The document discusses Scala's ability to combine functional and object-oriented programming paradigms. It provides an example of an Emitter class that uses mutable internal state carried between method invocations to improve performance, while maintaining an immutable/functional API that encapsulates the mutable state so it is not observable from outside the class. This approach allows algorithms with mutable data structures internally for readability while preserving the benefits of immutable/functional programming in the public interface.

1. Functional OO ImperativeFunctional OO Imperative
ScalaScala
関数型オブジェクト指向命関数型オブジェクト指向命
令型 Scala令型 Scala
Sébastien Doeraene -- セバスチャン·ドゥラン
June 28, 2019 -- Scala Matsuri -- 2019年6⽉28⽇
@sjrdoeraene

2. Scala Center,
École polytechnique fédérale de Lausanne
scala.epfl.ch
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3. BasicsBasics
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4. def times2(xs: List[Int]): List[Int] = {
var result: List[Int] = Nil
var i = 0
while (i < xs.length) {
result = result :+ (xs(i) * 2)
i += 1
}
result
}
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5. def times2(xs: List[Int]): List[Int] = {
var result: List[Int] = Nil
for (i <- 0 until xs.length) {
result = result :+ (xs(i) * 2)
}
result
}
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6. def times2(xs: List[Int]): List[Int] = {
val builder = List.newBuilder[Int]
for (i <- 0 until xs.length) {
builder += xs(i) * 2
}
builder.result()
}
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7. def times2(xs: List[Int]): List[Int] = {
val builder = List.newBuilder[Int]
for (x <- xs) {
builder += x * 2
}
builder.result()
}
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8. def times2(xs: List[Int]): List[Int] = {
for (x <- xs)
yield x * 2
}
def times2(xs: List[Int]): List[Int] = {
xs.map(x => x * 2)
}
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9. Sometimes, builders are better
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10. def fromPathClasspath(classpath: Seq[Path]): (Seq[IRContainer], Seq[Path]) = {
val containers = Seq.newBuilder[IRContainer]
val paths = Seq.newBuilder[Path]
for (entry <- classpath if Files.exists(entry)) {
val attrs = Files.readAttributes(entry, classOf[BasicFileAttributes])
if (attrs.isDirectory()) {
walkIR(entry) { (path, attrs) =>
containers += IRContainer.fromIRFile(...)
paths += path
}
} else if (entry.getFileName().toString().endsWith(".jar")) {
containers += new JarIRContainer(entry, attrs.lastModifiedTime())
paths += entry
} else {
throw new IllegalArgumentException("Illegal classpath entry " + entry)
}
}
(containers.result(), paths.result())
}
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11. def fromPathClasspath(classpath: Seq[Path]): (Seq[IRContainer], Seq[Path]) = {
val containers = Seq.newBuilder[IRContainer]
val paths = Seq.newBuilder[Path]
for (entry <- classpath if Files.exists(entry)) {
val attrs = Files.readAttributes(entry, classOf[BasicFileAttributes])
if (attrs.isDirectory()) {
walkIR(entry) { (path, attrs) =>
containers += IRContainer.fromIRFile(...)
paths += path
}
} else if (entry.getFileName().toString().endsWith(".jar")) {
containers += new JarIRContainer(entry, attrs.lastModifiedTime())
paths += entry
} else {
throw new IllegalArgumentException("Illegal classpath entry " + entry)
}
}
(containers.result(), paths.result())
}
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12. Sometimes, s are better
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13. var test = {
genIsScalaJSObject(obj) &&
genIsClassNameInAncestors(...)
}
def typeOfTest(typeString: String): js.Tree = ...
if (isAncestorOfString)
test = test || typeOfTest("string")
if (isAncestorOfHijackedNumberClass) {
test = test || typeOfTest("number")
if (useBigIntForLongs)
test = test || genCallHelper("isLong", obj)
}
if (isAncestorOfBoxedBooleanClass)
test = test || typeOfTest("boolean")
if (isAncestorOfBoxedCharacterClass)
test = test || (obj instanceof envField("Char"))
test
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14. var test = {
genIsScalaJSObject(obj) &&
genIsClassNameInAncestors(...)
}
def typeOfTest(typeString: String): js.Tree = ...
if (isAncestorOfString)
test = test || typeOfTest("string")
if (isAncestorOfHijackedNumberClass) {
test = test || typeOfTest("number")
if (useBigIntForLongs)
test = test || genCallHelper("isLong", obj)
}
if (isAncestorOfBoxedBooleanClass)
test = test || typeOfTest("boolean")
if (isAncestorOfBoxedCharacterClass)
test = test || (obj instanceof envField("Char"))
test
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15. val test0 = {
genIsScalaJSObject(obj) &&
genIsClassNameInAncestors(...)
}
def typeOfTest(typeString: String): js.Tree = ...
val test1 =
if (isAncestorOfString) test0 || typeOfTest("string")
else test0
val test3 = if (isAncestorOfHijackedNumberClass) {
val test2 = test1 || typeOfTest("number")
if (useBigIntForLongs) test2 || genCallHelper("isLong", obj)
else test2
}
val test4 =
if (isAncestorOfBoxedBooleanClass) test3 || typeOfTest("boolean")
else test3
val test5 =
if (isAncestorOfBoxedCharacterClass) test4 || (obj instanceof envField("Char"))
else test4
test5
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16. val test0 = {
genIsScalaJSObject(obj) &&
genIsClassNameInAncestors(...)
}
def typeOfTest(typeString: String): js.Tree = ...
val test1 =
if (isAncestorOfString) test0 || typeOfTest("string")
else test0
val test3 = if (isAncestorOfHijackedNumberClass) {
val test2 = test1 || typeOfTest("number")
if (useBigIntForLongs) test2 || genCallHelper("isLong", obj)
else test2
}
val test4 =
if (isAncestorOfBoxedBooleanClass) test3 || typeOfTest("boolean")
else test3
val test5 =
if (isAncestorOfBoxedCharacterClass) test4 || (obj instanceof envField("Char"))
else test4
test5
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17. val test0 = {
genIsScalaJSObject(obj) &&
genIsClassNameInAncestors(...)
}
def typeOfTest(typeString: String): js.Tree = ...
val test1 =
if (isAncestorOfString) test0 || typeOfTest("string")
else test0
val test3 = if (isAncestorOfHijackedNumberClass) {
val test2 = test1 || typeOfTest("number")
if (useBigIntForLongs) test2 || genCallHelper("isLong", obj)
else test2
}
val test4 =
if (isAncestorOfBoxedBooleanClass) test3 || typeOfTest("boolean")
else test3
val test5 =
if (isAncestorOfBoxedCharacterClass) test4 || (obj instanceof envField("Char"))
else test4
test5
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18. val test0 = {
genIsScalaJSObject(obj) &&
genIsClassNameInAncestors(...)
}
def typeOfTest(typeString: String): js.Tree = ...
val test1 =
if (isAncestorOfString) test0 || typeOfTest("string")
else test0
val test3 = if (isAncestorOfHijackedNumberClass) {
val test2 = test1 || typeOfTest("number")
if (useBigIntForLongs) test2 || genCallHelper("isLong", obj)
else test2
}
val test4 =
if (isAncestorOfBoxedBooleanClass) test3 || typeOfTest("boolean")
else test3
val test5 =
if (isAncestorOfBoxedCharacterClass) test4 || (obj instanceof envField("Char"))
else test4
test5
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19. In the (super) smallIn the (super) small
Immutable/functional API
Locally imperative implementation (if more readable)
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20. Algorithms with mutable internal dataAlgorithms with mutable internal data
structuresstructures
The of Scala.js
The change detection algorithm of the optimizer
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21. At the instance levelAt the instance level
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22. final class Emitter(config: CommonPhaseConfig) {
def emitAll(unit: LinkingUnit, builder: JSLineBuilder,
logger: Logger): Unit = {
...
}
}
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23. final class Emitter(config: CommonPhaseConfig) {
private class State(val lastMentionedDangerousGlobalRefs: Set[String]) {
val jsGen: JSGen = new JSGen(..., lastMentionedDangerousGlobalRefs)
val classEmitter: ClassEmitter = new ClassEmitter(jsGen)
val coreJSLib: WithGlobals[js.Tree] = CoreJSLib.build(jsGen)
}
private var state: State = new State(Set.empty)
private def jsGen: JSGen = state.jsGen
private def classEmitter: ClassEmitter = state.classEmitter
private def coreJSLib: WithGlobals[js.Tree] = state.coreJSLib
private val classCaches = mutable.Map.empty[List[String], ClassCache]
def emitAll(unit: LinkingUnit, builder: JSLineBuilder,
logger: Logger): Unit = {
...
classCaches.getOrElseUpdate(..., ...)
...
val mentionedDangerousGlobalRefs = ...
state = new State(mentionedDangerousGlobalRefs)
...
}
}
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24. final class Emitter(config: CommonPhaseConfig) {
private class State(val lastMentionedDangerousGlobalRefs: Set[String]) {
val jsGen: JSGen = new JSGen(..., lastMentionedDangerousGlobalRefs)
val classEmitter: ClassEmitter = new ClassEmitter(jsGen)
val coreJSLib: WithGlobals[js.Tree] = CoreJSLib.build(jsGen)
}
private var state: State = new State(Set.empty)
private def jsGen: JSGen = state.jsGen
private def classEmitter: ClassEmitter = state.classEmitter
private def coreJSLib: WithGlobals[js.Tree] = state.coreJSLib
private val classCaches = mutable.Map.empty[List[String], ClassCache]
def emitAll(unit: LinkingUnit, builder: JSLineBuilder,
logger: Logger): Unit = {
...
classCaches.getOrElseUpdate(..., ...)
...
val mentionedDangerousGlobalRefs = ...
state = new State(mentionedDangerousGlobalRefs)
...
}
}
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25. final class Emitter(config: CommonPhaseConfig) {
private class State(val lastMentionedDangerousGlobalRefs: Set[String]) {
val jsGen: JSGen = new JSGen(..., lastMentionedDangerousGlobalRefs)
val classEmitter: ClassEmitter = new ClassEmitter(jsGen)
val coreJSLib: WithGlobals[js.Tree] = CoreJSLib.build(jsGen)
}
private var state: State = new State(Set.empty)
private def jsGen: JSGen = state.jsGen
private def classEmitter: ClassEmitter = state.classEmitter
private def coreJSLib: WithGlobals[js.Tree] = state.coreJSLib
private val classCaches = mutable.Map.empty[List[String], ClassCache]
def emitAll(unit: LinkingUnit, builder: JSLineBuilder,
logger: Logger): Unit = {
...
classCaches.getOrElseUpdate(..., ...)
...
val mentionedDangerousGlobalRefs = ...
state = new State(mentionedDangerousGlobalRefs)
...
}
}
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26. final class Emitter(config: CommonPhaseConfig) {
private class State(val lastMentionedDangerousGlobalRefs: Set[String]) {
val jsGen: JSGen = new JSGen(..., lastMentionedDangerousGlobalRefs)
val classEmitter: ClassEmitter = new ClassEmitter(jsGen)
val coreJSLib: WithGlobals[js.Tree] = CoreJSLib.build(jsGen)
}
private var state: State = new State(Set.empty)
private def jsGen: JSGen = state.jsGen
private def classEmitter: ClassEmitter = state.classEmitter
private def coreJSLib: WithGlobals[js.Tree] = state.coreJSLib
private val classCaches = mutable.Map.empty[List[String], ClassCache]
def emitAll(unit: LinkingUnit, builder: JSLineBuilder,
logger: Logger): Unit = {
...
classCaches.getOrElseUpdate(..., ...)
...
val mentionedDangerousGlobalRefs = ...
state = new State(mentionedDangerousGlobalRefs)
...
}
}
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27. At the instance levelAt the instance level
Immutable/functional object API
Mutable state carried between invocations
But not observable from the outside
The state is encapsulated using object-orientation
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28. At the instance levelAt the instance level
Not possible using pure functional programming
Difficult to reason about if the state is observable (using
an imperative API)
Unique power of combining functional programming,
object-orientation and imperative features
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29. Used at several levels:
The emitter
The optimizer
The caches for files
The all-encompassing method
etc.
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30. Open class hierarchiesOpen class hierarchies
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31. So far: traits and classes
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32. /** A backend of a standard Scala.js linker. */
abstract class LinkerBackend {
/** Core specification that this linker backend implements. */
val coreSpec: CoreSpec
/** Symbols this backend needs to be present in the linking unit. */
val symbolRequirements: SymbolRequirement
/** Emit the given LinkingUnit to the target output. */
def emit(unit: LinkingUnit, output: LinkerOutput, logger: Logger)(
implicit ec: ExecutionContext): Future[Unit]
}
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33. /** The basic backend for the Scala.js linker. */
final class BasicLinkerBackend(config: LinkerBackendImpl.Config)
extends LinkerBackend {
val coreSpec = config.commonConfig.coreSpec
private[this] val emitter = new Emitter(config.commonConfig)
val symbolRequirements: SymbolRequirement = emitter.symbolRequirements
def emit(unit: LinkingUnit, output: LinkerOutput, logger: Logger)(
implicit ec: ExecutionContext): Future[Unit] = {
...
val builder = new JSFileBuilder
emitter.emitAll(unit, builder, logger)
OutputFileImpl.fromOutputFile(output.jsFile)
.writeFull(builer.complete())
...
}
}
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34. /** The Closure backend of the Scala.js linker. */
final class ClosureLinkerBackend(config: LinkerBackendImpl.Config)
extends LinkerBackend {
val coreSpec = config.commonConfig.coreSpec
private[this] val emitter = new Emitter(config.commonConfig)
val symbolRequirements: SymbolRequirement = emitter.symbolRequirements
def emit(unit: LinkingUnit, output: LinkerOutput, logger: Logger)(
implicit ec: ExecutionContext): Future[Unit] = {
...
val builer = new ClosureModuleBuilder
emitter.emitAll(unit, builer, logger)
val closureModules = makeClosureModules(builder.result())
val result = closureCompiler.compileModules(..., closureModules, ...)
writeResult(result, ...)
...
}
}
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35. Possible to write your own Scala.js backend in user-space
actually does that,
to collect imported modules for Webpack
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36. /** A JavaScript execution environment.
*
* This can run and interact with JavaScript code.
*
* Any implementation is expected to be fully thread-safe.
*/
trait JSEnv {
/** Human-readable name for this [[JSEnv]] */
val name: String
/** Starts a new (asynchronous) JS run. */
def start(input: Input, config: RunConfig): JSRun
/** Like [[start]], but initializes a communication channel. */
def startWithCom(input: Input, config: RunConfig,
onMessage: String => Unit): JSComRun
}
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37. In the main repo, one implementation:
Completely unrelated but compatible implementations
are in other repos:
Custom environments in users' builds
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38. Custom for comprehensionsCustom for comprehensions
the M word
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39. def genMethod(className: String, method: MethodDef)(
implicit globalKnowledge: GlobalKnowledge): WithGlobals[js.Tree] = {
val methodBody = method.body.getOrElse(
throw new AssertionError("Cannot generate an abstract method"))
implicit val pos = method.pos
val namespace = method.flags.namespace
val methodFunWithGlobals: WithGlobals[js.Function] =
desugarToFunction(className, method.args, methodBody, method.resultType)
methodFunWithGlobals.flatMap { methodFun =>
if (namespace != MemberNamespace.Public) {
...
} else {
if (useClasses) {
for (propName <- genPropertyName(method.name)) yield {
js.MethodDef(static = false, propName, methodFun.args,
methodFun.body)
}
} else {
genAddToPrototype(className, method.name, methodFun)
}
}
}
}
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40. def genMethod(className: String, method: MethodDef)(
implicit globalKnowledge: GlobalKnowledge): WithGlobals[js.Tree] = {
val methodBody = method.body.getOrElse(
throw new AssertionError("Cannot generate an abstract method"))
implicit val pos = method.pos
val namespace = method.flags.namespace
val methodFunWithGlobals: WithGlobals[js.Function] =
desugarToFunction(className, method.args, methodBody, method.resultType)
methodFunWithGlobals.flatMap { methodFun =>
if (namespace != MemberNamespace.Public) {
...
} else {
if (useClasses) {
for (propName <- genPropertyName(method.name)) yield {
js.MethodDef(static = false, propName, methodFun.args,
methodFun.body)
}
} else {
genAddToPrototype(className, method.name, methodFun)
}
}
}
}
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41. def genMethod(className: String, method: MethodDef)(
implicit globalKnowledge: GlobalKnowledge): WithGlobals[js.Tree] = {
val methodBody = method.body.getOrElse(
throw new AssertionError("Cannot generate an abstract method"))
implicit val pos = method.pos
val namespace = method.flags.namespace
val methodFunWithGlobals: WithGlobals[js.Function] =
desugarToFunction(className, method.args, methodBody, method.resultType)
methodFunWithGlobals.flatMap { methodFun =>
if (namespace != MemberNamespace.Public) {
...
} else {
if (useClasses) {
for (propName <- genPropertyName(method.name)) yield {
js.MethodDef(static = false, propName, methodFun.args,
methodFun.body)
}
} else {
genAddToPrototype(className, method.name, methodFun)
}
}
}
}
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42. def genMethod(className: String, method: MethodDef)(
implicit globalKnowledge: GlobalKnowledge): WithGlobals[js.Tree] = {
val methodBody = method.body.getOrElse(
throw new AssertionError("Cannot generate an abstract method"))
implicit val pos = method.pos
val namespace = method.flags.namespace
val methodFunWithGlobals: WithGlobals[js.Function] =
desugarToFunction(className, method.args, methodBody, method.resultType)
methodFunWithGlobals.flatMap { methodFun =>
if (namespace != MemberNamespace.Public) {
...
} else {
if (useClasses) {
for (propName <- genPropertyName(method.name)) yield {
js.MethodDef(static = false, propName, methodFun.args,
methodFun.body)
}
} else {
genAddToPrototype(className, method.name, methodFun)
}
}
}
}
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43. /** A monad that associates a set of global variable names to a value. */
private[emitter] final case class WithGlobals[+A](
value: A, globalVarNames: Set[String]) {
def map[B](f: A => B): WithGlobals[B] =
WithGlobals(f(value), globalVarNames)
def flatMap[B](f: A => WithGlobals[B]): WithGlobals[B] = {
val t = f(value)
WithGlobals(t.value, globalVarNames ++ t.globalVarNames)
}
}
private[emitter] object WithGlobals {
/** Constructs a `WithGlobals` with an empty set `globalVarNames`. */
def apply[A](value: A): WithGlobals[A] =
new WithGlobals(value, Set.empty)
def list[A](xs: List[WithGlobals[A]]): WithGlobals[List[A]] =
...
def option[A](xs: Option[WithGlobals[A]]): WithGlobals[Option[A]] =
...
} 31

44. Further exploration of the codeFurther exploration of the code
Time permitting
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45.
scala-js.org scala.epfl.ch
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