Coding in Style

Coding in Style
J. Suereth
Senior Software Engineer

粋なコード

Agenda
● The Scala mindset
● Scala 2.10 new features
● A brave new venture

Scala 風に考える／Scala 2.10 の新機能／その先へ

Favor Expressions

文より式

def isAwesome(data: Data): Boolean = {
if(data.awesome) return true
if(data.lame) return false
return data.contains("Tasty Sandwich")
}

def isAwesome(data: Data): Boolean =
if(data.awesome) true
else if(data.lame) false
else (data contains "Tasty Sandwich")
}

def isAwesome(data: Data): Boolean =
(data.awesome ||
(!data.lame) ||
(data contains "Tasty Sandwich"))

Embrace Operator Notation

演算子（中置）記法を取り入れよう

def privs(user: Option[User]): Privileges =
user.map(findUserPrivileges).getOrElse(
NoPrivs)

def privs(user: Option[User]): Privileges =
(user map findUserPrivileges
getOrElse NoPrivs)

Let the language do the
work
SUDO Make me a variable

作業を言語に任せる

def slurp(file : File): String = {
var input: InputStream = null
var output: OutputStream = null
var read = 0
try {
input = new FileInputStream(file)
output = new StringOutputStream()
val buf = new Array[Byte](BUF_SIZE)
read = input.read(buf)
while(read > 0) {
output.write(buf, 0, read)
read = input.read(buf)
}
} finally {
if(input != null) input.close()
if(output != null) output.close()
}
if(output != null) return output.toString
return null
}

def slurp(in: File): String = {
val in = new FileInputStream(in)
val out = new StringWriter()
val buf = new Array[Byte](BUF_SIZE)
def read(): Unit =
(in read buf) match {
case 0 => ()
case n =>
out.write(buf, 0, n)
read()
}
try read()
finally in.close()
out.toString
}

Tail-recursioN!
@tailrec
def read(): Unit =
case 0 => ()
case n =>
read()
}

Tail-recursioN!
@tailrec
def read(): Unit =
(in read buf) match { Base Case
case 0 => ()
case n =>
read()
}

Tail-recursioN!
@tailrec
def read(): Unit =
case 0 => () Recursive Case
case n =>
read()
}

If it seems useful, look for it

便利そうであれば、まずは探そう

def countEm(counts: Seq[Int]): Int = {
var total = 0
for(count <- counts) total += count
total
}

def countEm(counts: Seq[Int]): Int =
counts.sum

Collections!
● Scala collections have > 100 useful methods
● By far the biggest reason to use Scala is
manipulating data using collections

Scala を使う最大の理由: コレクションを使ったデータ処理

Aim Higher
higher-order-functions, that is

上を狙おう (高階関数)

def sort[T](
data: Seq[T]
)(
lessThan: (T,T) => Boolean
): Seq[T] = ...

Why higher order functions?
def sort[T](
data: Seq[T]
)(
lessThan: (T,T) => Boolean
): Seq[T] = ... Abstract the algorithm (how to
do something) from the goal
(what to do)

目的(何がしたいか) とアルゴリズム(どうやるか) を分離する

New w/ Scala 2.10
Features that alter your daily code

Scala 2.10 の新機能

Build strings the nice way
interpolators

文字列生成を楽に

val HAI = "HI"
val name = "SQUARE"
val height = 100
val width = 20

scala> val x = s"${HAI}"
x: String = HI

scala> f"$name is $height%04d meters by
$width%04d meters"
res0: String = SQUARE is 0100 meters by 0020
meters

Keep the abstract....
.... well abstract.

抽象的なものは抽象的に

A Graph Library API
trait Node[N] { trait Graph[N,E] {
def value: N type Nd = Node[N]
} type Ed = Edge[N,E]
trait Edge[N,E] {
def value: E def nodes: Set[Nd]
def from: Node[N] def edges(n: Nd): Seq[Ed]
def to: Node[N] }
}

A Graph Library API
trait Node[N] { trait Graph[N,E] {
def value: N type Nd = Node[N]
} Great for implementers,
type Ed = Edge[N,E]
trait but where's all the
Edge[N,E] {
def friendly user methods? Set[Nd]
value: E def nodes:
def from: Node[N] def edges(n: Nd): Seq[Ed]
def to: Node[N] }
}

実装する人にはいいけど、trait を使う人にやさしくない

Add behavior
with shiny new value classes and implicit classes

値クラスとimplicit クラスで振る舞いを追加する

object Graph {
implicit class Ops[N,E](val g: Graph[N,E])
extends AnyVal {

def isAcyclic: Boolean =
cycles forall (c => (c drop 1).
isEmpty)

def cycles: Set[Set[Node[N]]] =
... compute using tarjan or
other algorithm ...
}
}

Fast at runtime, too
val graph: Graph[N, E] = ...
if(graph.isCyclic) sys.error("O NOES")

Translates to static helper
method call

val graph: Graph[N, E] = ...
if(Graph.Ops.isCyclic(graph))
sys.error("O NOES")
実行時には static メソッドの呼び出しへと変換される

Cheat in an Emergency
With Dynamic Types

いざという時の動的型

class MapHelper(json: Map[String,AnyRef])
extends Dynamic {

def selectDynamic(name: String) =
(json get name
getOrElse sys.error(s"Attribute $name
not found in json: $json"))

}

scala> val m = new MapHelper(Map[String,
AnyRef]("name" -> "Jimmy", "age" -> "10"))
m: MapHelper = MapHelper@683f67e0

scala> m.name
res1: AnyRef = Jimmy

scala> m.sex
java.lang.RuntimeException: Attribute sex
not found in json: Map(name -> Jimmy, age ->
10)

Promise the Future!
Make sure to deliver

Promise とFuture (約束は守りましょう)

import scala.concurrent._
import scala.concurrent.ExecutionContext.
Implicits.global
scala> val x = promise[Int]
scala> val y = x.future
scala> y.isCompleted
res5: Boolean = false
scala> x success 5
scala> y.isCompleted
res7: Boolean = true
scala> y.value
res9: Option[Either[Throwable,Int]] = Some
(Right(5))

Why not Java Futures?
future { executor.submit(
Thread sleep 30000L
5 new
} Callable<Integer>(
) {
def call():
Creating! Integer = {
Thread sleep
3000L
new Integer(5)
}

val x: Future[Int] = val x: Future[Integer]
... =
...
Await.result(x,
10 seconds)
x.get(10,
Blo TimeUnit.SECONDS)
cki
ng
!

val x: Future[Int] = N/A
...
val y: Future[Int] =
x map { result =>
result + 5 king
} l oc
-B !
on ins
N a
ch

Scala's Futures
Provide a lightweight model for asynchronous,
non-blocking execution that can be chained and
joined throughout your application.

Future: 軽量な非同期・ノンブロッキング実行モデル

Staying Async!
See http://jsuereth.com/intro-to-fp/

いつも非同期で

trait AsynchService {
def users: Future[Seq[User]]
def projects(user: User):Future[Seq[Project]]
}

def allprojects(api: AsynchService): Future[Set
[Project]] =
(Future.traverse(api.users)(api.projects)
map (_.flatten.toSet))

Traverse is fork join
def traverse[A](
seq: Future[Seq[A]]
)(
op: A => Future[B]
): Future[Seq[B]] = ....

traverse は fork/join を用いて行われる

Feeling Crazy?
Manipulate some ASTs

ちょっと遊ぶ？AST をいじろう

object MacroAsserts {

def assert(cond: Boolean, expr: String) =
macro assertImpl

def assertImpl(c: Context)(
cond: c.Expr[Boolean],
msg: c.Expr[String]
): c.Expr[Unit] =
if(sys.env("SCALA_ASSERT") == "true")
c.reify(if(cond.splice)
sys.error(msg.splice))
else c.reify(())
}

What the user sees
macro assertImpl

msg: c.Expr[String]
): c.Expr[Unit] =
else c.reify(())
}


macro assertImpl

msg: c.Expr[String]
): c.Expr[Unit] =
else c.reify(())
}
What the compiler uses

Tree Transformation
object Bar {
assert(false,
"THIS IS A BAD OBJECT")
}

構文木の変換

object Bar {

Expr[Boolean]
} Expr[Any]

object Bar {

Macro
Expr[Boolean]
} Expr[Any]

object Bar {

Expr[Unit] Macro

}

A Brave New World
Welcome to

すばらしい新世界へようこそ

What's coming in Scala 2.11?
Nothing certain, but a list of possibilities
● Smaller
○ Modularization of components?
○ Eviction of deprecated libraries?
● Faster
○ More reliable inlining?
○ Method-Handle based closures?
● Stabler
○ Fix ALL THE BUGS
○ Reflection API Stabilizes?

より小さく、高速で、安定した Scala 2.11

Resources
Collections: http://docs.scala-lang.org/overviews/collections/introduction.html
String Interpolation: http://docs.scala-lang.org/overviews/core/string-
interpolation.html
Value Classes: http://docs.scala-lang.org/overviews/core/value-classes.html
Futures: http://docs.scala-lang.org/overviews/core/futures.html
Reflection: http://docs.scala-lang.org/overviews/reflection/overview.html
Macros: http://docs.scala-lang.org/overviews/macros/overview.html

Coding in Style

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