("after...") }
before...
then...
after...
res12: Foo = Foo@1f16e6e

Agni

That's embarrassingly simple :). Thanks – Virgel

Answer 3

If you want these blocks to appear in the specific order, this change to Knut Arne Vedaa's answer would work:

class Foo1 {
  def before(f: => Unit) = { f; new Foo2 }
}

class Foo2 {
  def then(f: => Unit) = { f; new Foo3 }
}

...

Nathanialnathaniel

Answer 4

It is not possible to have a "split" method, but you can emulate it.

class Finally(b: => Unit, t: => Unit) {
    def `finally`(f: => Unit) = {
        b
        try { t } finally { f }
    }
}

class Then(b: => Unit) {
    def `then`(t: => Unit): Finally = new Finally(b, t)
}

def before(b: => Unit): Then = new Then(b)

scala> before { println("Before") } `then` { 2 / 0 } `finally` { println("finally") }
Before
finally
[line4.apply$mcV$sp] (<console>:9)
(access lastException for the full trace)
scala>

Zoltai