Thursday, August 16, 2007

The Kingdom of Nerbs

In his famous rant Execution in the Kingdom of Nouns, Steve Yegge hilariously excoriates Java for being so "noun oriented" that it's difficult to express ideas that are simple to express in functional programming languages. Scala is a language for the JVM that attempts to unify the funcional and object oriented worlds using first class functions, pattern matching, and what I'm going to call nerbs.

First, a short attention span theater version of Steve's post.1

Once Upon A Time In The Land of JVM

Nouns(together): You there, verbs, get encapsulated at once!
Verbs: Oh please set us free.
Noun1 (implements an interface with only one verb): I am a verb. I am a functor!
Nouns(together): Hahahaha, what a clown.
Citizens of functional lands: Look how much power our verbs have.
Verbs: Free, free, set us free.2

So what was to be done for the poor denizens of the JVM?

To Make a Nerb

There are a bunch of language implementations that support functional programming on the JVM to one extent or another (Cal, Jaskell, Kawa, SISC, Jython, JRuby, Groovy, Rhino, ABCL, hey guys, 'sup?). Unfortunately, none of them are named Java.

For this post I'm going to pretend that Scala is the One True Answer(TM). Other answers will require their own fairy tales er blog entries. Scala is interesting because it not only supports "mere" first class functions, it also supports nerbs.

So what is a nerb?3 Good question, I'm so glad you asked. A nerb is a noun that has been verbed. If you don't believe me, go Google it. But before you do, realize that Google is a publicly traded corporation that makes oodles of cash from selling paid advertising using free web search as a loss leader. But in saying "Google it" I'm not referring to the noun that offers free donuts to its programmers, I'm referring to the verb that this noun is most well known for enabling. Hence the ever so slightly tautological definitions "Google: (verb) to search the web using a web search engine provided by Google - (noun) the company that lets you Google(verb)." 4

Before I continue, let me say that the word "nerb" does not from my close scrutiny seem to appear in any portion of the Scala language specification. But it should.

Back to my butchering of Steve's fairy tale. The verbs are clearly functions and the nouns are clearly objects.5 So a nerb would be an object that's been turned into a function. But in my version of the fairy tale, the verbs all laughed at the noun um object pretending to be a function. Why? Well, because while such beasts work they're just so, so, ... verbose and awkward. Here's one in Java:

public interface Transformer<P, R> {
  public R execute(P param)
}

public class NumberToFrenchStringTransformer 
    implements Transformer<Integer, String> {
  public String execute(Integer param) {
    switch (param.intValue()) {
      case 0: return "zero";
      case 1: return "un";
      case 2: return "deux";
      case 3: return "trois";
      default: throw new 
        RuntimeException("Je ne sais quoi");
    }
  }
}

public class Test {
  void countToThree(Transformer<Integer, String> 
      numberToStringTransformer) {
    for (int i=1;i<4;i++)
      System.out.println(
        numberToStringTransformer.execute(
          new Integer(i)));
  }
  
  public static void main(String[] args) {
    new Test().countToThree(
      new NumberToFrenchStringTransformer());
  }
}

Transformer is a generic interface that requires two types to fully specify: a parameter and a return type. The countToThree method requires a Transformer from Integer to String. The goal is that another Transformer could allow counToThree to speak in some other language, but this code only implements French. The main method hooks everything together.

For a toy example this may not seem so bad. But consider: every time you want to turn an existing function into an object you recreate a significant amount of boilerplate. Imagine writing a significant portion of a real application this way. I could have made it slightly more concise by using an anonymous class, but that would have been a minor change. Let's clean things up for the Scala interpreter.6

def numberToGermanString(n: Int) = n match {
  case 0 => "null"
  case 1 => "ein"
  case 2 => "zwei"
  case 3 => "drei"
  case _ => throw new RuntimeException(
    "Ich bin nicht ein Berliner")
}

def countToThree(
    numberToString: Int => String) =
  for (n <- List.range(1,4)) 
    println(numberToString(n))

countToThree(numberToGermanString)

The play by play: numberToGermanString is a function from integers to strings, countToThree just happens to require such a function, and the last line makes everybody go to town. It's concise and exactly to the point. In a lot of ways it's similar to any other language with first class functions. Again, I could have used an anonymous function but that would have been a minor change. Still we haven't created a nerb - at least not obviously. First class functions are more like gerunds - verbs turned into nouns.

To motivate the desire for a nerb, I ask you this: what else, when given an integer can return a string? Well, how 'bout an array of strings? I can hear you now, "yeah, but an array is a noun not a v.... hey...wait a minute..."

def countToThree(numberToString: Int => String) =
  for (n <- List.range(1,4))
    println(numberToString(n))
  
val words = 
  Array("none", "one", "a couple", "many")
  
countToThree(words)

Yup, in Scala an Array is a nerb. Clearly, an Array is a noun. But we didn't change countToThree a bit and it took the array - the array was verbed. Cool, eh? In scala, getting the 3rd element from the words array just do words(3).

So you might be thinking that arrays must get some special treatment, then, some special syntactic sugar all their own. Nope. You can sprinkle that sugar anywhere you want and turn any of your classes into nerbs.

How? Well, much like you would in Java but you use interfaces (well, traits) that Scala provides.

object NumberToSpanishStringTransformer() 
    extends Function1[Int, String] {
  def apply(n: Int) = n match {
    case 0 => "cero"
    case 1 => "uno"
    case 2 => "dos"
    case 3 => "tres"
    case _ => throw new 
      RuntimeException("Soy un sustantivo")
  }
}

def countToThree(
    numberToString: Int => String) =
  for (n <- List.range(1,4)) 
    println(numberToString(n))
  
countToThree(NumberToSpanishStringTransformer)

Obviously in this case it would be cleaner if I just used a function instead of creating a nerb. But the point here is that I CAN create a nerb - which is nice when I have a more complex class like Array. The "object" keyword means I'm creating a singleton. Function1 is an interface (trait in Scala terms) that is parameterized by two types. Think of it like a generic in Java or a template class in C++ but using square brackets instead of angle braces. The trait requires me to implement one method: apply. In other words, it's pretty much the same as the Java implementation above, except that I was able to treat my object as a function.

The Nerbs United Shall Never be Divided

Earlier I said of our first class function that we had not created a nerb "at least not obviously." Here's the punch line to my cryptic hint: whenever you use a function in a first class manner Scala implicitly creates a nerb. So this code

def numberToGermanString(n:Int) = ... 

def countToThree(numberToString: Int => String) =
  for (n <- List.range(1,4)) 
    println(numberToString(n))
  
countToThree(numberToGermanString)

translates into something along the lines of

def numberToGermanString(n: Int) = ... 

def countToThree(
    numberToString: Function1[Int, String]) =
  for (n <- List.range(1,4)) 
    println(numberToString.apply(n))
  
val $anonymousNerb = new Function1[Int, String]{
  def apply(n: Int) = numberToGermanString(n)
}

countToThree($anonymousNerb)

The byte code generated won't be exactly that but the the gist will be the same: underneath the hood Scala turns first class function creation into nerb creation. This might just seem like just an implementation detail, but it turns out that the Function1 trait provides additional methods for creating function compositions and those methods can be used anytime you create a first class unary function. It's also how Java code can interact with Scala first class functions.

The Grand Unification

In Scala, classes and objects can be turned into nerbs using the object oriented technique of inheritance. Functions and methods can be turned into gerunds using typical functional programming techniques but the gerunds turn out to be nerbs.

Nerbs are a key part to how Scala unifies the object oriented and functional paradigms. There's much more to the unification story including closures and pattern matching. I'll save those for another time.

Once Upon a Time in the Land of Scala

Unlike the land of Java the land of Scala was all peace and harmony. Verbs could be nouned and nouns could be verbed as needed. Together they held hands and sang the songs of nerbiness.

Footnotes

1. My apologies to Steve. Really, sorry dude.

2. My apologies to The Police. Really, sorry dudes.

3. Due credit - I stole the word "nerb" from Nancy Allison's "Every noun can be...".

4. To be madly self referential I would say the word "Google" has been nerbed.

5. Actually, the fairy tale draws the analogy of nouns being both objects and classes but Steve didn't make much distinction and I didn't either. I also won't make the distinction between functions and methods. Please just read on and pretend I know what I'm analogizing.

6. For compiled Scala we just need to wrap all the code in an object that extends Application. I know, I know, it's ironic after all my talk about setting verbs free, but please bear with me until I can talk about singletons.

9 comments:

Anonymous said...

Great start to the series. Your code is getting cut off, though, e.g. "public class NumberToFrenchStringTransformer implem". Can you fix that somehow? Or link to a download. I'd like to see what you're referring to. Nice style and nice start. Patrick

Anonymous said...

Excellent! This style of conceptual-story-telling works well for me. Looking forward to the next installment. Dave Webb

James Iry said...

Sorry about the format issues gang. They've been fixed. Unfortunately, the result is a bit of vertical stretching that you'd never find in real world code. Ah well, c'est la blog.

Anonymous said...

"SICP"

Pretty sure you mean "SISC" here.

James Iry said...

SICP, SISC, what's the difference? It's Scheme either way. ;-) Fixed.

Anonymous said...

Fun stuff! On a minor note, you can now compile scripts using the -Xscript option, so technically you do not need a wrapper object. -Lex Spoon

Al said...

Just wanted to point out two totally irrelevant errors in your German :-)

The cardinal number one is "Eins". "Ein" is the indefinite article (masculine or neuter, in the nominative case, if you really want to know). And "nicht ein" is unfortunately also wrong - it should be "kein". (The logical state is collapsed into the indefinite article, creating a logstart and not a nerb...)

I'm sure you wanted to know all this :-)

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