Scala DSL - implicit methods of a function object

Question

In my DSL I want to be able to do like this:

val a = 5

Parse("TYPE") {
  a match {
    case 3 => info("almost")
    case s: String => error("wrong type")
    case 5 => info("you won!")
    case _ => error("omg")
  }
}

with output

[INFO] TYPE: you won!

where Parse is a function object, which has the apply(String), info(String) and error(String) methods:

object Parse {

  def apply(name: String)(parseF: => Unit) { parseF }
  def info(text: String) { println("[INFO] " + name + ": " + text) }
  def error(text: String) { println("[ERROR] " + name + ": " + text) }

}

The trick is that the output of the info and error methods should be somehow native to the Parse object and construct the message by the example as shown above. Therefore, they

have to be accessible like in the example, without any imports.
have to have the access to the first argument passed to Parse() ("TYPE" in the example).
must not create any instances or extra objects during their working.

This is an ideal description. I think that it will require some more boilerplate. Please suggest, how can I acheive that?

EDIT: my initial guess to declare the info and error methods inside the apply method does not make them visible in the parseF method being passed. Well, unsurprisingly..

Rogach · Accepted Answer · 2012-09-19 11:29:26Z

2

Maybe something like this:

object Parse {
  val currentName = new util.DynamicVariable("<none>")
  def apply(name: String)(parseF: => Unit) = currentName.withValue(name)(parseF)
  def info(text: String) = println("[INFO] %s: %s" format (currentName.value, text)
}

// usage
import Parse._ // there's no other way to get ability to call 'info' without 'Parse'.
Parse("TYPE") {
  // some code
  info("some info") // prints: [INFO] TYPE: some info
}

info("some other info") // prints: [INFO] <none>: some other info

If needed, it's easy to make info throw exception if it is called outside Parse {} block.

answered Sep 19, 2012 at 11:29

Rogach

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Comments

Kim Stebel · Accepted Answer · 2012-09-19 12:12:14Z

1

There is no solution that satisfies 1-3. You can have 1 and 2 or 2 and 3 with slightly different syntax.

1 and 2)

class Parse(val s:String) {
  def info....

}

new Parse("TYPE") {

}

2 and 3)

object Parse {
  def apply(s:String)(f:String=>Unit) = ...
}

Parse("TYPE")(s => {

})

answered Sep 19, 2012 at 12:12

Kim Stebel

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