Scala 2.10.0 (english version)

Where did we came from, and where are we going to?

Daniel Capó Sobral

Last revision: Jan 05, 2013

Revision History
 Jul 20, 2012: add IsSeqLike and Searching
 Aug 08, 2012: removed previous revision – it didn’t
make 2.10.0; added raw interpolator; added :warnings;
renamed makro to macros; @static; scaladoc grouping
and link to anything
 Aug 09, 2012: fixed stuff on macros and reflect; fixed
stuff with Try; added references to other presentations
 Sep 20, 2012: rescue replaced with recoverWith
 Jan 05, 2014: Incorporate feedback about macros;
improve actors; add links to some docs;

Who am I?
 Daniel C. Sobral @ Stack Overflow
 dcsobral @ Twitter, Gmail/Gtalk, Skype, SliderShare
 Daniel Sobral @ Facebook, Speaker Deck, LinkedIn
 http://www.linkedin.com/in/danielsobral
 http://www.slideshare.net/dcsobral
 https://speakerdeck.com/u/dcsobral
 http://dcsobral.blogspot.com

What do I know about Scala?
 Scala gold badge @ Stack Overflow
 Small contributions to Scala:
 code optimization
 enhancements
 bug fixes
 documentation (not that small)
 Active member of the Scala community (mailing lists,
irc)
 Blog about Scala

IMPORTANT!
 Scala 2.10.0 has not been released!
 At the time this presentation has been written... (July,
07, 2012)

 The information that follows may be changed before
2.10.0 is released.

Summary
 History
 Near Future
 Versioning
 Release Process
 Bug fixes
 Enhancements
 SIPs (Scala Improvement Process)
 Reflection
 Actors and Akka
 Distant Future

Scala 2.10.0 (english version)

Release History
2003 2004 2005 2006 2007 2008 2009 2010 2011 2012

• 0.8.1 • 1.0.0 • 1.4.0 • 2.0.0 • 2.3.2 • 2.7.1 • 2.7.3 • 2.8.0 • 2.9.0 • 2.9.2
• 0.8.2 • 1.1.0 • 2.1.0 • 2.3.3 • 2.7.2 • 2.7.4 • 2.8.1 • 2.9.1
• 0.9.0 • 1.1.0 • 2.1.1 • 2.4.0 • 2.7.5 • 2.8.2
• 0.9.1 • 1.1.1 • 2.1.2 • 2.5.0 • 2.7.6
• 1.2.0 • 2.1.3 • 2.5.1 • 2.7.7
• 1.3.0 • 2.1.5 • 2.6.0
• 2.1.6 • 2.6.1
• 2.1.7
• 2.1.8
• 2.2.0
• 2.3.0
• 2.3.1

Some important landmarks

2011
• Parallel Collections
2010
• New Collections Design
• Binary Compatibility of
Minor Releases
2008
• Java Generics
• Case Class Extractors

2007
• Extractors
• Private/protected
primary constructors
• Private[this]
• Placeholder
2006 parameters
• Scalac written in • Early Initialization
Scala • Abstract Type
• Implicits Constructors
• Traits e • Existential Types
linearization • Lazy values
• Structural Types
Not comprehensive, by any means!
• Multilline Strings

Explaining Versioning
2 .9 .1 -1

Bug
Epoch Major Minor
fix

Versioning – Scala 2.10.0
 Major release
 Binary incompatible with previous versions
 Language changes
 Library changes
 New deprecations
 Removal of deprecated features
 Classes
 Methods
 Language syntax

Examples of New Deprecations
 Octal numbers!
scala> 077
<console>:1: warning: Treating numbers with a leading zero as octal is
deprecated.
077
^
res10: Int = 63
 Double ending in dot
scala> 1.
<console>:1: warning: This lexical syntax is deprecated. From scala
2.11, a dot will
only be considered part of a number if it is immediately followed by
a digit.
1.
^
res11: Double = 1.0

Examples of removal of deprecated
features
 Language syntax
for ( val i <- 1 to 10) println(i)

 Class
scala.collection.mutable.CloneableCollection

 Method
scala.collection.immutable.Queue.+

 Package
scala.testing.SUnit

Improvements of the release
process
 Process becoming more and more automated
 Package for RPM, APT, Brew, etc
 Automatically generated!
 Milestones used to help integrate libraries and
frameworks
 M4 June 12
 M5 soon!
 Release Candidates to consolidate and fix bugs

Bug fixes
 Many fixes!
 New pattern matcher!
 Ticket priorization!
 New warnings:
 Detecting bugs in the compiler itself!
 More third party contributions!
 git + pull request == FTW
 ...besides the natural improvements of the codebase.

Closed Tickets
102 451 490 622 896 963 1118 1133 1195 1201 1247 1336
1430 1431 1432 1439 1510 1785 1799 2089 2094 2171 2196 2308
2322 2337 2388 2405 2435 2442 2460 2764 2782 2807 3047 3048
3098 3240 3272 3326 3343 3371 3392 3481 3566 3569 3628 3702
3708 3718 3755 3758 3761 3770 3798 3853 3854 3880 3898 3929
3960 3999 4018 4019 4025 4027 4063 4070 4098 4110 4124 4134
4147 4171 4172 4176 4216 4262 4270 4271 4273 4319 4326 4336
4355 4372 4391 4398 4417 4419 4425 4430 4441 4461 4467 4469
4478 4482 4490 4494 4501 4510 4512 4515 4535 4540 4541 4542
4547 4553 4561 4568 4570 4573 4574 4578 4579 4584 4592 4593
4595 4599 4601 4627 4636 4642 4647 4651 4656 4658 4661 4692
4696 4697 4709 4712 4713 4716 4717 4723 4727 4731 4737 4740
4749 4753 4757 4758 4759 4761 4764 4766 4770 4777 4780 4792

Closed Tickets
4794 4800 4802 4804 4807 4809 4811 4818 4823 4827 4828 4831
4833 4839 4842 4846 4851 4853 4857 4858 4859 4860 4861 4869
4871 4874 4875 4877 4879 4882 4891 4894 4898 4899 4909 4910
4911 4925 4928 4929 4933 4935 4938 4954 4957 4959 4961 4963
4970 4975 4976 4979 4981 4985 4987 4989 5005 5009 5012 5018
5020 5023 5026 5029 5032 5033 5034 5037 5040 5041 5053 5054
5056 5062 5063 5066 5071 5072 5077 5078 5080 5083 5084 5085
5093 5096 5097 5099 5104 5105 5108 5117 5119 5121 5125 5127
5135 5137 5147 5148 5152 5156 5162 5165 5167 5168 5175 5176
5178 5189 5199 5201 5205 5206 5210 5212 5215 5223 5226 5229
5230 5239 5245 5248 5256 5259 5266 5267 5272 5284 5287 5291
5293 5295 5300 5305 5313 5317 5318 5328 5334 5336 5341 5343
5344 5352 5354 5356 5358 5359 5373 5374 5375 5377 5382 5384

Closed Tickets (442 – July 2nd)
5387 5399 5405 5406 5407 5426 5428 5429 5441 5444 5446 5451
5452 5453 5455 5471 5488 5489 5497 5500 5510 5514 5522 5528
5530 5532 5535 5537 5542 5543 5544 5545 5552 5553 5554 5564
5571 5572 5577 5578 5580 5589 5590 5591 5593 5599 5608 5609
5610 5612 5614 5617 5623 5626 5627 5629 5632 5640 5641 5644
5648 5652 5654 5655 5656 5663 5666 5667 5672 5676 5677 5680
5682 5683 5688 5689 5690 5693 5696 5702 5703 5704 5706 5707
5708 5713 5715 5720 5728 5729 5735 5738 5742 5760 5761 5763
5769 5777 5779 5796 5801 5804 5805 5809 5816 5821 5829 5839
5840 5843 5845 5846 5853 5857 5862 5867 5879 5880 5881 5899
5910 5912 5914 5932 5953 5966 5967 5968 5971 5986 etc?

Enhancements
 New collections
 Mutable SortedSet/SortedMap (AVL-based)
 Mutable Concurrent Map (TrieMap)
 Parallel Mutable Concurrent Map (ParTrieMap)
 Performance:
 Immutable SortedSet/SortedMap
 New (private) RedBlackTree
 TreeIterator
 PartialFunction
 applyOrElse

 BitSet
 MurmurHash3 (collections, xml, case classes)

New stuff
 Type Classes:
 Hashing (there’s an Equiv already)
 IsTraversableOnce and IsTraversableLike
 Try (Twitter’s alternative to Either)
 Package scala.util.hashing
 Configurable Pools for parallel collections
 @static
 @unspecialized
 to[Collection]
 ???

New compile parameters
 override object (-Yoverride-objects)
 bytecode version 49, 50 and 51 (-target:jvm-1.x-asm)
 -optimize faster and more effective (ok, not new)
 -Dscala.timings=true shows what’s is taking so long to
compile
 SIP-18: -language:XXX, -feature
 various logs
 various macro
 various patmat
 etc

New REPL commands
 The :warnings command will turn on all hidden
warnings, such as deprecation (that one is going to my
finger’s muscle memory...)

scala.util.hashing.Hashing
def hashingOf[T : Hashing](obj: T): Int =
implicitly[Hashing[T]].hash(obj)

// A bad hashing function for Seqs
implicit val seqHashing =
Hashing fromFunction ((_: Seq[_]).size)

IsTraversableOnce and
IsTraversableLike
class FilterMapImpl[A, Repr]
(val r: GenTraversableLike[A, Repr]) {
final def filterMap[B, That]
(f: A => Option[B])
(implicit cbf: CanBuildFrom[Repr, B, That])
: That =
r.flatMap(f(_).toSeq)
}

// How to write the implicit conversion???

IsTraversableLike
 Obstacles to writing the implicit conversion:
 Array and String are not GenTraversable
 String does not have a type parameter (A)
 Nor BitSet and other collections
 Inference over view bounds is difficult
 IsTraversableOnce and IsTraversableLike
provides a solution

IsTraversableLike
class FilterMapImpl[A, Repr]
(val r: GenTraversableLike[A, Repr]) {
final def filterMap[B, That]
(f: A => Option[B])
(implicit cbf: CanBuildFrom[Repr, B, That])
: That =
r.flatMap(f(_).toSeq)
}

implicit def filterMap[Repr, A](r: Repr)
(implicit fr: IsTraversableOnce[Repr])
: FilterMapImpl[fr.A,Repr] =
new FilterMapImpl(fr.conversion(r))

scala.util.Try
def percentCompleted(total: Int,
done: Int): Int =
Try ( done * 100 / total ) getOrElse 100

scala.util.Try
Try {
new FileInputStream(a)
} recoverWith {
case _: FileNotFoundException =>
Try(new FileInputStream(b))
} recover {
case _: FileNotFoundException =>
defaultInputStream
} andThen {
stream =>
in = stream.read(); stream.close(); in
}

@static
 Turn object fields into static fields of the object’s
companion class
 Fields are initialized through the static initializer of
the class
 Access to these fields are turned into direct static field
access

Configurable Pools on Parallel
Collections
import scala.collection.parallel._
val pc = mutable.ParArray(1, 2, 3)

pc.tasksupport = new ForkJoinTaskSupport(
new scala.concurrent.ForkJoinPool(2))

pc map { _ + 1)

pc.tasksupport = new ThreadPoolTaskSupport()

Configurable Pools on Parallel
Collections – Customizing
class customTaskSupport extends TaskSupport {
def execute[R, Tp]
(task: Task[R, Tp]): () => R = ???

def executeAndWaitResult[R, Tp]
(task: Task[R, Tp]): R = ???

def parallelismLevel: Int = ???
}

to[Collection]
scala> List(2, 1, 3).to[Seq]
res17: Seq[Int] = Vector(2, 1, 3)

scala> List(2, 1, 3).to[Vector]
res18: Vector[Int] = Vector(2, 1, 3)

scala> List(2, 1, 3).to[collection.mutable.Seq]
res19: scala.collection.mutable.Seq[Int] = ArrayBuffer(2,
1, 3)

scala> List(2, 1, 3).to[collection.immutable.SortedSet]
res20: scala.collection.immutable.SortedSet[Int] =
TreeSet(1, 2, 3)

???
 ??? it‘s a Nothing-returning that throws exceptions
 Because it’s type is Nothing, ??? may replace any
expression
 Excellent for stubs, exercises and presentations!

???
trait Opt [A] { // Exercise by Tony Morris
def fold[X](some: A => X, none: => X): X = ???
def map[B](f: A => B): Opt[B] = ???
def get: A = ???
def flatMap[B](f: A => Opt[B]): Opt[B] = ???
def mapAgain[B](f: A => B): Opt[B] = ???
def getOrElse(e: => A): A = ???
def filter(p: A => Boolean): Optional[A] = ???
def exists(p: A => Boolean): Boolean = ???
def forall(p: A => Boolean): Boolean = ???
def foreach(f: A => Unit): Unit = ???
def isDefined: Boolean = ???
def orElse(o: => Opt[A]): Opt[A] = ???
// etc
Adapted from Tony Morris, http://blog.tmorris.net/further-understanding-scalaoption/

Enhancements
 Scaladoc
 Implicits!
 Diagrams!
 Link to anything!
 Group things!
 API docs
 Steady stream of improvements on the documents
themselves
 See also:
 http://docs.scala-lang.org/

ScalaDoc & Implicits

scaladoc -implicits

Class Diagrams!

scaladoc -diagrams

Parallel Collections Docs!

Available at http://docs.scala-lang.org/

Scaladoc – Link to anything!
 [[x.y.Z.m]] links to m on Z, defaulting to the type Z
if m is present on both type and object
 Z! forces type and Z$ forces object, to disambiguate
 m can be written with type signature, to disambiguate
 [[m]] refers to m on the current scope
 If dots or hashes are required in the type signature,
escape them with

Scaladoc – Group things!
 Scaladoc now has attributes that allow members to be
grouped, and the groups described and prioritized
 Members of the same group are displayed together
 Groups of lower priority are seen before groups of
higher priorities
 See: @group, @groupdesc, @groupname and
@groupprio

SIP – Scala Improvement Process
 SIP-11: String Interpolation
 SIP-12: Uncluttering Scala’s syntax for control
structures
 SIP-13: Implicit Classes
 SIP-14: Futures and Promises
 SIP-15: Value Classes
 SIP-16: Self-cleaning Macros
 SIP-17: Type Dynamic
 SIP-18: Modularizing Language Features
 SIP-19: Implicit Source Locations

SIP – Scala Improvement Process
 SIP-11: String Interpolation (accepted!)
 SIP-12: Uncluttering Scala’s syntax for control
structures (postponed)
 SIP-13: Implicit Classes (accepted!)
 SIP-14: Futures and Promises (accepted!)
 SIP-15: Value Classes (accepted!)
 SIP-16: Self-cleaning Macros (postponed)
 SIP-17: Type Dynamic (accepted!)
 SIP-18: Modularizing Language Features (accepted!)
 SIP-19: Implicit Source Locations (not accepted)

SIP-11: String Interpolation
 Lack of string interpolation were a source of constant
complains
 Even though very few languages support them!
 Odersky says the difference between ${x} and "+x+" is a
single character
 If you ignore the shift-typing...
 Someone says " and + can be typed without shift on a swiss
keyboard
 True!
 About-face:
 What if string interpolation was more than string
interpolation?

def hello(name: String = "world"): String =
"Hello, " + name + "! "

// Interpolation!
def hello(name: String = "world"): String =
s"Hello, $name! "

def percentCompleted(total: Int, done: Int): String =
Try ( done * 100 / total ) map {
percent => f"$percent%2d% completed"
} getOrElse "100% completed"

def percentCompleted(total: Int, done: Int): String =
Try (
f"${ done * 100 / total }%2d% completed"
) getOrElse "100% completed"

SIP-11: String Interpolation:
How does it work?
s"Hello, $name!" // becomes
StringContext("Hello, ", "!").s(name)

f"${ done * 100 / total }%2d% completed" // becomes
StringContext("", "%2d% completed").f(done * 100 / total)

raw"String with n in it"

 Multiline literals work too
 s"""Ok!"""
 Interpolation may be nested
 s"${ s"$x" }"
 Backslash is not used as escape character
 r"d+/$month" // not an actual interpolator!
 StringContext("""d+/""").r(month)
 But both s and f interpolators add the escaping themselves!
 s"Hello, $name!n" // newline normal
 But the raw interpolator does not!
 How to add/modify behavior:
 Enrich My Library on StringContext
 Shadowing of object StringContext
 Added interpolators don’t actually need to be methods!

// Enrich My Library
class RegexContext(sc: StringContext) {
def r(args: Any*) = {
sc.checkLengths(args: _*)
val res = (args, sc.parts.tail).zipped map {
(arg, part) => s"Q$argE$part"
} mkString ""
(sc.parts.head + res).r
}
}

implicit def toRC(sc: StringContext) = new RegexContext(sc)

// Interpolation nesting
class RegexContext(sc: StringContext) {
s"${sc.parts.head}${
(args, sc.parts.tail).zipped map {
(arg, part) => s"Q$argE$part"
} mkString ""
}".r
}
}

implicit def toRC(sc: StringContext) = new RegexContext(sc)

// Shadowing of StringContext object
object StringContext(parts: String*) {
require(parts.length == args.length + 1)
val res = (args, parts.tail).zipped map {
"Q" + _ + "E" + _
} mkString ""
(parts.head + res).r }
}

// One can change s and f interpolators!

// apply and unapplySeq

def hello(name: String = "world"): String = i"Hello, $name!"

def who(message: String): String = message match {
case i"Hello, $name!" => name
case _ => “no clue"
}

who(hello("Daniel")) == "Daniel"

// apply and unapplySeq
implicit class SI(sc: StringContext) {
object i {
def apply(args: Any*): String =
sc.parts.head +
(args,sc.parts.tail).zipped.map(_+_).mkString
def unapplySeq(s: String): Option[Seq[String]] = {
val partsr = sc.parts map (p => s"Q$pE")
val r = (partsr mkString "(.*)").r
s match { case r(xs @ _*) => Some(xs) case _ => None }
}
}
}

SIP-13: Implicit Classes
 “Enrich My Library” (formely “Pimp My Library”) very
popular
 But with lots of cerimony...
 “Extension methods” becoming common in other
languages (even Java!)
 And with less cerimony at that!

 Solution: implicit class

SIP-13: Implicit Classes
// String interpolation with Implicit Classes
implicit class RegexContext(sc: StringContext) {
"Q" + _ + "E" + _
} mkString ""
}
}

SIP-14: Futures and Promises
 Non-blocking asynchronous of operations is a common
task
 Evidence: many libraries have a Future implementation
 Including Scala Actors library!
 Problem:
 Non-standard interface
 Interface tied to implementation
 Solution:
 Powerful and flexible API implementing the concepts of
Future and Promise

// TODO – Sorry! Examples from SIP:

import scala.concurrent._
val f: Future[List[String]] = future {
session.getRecentPosts
}
f onComplete {
case Right(posts) => for (post <- posts) render(post)
case Left(t) => render("An error has occured: " +
t.getMessage)
}


val f: Future[List[String]] = future {
session.getRecentPosts
}
f onFailure {
case t => render("An error has occured: " + t.getMessage)
} onSuccess {
case posts => for (post <- posts) render(post)
}


def main(args: Array[String]) {
val rateQuote = future {
connection.getCurrentValue(USD)
}

val purchase = rateQuote map {
quote => if (isProfitable(quote)) connection.buy(amount,
quote)
else throw new Exception("not profitable")
}

blocking(purchase, 0 ns)
}


import scala.concurrent.{ future, promise }
val p = promise[T]
val f = p.future
val producer = future {
val r = produceSomething()
p success r
continueDoingSomethingUnrelated()
}
val consumer = future {
startDoingSomething()
f onSuccess {
case r => doSomethingWithResult()
}
}

SIP-15: Value Classes
 Curse of references
 Curse of boxing:
 case class Meter(n: Int) much more “expensive”
than int
 Curse of implicit classes:
 "abc“.r creates an unnecessary object just to call new
Regex("abc")
 We want a class that is not a reference!
 Solution: value classes.

// String interpolation with implicit value class!
implicit class RegexContext(sc: StringContext) extends AnyVal {
"Q" + _ + "E" + _
} mkString ""
}
}

 Can have only one parameter
 Which must be a public val
 For instance, [T: Ordering](v: T) is not acceptable
 Can be extended by traits
 If they are either Any or AnyVal
 Cannot define equality or hash code
 But may be case classes!
 Equality and hash code are applied over the parameter
 They are effectively “final”
 On a local scope, the class is optimized away!
 If they escape scope, they are boxed

SIP-16: Macros
 Acceptance postponed, but available experimentally on
Scala 2.10.0
 Large compilers are hard to maintain
 Pressure against new features on the compiler
 Compiler Plugins can be used to extend Scala...
 ...but they are difficult to keep in sync with scalac
 Possible solution: Macros
 “Macros? Ugh!” –C/C++ trauma!
 Something new: scala.reflection
 Macros become almost free!

SIP-16: Macros
 Scala CATs!
 Compile-time AST Transformations
 Abstract Syntax Tree
 Inspired by Nemerle macros
 Four kind looked into:
 Typed def macros – This is the only made available!
 Untyped def macros
 Type (class/trait) macros
 Annotation macros
 Official Overview:
 http://docs.scala-lang.org/overviews/macros/typemacros.html

SIP-16: Macros
// Typed def macros

(1 to 5).foreach(println)
^ ^
| + Type check
+ Macro execution
+ New type check

SIP-16: Macros
// Untyped def macros

for {val i = 0; i < 10; i += 1} println(i)
^ ^
| |
| + Arguments not type checked!
+ Macro execution
+ Type check

SIP-16: Macros
// Untyped def macros – why?

for {val i = 0; i < 10; i += 1} println(i)
^ ^
| “i” not visible in this scope! +
+ “i” only visible in this scope.

SIP-16: Macros
// Type (class/trait) macros

class User extends DbTable(jdbcUrl, “t_user”)
^
+ Macro

// Connects to database at compile time
// and gets table description to create
// class “User”

SIP-16: Macros
 Type macros are similar to F#’s type providers

 Next in line of development
 First user likely to be Typesafe’s Slick (formely
ScalaQuery)
 http://slick.typesafe.com/

SIP-16: Macros
// Macro annotations

@memoize
def fat(n: Int) =
if (n > 0) n * fat(n – 1)
else 1

// Macro applied over the annotated definition,
// be it class, method, parameter, etc

SIP-16: Macros
 Only typed macros made available...
 And with experimental status, at that...
 Behind flag –language:experimental.macros...
 But already used in some places on the library!
 C/C++ macros trauma very widespread...
 ...but the true C++ macros are templates! (imho)
 Macro Paradise used to experiment with other types of
macros:
 http://docs.scala-lang.org/overviews/macros/paradise.html

SIP-16: Macros
 Typed Macros Advantages:
 Relatively simple to implement
 There’s no difference in the type signature of a method
implemented with a macro!
 Easy to test
 Hygienic macros provided through a macro!
 Therefore, self-cleaning
 Non-hygienic Macros are also easy to create

SIP-16: Macros
 Disadvantages:
 Somewhat limited
 Two-stages compilation
 It’s not possible to define and use a macro on the same
compilation
 Non-hygienic by default
 Nay-sayers will go on nay-saying.

SIP-16: Macros
object Trimmer {
def trim(s: String): String = macro trimImpl

import scala.reflect.macros.Context
def trimImpl(c: Context)(s: c.Expr[String]): c.Expr[String] = {
import c.universe._ // see reflection

val Literal(Constant(untrimmed: String)) = s.tree
val trimmed = untrimmed.lines.map(_.trim).mkString("n")

c.Expr(Literal(Constant(trimmed)))
}
}

SIP-16: Macros
 Path dependent method types:
 def m(c: Context)(param: c.Expr[Any]):
c.Expr[Any]
 Through –Ydependent-method-types parameter on
previous versions
 Reflection, reflection and reflection! 
 “Once we do reflection right, we get macros for free!” –
paraphrasing Martin Odersky

SIP-17: Type Dynamic
 Problems:
 Scala’s integration to other JVM languages assumes
static typing
 Lots of dynamically typed languages on the JVM!
 Static typing also hinders some flexible DSLs
 Solution: trait Dynamic
 Similar to “missing method”
 Makes it easy to integrate with other JVM languages
 Has great DSL potential
 And other uses as well...

scala> class xmlPath(xml: scala.xml.Node) extends Dynamic {
| def selectDynamic(path: String) = xml path
| }
defined class xmlPath

scala> new xmlPath(<root><a><b><c/></b></a></root>).b
res9: scala.xml.NodeSeq = NodeSeq(<b><c/></b>)

 Method call conversions:

 Standard methods:
 applyDynamic
 Methods with named parameters:
 applyDynamicNamed
 Setters:
 updateDynamic
 Getters (nullary methods):
 selectDynamic

 Also in the works is DynamicProxy
 Add/Change methods, have everything else be
automatically redirected
 Might (maybe) even do so with static typing, through
macros!
 However, not yet in the library
 Might make 2.10.0 or not – time will tell

SIP-18: Modularizing Language
Features
 Problems:
 Some Scala features are hard to understand
 Others induce errors
 And some are experimental
 But they are all necessary, for one reason or another!
 Solution: SIP-18

Features
 Controls access to some language features
 Using these features cause compilation warnings
 Which might become compilation errors in the future
 Access to features allowed through:
 compile flags
 implicit in scope
 Access can be allowed through inherited
 ScalaDoc explains why control and why make available

Features
 Contentious features:
 Postfixed operators
 Method invocation made using implicit reflection
 Implicit conversions
 Higher kinded types
 Existentials
 Except for those equivalent to Java wildcard types
 Type Dynamic
 Experimental Features
 Macros

Features
// Postfix operator

scala> "abc" length
warning: there were 1 feature warnings; re-run with -feature for
details
res0: Int = 3

Features
// Postfix operator, using -feature

scala> "abc" length
<console>:8: warning: postfix operator length should be enabled
by making the implicit value language.postfixOps visible.
This can be achieved by adding the import clause 'import
language.postfixOps'
or by setting the compiler option -language:postfixOps.
See the Scala docs for value scala.language.postfixOps for a
discussion
why the feature should be explicitly enabled.
"abc" length
^
res0: Int = 3

Features
// Methods called using reflection
scala> val x = new AnyRef { def hello = println("world") }
x: Object{def hello: Unit} = $anon$1@7d628303

scala> x.hello
<console>:10: warning: reflective access of structural type member method hello
should
be enabled
by making the implicit value language.reflectiveCalls visible.
This can be achieved by adding the import clause 'import language.reflectiveCalls'
or by setting the compiler option -language:reflectiveCalls.
See the Scala docs for value scala.language.reflectiveCalls for a discussion
x.hello
^
world

Features
// Implicit conversions

scala> implicit def f(s: String): Int = Predef.augmentString(s).toInt
<console>:8: warning: implicit conversion method f should be enabled
by making the implicit value language.implicitConversions visible.
language.implicitConversions'
or by setting the compiler option -language:implicitConversions.
See the Scala docs for value scala.language.implicitConversions for a
discussion
implicit def f(s: String): Int = Predef.augmentString(s).toInt
^
f: (s: String)Int

Features
 The feature being limited are implicit conversions
 Other implicit usages are still allowed
 Implicit conversions through implicit classes works too

Features
// Higher Kinded Types

scala> class Monad[M[_]]
<console>:9: warning: higher-kinded type should be enabled
by making the implicit value language.higherKinds visible.
language.higherKinds'
or by setting the compiler option -language:higherKinds.
See the Scala docs for value scala.language.higherKinds for a
discussion
class Monad[M[_]]
^
defined class Monad

Features
// Existentials

scala> val l: List[ T forSome { type T }] = List(1)
<console>:7: warning: the existential type T forSome { type T },
which cannot be expressed by wildcards, should be enabled
by making the implicit value language.existentials visible.
language.existentials'
or by setting the compiler option -language:existentials.
See the Scala docs for value scala.language.existentials for a
discussion
val l: List[ T forSome { type T }] = List(1)
^
l: List[T forSome { type T }] = List(1)

Features
// Type Dynamic

TODO

Features
// Macros

scala> def g(d: Any): Any = macro f
<console>:12: error: macro definition needs to be enabled
by making the implicit value language.experimental.macros
visible.
language.experimental.macros'
or by setting the compiler option -language:experimental.macros.
See the Scala docs for value scala.language.experimental.macros
for a discussion
why the feature needs to be explicitly enabled.
def g(d: Any): Any = macro f
^

scala.reflect
 Allows us to explore the structure of a program,
 and interact with that structure
 Wikipedia: reflection happens at compile time
 Scala: you use the same API at compile time as well, with
macros
 Main Concepts:
 Reflection Libraries and Universes
 Names, symbols, types and trees
 Mirrors
 Overview will be provided at:
 http://docs.scala-lang.org/overviews/reflection/overview.html

scala.reflect – Universes

Source: Scala Reflection Pre-SIP
Notice: all of the classes on scala-library.jar depicted above were removed.

 Contains a number of interrelated elements which,
together, make up the main components of reflection
 Symbol
 Tree
 Name
 etc...
 Use of path dependent types to relate types with the
universes they came from

 Built in layers with different degrees of detail using the
“cake pattern” :
 Base:
 Fundamental concepts
 Identity, e and little else

 JavaUniverse
 Manipulation through JVM-provided reflection, enhanced with
information provided by Scala
 Macros
 Manipulation at compile time
 Compiler’s universe, but presented with a more restricted API

 Others: compiler

scala.reflect – Main Types
Name

FlagSet Symbol

Universe
AnnotationInfo
Type

Position Tree

scala.reflect – Name

Name

TermName TypeName

scala.reflect – Name
 Scala has two namespaces:
 Types
 Values (Term)
 Name ties a string to one of those namespaces
 Name also enables convertion of names from language-
representation to class-file representation

scala.reflect – Symbol
Symbol
Type owner Name

Symbol Type ... Symbol TermName TypeName

scala.reflect – Symbol
 All definitions are tied to symbols
 Classes
 Methods
 Parameters
 Variables
 If you named it, it has a symbol
 If you did not name it, but refer to it as being
“anonymous”, it also has a symbol
 Immutable at run time
 At macro time, one can change its flags

scala.reflect – Type
 Structure associated with a symbol:
 Class members
 Parameters and return types of a method
 etc
 Made up of case classes
 Handled through pattern matching

scala.reflect – Types and Symbols
scala> import scala.reflect.runtime.universe._
import scala.reflect.runtime.universe._

scala> typeOf[List[Int]]
res0: reflect.runtime.universe.Type = List[Int]

scala> res0.member(newTermName("head"))
res1: reflect.runtime.universe.Symbol = method head

scala> res1.typeSignature
res2: reflect.runtime.universe.Type = => A

scala> res1.typeSignatureIn(res0)
res3: reflect.runtime.universe.Type = => Int

def intMethods[T : TypeTag](v: T) = {
val IntType = typeOf[Int]
val vType = typeOf[T]
val methods = vType.members.collect {
case m: MethodSymbol if m.isPublic =>
m -> m.typeSignatureIn(vType)
}
methods collect {
// Pattern-matching various method types:
case (m, mt @ NullaryMethodType(IntType)) => m -> mt
case (m, mt @ MethodType(_, IntType)) => m -> mt
case (m, mt @ PolyType(_, MethodType(_, IntType))) => m -> mt
}
}

}
methods collect {
// Since m is a MethodSymbol, we can do it like this instead:
case (m, mt) if m.returnType == IntType => m -> mt
}
}

}
methods collect {
// Actually, you need this to account for type aliases:
case (m, mt) if m.returnType =:= IntType => m -> mt
}
}

scala.reflect – Tree
Tree
children
• Tree
• Tree Type Position Symbol
• ...
• Tree

TypTree TermTree

 Compiler’s internal representation of source code
 Immutable at runtime, made up of case classes
 However, at macro time one can change its symbol, type
and position
 Essential when writing macros
 Also used for annotated typed

scala> import scala.tools.reflect.ToolBox
import scala.tools.reflect.ToolBox

scala> import scala.reflect.runtime.{currentMirror => m}
import scala.reflect.runtime.{currentMirror=>m}

scala> val tb = m.mkToolBox()
tb: scala.tools.reflect.ToolBox[reflect.runtime.universe.type] =
scala.tools.reflect.ToolBoxFactory$ToolBoxImpl@5bbdee69

scala> val tree = tb.parseExpr("1 to 3 map (_+1)")
tree: tb.u.Tree = 1.to(3).map(((x$1) => x$1.$plus(1)))

scala> val eval = tb.runExpr(tree)
eval: Any = Vector(2, 3, 4)

scala> import scala.reflect.runtime.universe._
import scala.reflect.runtime.universe._

scala> showRaw(reify({ def f(x: Int, y: Int) = x + y }).tree)
res4: String = Block(List(DefDef(Modifiers(), newTermName("f"),
List(), List(List(ValD
ef(Modifiers(PARAM), newTermName("x"), Ident(scala.Int),
EmptyTree), ValDef(Modifiers(
PARAM), newTermName("y"), Ident(scala.Int), EmptyTree))),
TypeTree(), Apply(Select(Ide
nt(newTermName("x")), newTermName("$plus")),
List(Ident(newTermName("y")))))), Literal
(Constant(())))

scala.reflect – Mirror

InstanceMirror ClassMirror

MethodMirror
ClassMirror FieldMirror MethodMirror ModuleMirror
(constructors)

 Mirrors allow one to manipulate classes and instances
at run time
 Each Universe may have one or mirrors
 Each mirror is tied to a class loader
 Classes with the same name may be loaded by distinct
class loaders
 Mirrors allows reflection over the Scala language,
instead of its JVM implementation

val obj = "String"
val objClass = obj.getClass
val classClassLoader = objClass.getClassLoader
val classLoaderMirror = runtimeMirror(classClassLoader)
val classSymbol = classLoaderMirror.classSymbol(objClass)
val classType = classSymbol.typeSignature
val methodName = newTermName("length")
val methodSymbol = classType.member(methodName).asMethodSymbol
val instanceMirror = classLoaderMirror.reflect(obj)
val methodMirror = instanceMirror.reflectMethod(methodSymbol)
methodMirror.apply() // == (obj.length: Any)

Actors and Akka
 The standard actors library will be removed
 Replaced by Akka’s
 Akka will be part of 2.10’s distribution
 Only binaries – source code will remain apart
 The old library will also be available, on a separate jar
 There is a migration guide:
 http://docs.scala-lang.org/overviews/core/actors-migration-guide.html

 The standard actors has a migration kit
 This provides a layer over standard and Akka actors to
make them source code compatible with each other

Wild Speculations
 untyped macros
 for { val x = 0; x < 10; x += 1 } println(x)
 macro types
 class X extends MacroClassY(parm)
 macro annotations
 @memoize def fat(n: Int) = if (n > 0) n *
fat(n -1) else 1
 effect system
 abstract types/type parameters unification
 pre-processor (DOA!)

Other presentations
 Alex Boisvert’s Scala Roadmap:
 https://speakerdeck.com/u/boia01/p/scala-roadmap-as-
of-april-2012
 Goes into much more depth about “distant future”
 Simon Ochsenreither’s What’s New in 2.10
 http://ochsenreither.posterous.com/whats-new-in-210
 No explanations, but more comprehensive, and links to
commits

Scala 2.10.0 (english version)

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Scala 2.10.0 (english version)