Scala Exchange 2011 - Day 1

I’ve attended a day full of Scala-related talks at Scala Exchange 2011, and, as was the case with the previous events organised by SkillsMatter, I felt that relating what was said and what my impressions were will help me digest all the new pieces of information. So, here it goes.

State of Scala

In the keynote talk Martin Odersky explained how his perception of usefulness of functional programming in the real world shifted from a good fit for XML manipulation to concurrent and parallel computing. Most of the time was spent on explaining how persistent (i.e. immutable) collections provide a good model for implicit parallelism. Starting from a concise solution to a phone number to sentences encoding problem, Martin showed how the parallel collections introduced in Scala 2.9 make it possible to take advantage of multiple cores with minimal code changes; all it took was changing the type of a word list from List[String]to ParVector[String] and calling par method on a range to obtaine 2.5x speed-up on a four-core machine. According to Martin, this is a major step towards solving the Popular Parallel Programming challenge. At the first glance, it might seem like this sort of parallelisation could be done in a way that is transparent to the users of the collections. Unfortunately, this is not the case; for instance, users of Seq expect the order of processing elements to be stable, but ParSeq cannot guarantee this. Because of that, Scala’s collection hierarchy has been expanded to support General* traits, which are used as a base for sequential (as in Scala 2.8) and parallle (Par*) collections. A natural next step for implicit parallelism would be to scale the collection operations out to multiple nodes. Arguably the most popular model for distributed computing nowadays is map/reduce, first popularised by Google and made widely available by Apache Hadoop. However, some problems require hundreds of map/reduce steps and Martin’s hope is that putting collection abstractions on top of map/reduce can make distributed computation accessible to the wider developer community.

On the explicit parallelism side, an interesting twist was mentioning Akka as the source of tools like actors, software transactional memory and futures. This is obviously the consequence of Martin’s commercial alliance with Jonas Bonér under the TypeSafe umbrella. Martin also recognised problems with existing actor implementation in Scala (most notably performance issues and memory leak associated with storing unprocessed messages in the inbox) and announced that Akka’s and Scala’s actor implementation will be merged in future releases.

Carrying on the subject of parallelism, Martin accepted that neither parallel collections nor actors would be a good fit for massively parallel machines, like the NVIDIA Fermi, which requires in the order of 24000 threads to fully utilise its computing power. An approach currently investigated by researchers from EPFL and Standford is to provide domain-specific languages embedded in Scala that will use the knowledge of the domain for parallelisation – something Martin referred to as “language virtualisation”. An example given was Liszt, a language for calculating molecular flows, useful e.g. to model the air flows in a jet engine. The code in Liszt is converted to abstract syntax tree, which in turn gets processed by a specialised compiler generating code for the target platform. The compiler needs to be specialised to each domain, but Martin’s hope is large parts of it can be reused, making development of highly-parallel DSLs cheaper.

Mentioned among other plans for the future were a common framework for querying collections and databases (Linq for Scala?), libraries for Scala reflection and for reactive programming and improvements to Scala.NET. Other ideas being explored but not being commited to yet are including in the core library, a way of encoding information about side effects in the type system and a Scala to JavaScript translator. Modularisation of Scala library is being looked at, but given past failed efforts Martin is not overly optimistic this will be delivered any time soon.

Eclipse IDE for Scala

After the break Martin returned to the stage to show how his initial commercial venture, Scala Solutions, attempted to improve the tool support for Scala by taking over the development of Eclipse IDE for Scala. While the Scala support in IntelliJ IDEA was getting significantly better with each release, the Eclipse plug-in, suffering from lack of commercial support and limited number of contributors, was lagging behind and the popular opinion was that it is inadequate for any serious work. The involvement of Scala Solutions was meant to change this: Eugene Vigdorchik joined from JetBrains to provide IDE experience, Iulian Dragos became the project lead while Martin became involved in modifying the compiler to suit the requirements of the IDE.

It’s worth noting at this point that while IntelliJ maintains its own Scala typechecker, the decision for Eclipse was to use the EPFL Scala compiler for error reporting in the UI. The advantages of this approach was that it guaranteed consistency with the main compiler and saving of effort – according to Martin writing the type checker from scratch would take between one and two man-years. On the other hand, IDE-specific type checkers can be faster than fully-fledged compilers, simply because they can cut corners where it doesn’t affect the accuracy of error reporting. Another disadvantage of the chose solution was that it tied Eclipse plug-in to a particular version of scala compiler.

The chosen architecture was to run the compiler in a separate thread which would receive work requests – a request would require determining either the type at given source position or the completion list – and respond via a dataflow variable (SyncVar). The implementation proved tricky: first of all, it needed to ensure that all compiler activity took place on a single thread, so that the internal state was protected. Secondly, the requirement to run up to hunderds of compilation tasks per minute exposed a number of memory leaks in the type checker. Finally, to make the error reporting responsive, the type checking had to be made selective, so that only the path in the AST leading to the node currently being changed was type-checked.

I did make a couple of attempts at using Scala IDE for Eclipse. The last one was about a year ago and after another disappointment I settled on Emacs+Ensime. The result of Scala Solutions’ work look promising indeed, and while it still seems to be behind IntelliJ in terms of features and reliability (Martin’s demo of working with the scala compiler itself didn’t go particularly smoothly, to put it mildly), it is getting there and deserves a serious look. Coming up are integration with SBT builder, an improved REPL and proper Scala debugger – as a long time Eclipse JDT user I plan to watch the development of this plug-in closely.

Scalaz - Get Functional

Scalaz provides hard-core functional programmers with the tools they are used to from Haskell: a hierarchy of type classes an their instances for standard Scala types. Jason Zaugg, one of the most active committers of the library, provided an overview of how type classes are encoded in Scala and how they can be used. One of the highlights was a demonstration of how type classes are useful based on well-known Java Comparator interface:

<T> void sort(List<T> ts, Comparator<? super T> comparator)

What was missing was a clearer explanation of motivation for the set of type classes as provided by Scalaz. It got close to a really compelling example when Jason has shown how a Seq[Promise[Int]] can be turned into a Promise[Seq[Int]], but I can’t help but feel that this example could have been presented more effectively.

Referential transparency, modularity and compositionality are very nice properties indeed, but the syntax used to achieve them in Scala is somewhat unpleasant. Let’s see:

trait Monoid[A] {
  def zero: A
  def append(a1: A, a2: A): A

implict def T2Monoid[A, B](implicit ma: Monoid[A], mb: Monoid[B]): Monoid[A, B)] = new Monoid[(A, B)] {
  def zero = (,
  def append(a1: (A, B), a2: (A, B)) = (ma.append(a1._1, a2._1), mb.append(a1._2, a2._2))

while the equivalent Haskell code would be:

class Monoid a where
  zero :: a
  append :: a -> a -> a

instance (Monoid a, Monoid b) => Monoid (a, b) where
  zero = (zero, zero)
  (a1, b1) `append` (a2, b2) = (a1 `append` a2, b1 `append` b2)

Furthermore, while Scalaz provides the IO monad to separate the side-effecting code from pure one, it is only done by convention and not enforced by the compiler – unlike in Haskell. On the other hand, Scala’s implicits are slightly more expressive than Haskell’s type classes, allowing for usage of the default instance but also providing a custom one when required. All in all, the talk was a good introduction to Scalaz for those familiar with Haskell; for the others, the opaque syntax and lack of clearly defined benefit might have been off-putting.

Representing Polymorphic Function Values in Scala Using Type Classes

After handing over the day-to-day Scala IDE development to Scala Solutions/Typesafe team, Miles Sabin seems to be thriving in cutting-edge type system research. His Scrap Your boilerplate with Scala talk on Functional Programming Exchange, while presenting a Scala encoding of a fascinating result, left part of the audience confused, due to large amount of material being presented in a very short time. This time round the ratio was much better and we had a chance to at least understand the problem in reasonable detail.

The problem is that while Scala makes it easy to define a polymorphic method:

object Id {
  def id[A](a: A) = a

when converting this method to a function value (the distiction between a method and a function is one of Scala quirks) using the trailing underscore syntax we are forced to provide a concrete type (e.g. val fid = id[Int] _), otherwise we’ll end up with a useless function with inferred type Nothing => Nothing. One way to work around that would be to make use of the way Scala represents functions and provide a custom function object with polymorphic apply method:

trait PolyFn1[M[_]] {
  def apply[T](t: T): M[T]

val pf = new PolyFn1[List] {
  def apply[T](t: T) = List(t)

While the “function definition” is not particularly pleasing to the eye, we can still take advantage of the standard application syntax. We can write pf(23) and pf("str") and both will typecheck – i.e. pf is a parametrically polymorphic function.

But this was not sufficient for Miles; what he wanted to achieve was ad-hoc polymorphism, like the one allowed by Haskell’s type classes. The solution to this problem was the centrepiece of the talk, and, sadly, the part whose details I wasn’t able to follow. It seemed to boil down to using a single typeclass TransCase[F, A, B] whose instances represented an arbitrary function A => B and were selected by providing a phantom type as the first type parameter.

Below is what I hope is a faithful reproduction of one of Miles’s examples. In the definition of inc, the actual function to run is provided via a type parameter (Inc):

object IncFn {
  import PolyFun._

  class Inc extends PTrans[Inc]
  object Inc {
    def apply[T](f0: T => T) = new TransCase[Inc, T, T] { val f = f0 }

  implicit def incInt: TransCase[Inc, Int, Int] = Inc[Int](_ + 1)
  implicit def incString: TransCase[Inc, String, String] = Inc[String](_ + "*")

  def inc[T](t: T)(implicit f: TransCase[Inc, T ,T]) = f(t)


Finally, this mechanism was used to define a function that lifted a function of arbitrary arity to Option monad. Miles is planning to write a blog post on this soon so watch if you are interested in a coherent explanation.

Given conceptual difficulty, the practicality of this approach seems questionable. Miles admitted that the proposed encoding is not something he would recommend for production use; nevertheless, it was yet another demonstration of the power of Scala’s type system and a number of conference attendees twitted about having their “mind blown”, which I assume must be a geeky expression of utmost appreciation.

Apache ESME - Using Scala And Lift For Social Messaging

During FP Exchange back in March I disliked the Play framework and WebSharper presentations, but it clearly must be me: another web-related talk and again something I can’t seem to get excited about. Vassil Dichev talked about an Apache project ESME, which seems to be a Twitter-like application built on Lift web framework and targeted at corporate intranets. Vassil elaborated on the advantages of Lift: Comet for server-initiated updates and abstraction of underlying communication mechanism, tight security via automatic escaping of user inputs and type-checked SQL queries and lightweight actors. The application itself seemed to provide little beyond what Twitter does.

Lift is one of most widely used toolkits built on top of Scala, so I assume most of the audience was to some extent familiar with it and its advantages. The talk had the flavour of “what I liked about scala/Lift when I was implementing this project”, which can be helpful, but even more helpful is sharing experiences of what works and what doesn’t and some deeper reflection on how to approach introducing Scala in a “real” project.

Stairway to Heaven or Scala in the “Real World”

Which, incidentally, was what the following talk was about. Chris Marshall has been working in Java for over 10 years, implementing systems in the financial sector. Like many other Java developers at some point he got fed up with the boilerplate required in Java to e.g. express simple collection transformations. His initial requirement was for “java with closures” and Scala seemed like a natural choice. Chris has now been using Scala for three years and shared a number of insights on adopting it in the workplace:

One remark I found particularly interesting was about how programming in Scala changed the way Chris codes in Java. For example, he reconfigured his IDE so that the keywords and types are not prominent – he found that they obscure the structure of the program.

Fostering Scala Adoption in Your Enterprise

Glen Ford approached the issue of Scala adoption from a different angle: selling it to the decision-makers. He stated some rules which might seem obvious in retrospect, but which are often overlooked by the developers trying to push for the introduction of a new technology from the bottom-up. For example:

The Unfiltered Web Framework

Did I say that I don’t like web framework presentations? Scratch that; Dustin Whitney’s fifteen-minute overview of Unfiltered was interesting, well paced and to the point. First of all, Unfiltered is not a “framework”, but a set of libraries which can be included in an SBT project and used to implement stand-alone apps responding to HTTP requests using request matchers and response functions and combinators – a major selling point as far as I’m concerned. The API looked lightweight, a bit like Scalatra, but – according to the speaker – more idiomatic to Scala. It is mature enought for Dustin to use it in production apps. Finally, in response to a question from the audience, Dustin expressed his disregard for servlet containers with which he immediately bought me over. Executable jars FTW.

How to Build a High Performance, Scalable Infrastructure in Under 5 Minutes

David Savage attempted to explain how his employer, Paremus, deploys scalable and modular services. From what I could understand without being familiar with some of the technologies discussed, the platform consists of a minimal kernel that pulls all the dependencies in the form of OSGi modules. The system is defined as the components, transport, scaling and contract, with components and scaling requirements defined in an XML file and contracts expressed as LDAP filters. Overall Scala played a seemingly insignificant role of implementation language for one of the services – even its interface was defined in Java – so it felt a bit like an excuse to present an otherwise unrelated commercial product. David mentioned that Scala provided for the “high performance” part of the headline, but it was not elaborated on in any detail.