What is Composability
in Scala

@lyrical_logical

Composition

• Kinds of Composition in Scala
  • Functional Composition
    • ​compose, etc...
  • Objective Composition​​​​
    • e.g. DI, Inheritance, etc...
    • coined word, not a established term sry

Functional Composition

• FunctionN

val increment: Int => Int = n => n + 1
val double: Int => Int = n => n * 2
val composed = double.compose(increment)
// or increment.andThen(double)

composed(10) // => 22

• Parser Combinator

object Calculator extends RegexParsers {
  def number = """\d+(\.\d*)?""".r ^^ { _.toDouble }
  def factor = number | "(" ~> expr <~ ")"
  def term = factor ~ rep( "*" ~ factor | "/" ~ factor) ^^ {
    case number ~ list => ...
  }
  def expr = term ~ rep("+" ~ term | "-" ~ term) ^^ {
    case number ~ list => ...
  }
}

• Functional Composition
  • composite functions as processes
  • ​big function from small functions
  • complex function from trivial functions
  • reduce complexity of functions

• composability of functions
  • achieved from abstraction
  • abstraction makes codes more trivial

// complex
def complexFunction(s: String, n: Int): Int => Unit
// trivial
def identity[T](t: T): T

someFunction.compose(identity)

Objective Composition

• Monolithic

class LogRetriever {
  // hard wiring
  val auth = new OAuth2Authorizer
  val logApi = new LogApi
  def retrieve() = {
    auth.autorize match {
      case Success(token) => logApi.getLatests(token)
      case Failure => ...
    }
  }
}

• Traditional Dependency Injection

class LogRetriever(auth: OAuth2Authorizer, logApi: LogApi) {
  def retrieve() = {
    auth.autorize match {
      case Success(token) => logApi.getLatests(token)
      case Failure => ....
    }
  }
}

new LogRetriever(new OAuth2Authorizer, new LogApi)

• Traditional Dependency Injection

class LogRetriever(auth: OAuth2Authorizer, logApi: LogApi) {
  def retrieve() = {
    auth.autorize match {
      case Success(token) => logApi.getLatests(token)
      case Failure => ....
    }
  }
}

new LogRetriever(new OAuth2Authorizer, new LogApi)

• Traditional DI Lacks
  • inject only values and methods, not types
  • if injected objects' signature has a detail
    • exposure details like a `token`
    • type of `token` is public if no need
    • ​implementation hiding is failed​

• DI by self-type annotations and mix-in

class LogRetriever {
  this: {
    type Token
    def authorize(): Token
    def getLatests(token: Token): Seq[String]
  } =>
  def retrieve() = {
   autorize() match {
      case Success(token) => getLatests(token)
      case Failure => ....
    }
  }
}

new LogRetriever extends OAuth2Authorizer with LogApi

• DI by self-type annotations and mix-in
  • inject types by abstract type member
    • ​token's details are hided
  • self-type annotations as "implemented by"​
    • required signatures become more clear

• Objective Composition
  • composite classes as module
  • ​big module from small modules
  • complex module from trivial module
  • reduce complexity of programs

• composability of objects
  • achieved from abstraction
  • abstraction makes codes more trivial
  • abstract type member help your codes
    • implementation hiding, etc...
    • like opaque pointer

class SomeClass { this: {
    type T // implementation hiding
  } =>
  // implemented by T
  // but SomeClass doesn't needs know about T
}

• These techniques are Scala's original?
  • already existed
  • called Functor in ML's module programming
    • ​not subclass of Monad
  • ​Scala's object system can correspond
    • ​ML's module system, wow!

Scala

• object
• trait
• abstract type member
• DI, mixin, function, etc...
  • instantiation methods

ML

• module
• signature
• abstract type
• functor
  • create modules from an other module

Correspondences

Scala

ML

Correspondences(signature)

trait Validator {
  type T
  def validate(t: T): Boolean
}

// normally, use generics in Scala
trait Validator[T] {
  def validate(t: T): Boolean
}

module type Validator = sig
  type t
  val validate: t -> bool
end

Scala

ML

Correspondences(module)

object Configuration {
  val credential = ...
  val loggerName = ...
}

module Configuration = struct
  val credential = ...
  val loggerName = ...
end

Scala

ML

Correspondences(functor)

trait IntHashMap {
  self: {
    type T
    def hash(n: Int): Int
  } =>

  def get(k: Int): T = ...
  def insert(k: Int, v: T): T = ...
}
// need instantiation
new IntHashMap extends Details

module IntHashSet (
  M: sig
    type t
    val hash: int -> int
  end
) = struct
  let get k = ...
  let insert kv = ...
end

• Summary
  • Two kinds of compositions in Scala
    • ​Functional, Objective
  • Functional Composition makes
    • ​reduce your code's complexity
  • ​Objective Composition makes
    • ​reduce your program's complexity
    • corresponds ML's module programming
  • ​gain Composability by abstraction

Any questions?

がっこうぐらし！三巻より引用