Class/Object

com.thoughtworks.feature

Constructor

Related Docs: object Constructor | package feature

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final class Constructor[F] extends AnyVal

An implicit value for dynamically creating classes and traits, especially dynamic mixins.

Usage 

libraryDependencies += "com.thoughtworks.feature" %% "constructor" % "latest.release"
trait A
trait B

def makeAWithB()(implicit constructor: Constructor[() => A with B]): A with B = {
  constructor.newInstance()
}

val ab: A with B = makeAWithB()

Motivation

This feature is useful for library authors. A library author may ask his user to create a trait type, then dynamically mix-in it with the features provided by the library.

Suppose you are creating a DSL that compiles to JavaScript.

You want your DSL is extensible. For example, the DSL users should be able to create custom binary operators.

With the help of Constructor.scala, you can put the boilerplate code into a private class BinaryOperator:

trait Ast

object Ast {

  class Literal(val n: Int) extends Ast {
    override final def compile(): String = n.compile()
  }

  private[Ast] abstract class BinaryOperator(leftHandSide: Ast, rightHandSide: Ast) extends Ast {
    protected def symbol: String
    override final def compile() = s"($leftHandSide $symbol $rightHandSide)"
  }

  def binaryOperator[T](leftHandSide: Ast, rightHandSide: Ast)(
    implicit constructor: Constructor[(Ast, Ast) => BinaryOperator with T]): BinaryOperator with T = {
    constructor.newInstance(leftHandSide, rightHandSide)
  }

}

The users only need a very simple implementation for their custom binary operators.

import Ast._

trait Plus {
  protected final def symbol = "+"
}

trait Minus {
  protected final def symbol = "-"
}

val myAst = binaryOperator[Plus](
  new Literal(1),
  binaryOperator[Minus](
    new Literal(3),
    new Literal(5)
  )
)

print(myAst.compile()) // Output: "(1 + (3 - 5))"

An alternative approach

There is another approach to integrate partial implementation from users: asking users to provide custom callback functions or type classes.

However, the callback functions or type classes approach will create additional object instances and additional references for each instance at run-time. On the other hand, the Constructor.scala approach create classes at compile-time and no additional run-time references. As a result, at run-time, Constructor.scala approach will consume less memory, and performs less indirect access on memory.

Author:

杨博 (Yang Bo) <[email protected]>

Source
Constructor.scala
Linear Supertypes
AnyVal, Any
Type Hierarchy
Ordering
  1. Alphabetic
  2. By Inheritance
Inherited
  1. Constructor
  2. AnyVal
  3. Any
Implicitly
  1. by any2stringadd
  2. by StringFormat
  3. by Ensuring
  4. by ArrowAssoc
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Visibility
  1. Public
  2. All

Instance Constructors

  1. new Constructor(newInstance: F)

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Value Members

  1. final def !=(arg0: Any): Boolean

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    Definition Classes
    Any

  2. final def ##(): Int

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    Definition Classes
    Any

  3. def +(other: String): String

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    Implicit information
    This member is added by an implicit conversion from Constructor[F] to any2stringadd[Constructor[F]] performed by method any2stringadd in scala.Predef.
    Definition Classes
    any2stringadd

  4. def ->[B](y: B): (Constructor[F], B)

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    Implicit information
    This member is added by an implicit conversion from Constructor[F] to ArrowAssoc[Constructor[F]] performed by method ArrowAssoc in scala.Predef.
    Definition Classes
    ArrowAssoc
    Annotations
    @inline()

  5. final def ==(arg0: Any): Boolean

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    Definition Classes
    Any

  6. final def asInstanceOf[T0]: T0

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    Definition Classes
    Any

  7. def ensuring(cond: (Constructor[F]) ⇒ Boolean, msg: ⇒ Any): Constructor[F]

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    Implicit information
    This member is added by an implicit conversion from Constructor[F] to Ensuring[Constructor[F]] performed by method Ensuring in scala.Predef.
    Definition Classes
    Ensuring

  8. def ensuring(cond: (Constructor[F]) ⇒ Boolean): Constructor[F]

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    Implicit information
    This member is added by an implicit conversion from Constructor[F] to Ensuring[Constructor[F]] performed by method Ensuring in scala.Predef.
    Definition Classes
    Ensuring

  9. def ensuring(cond: Boolean, msg: ⇒ Any): Constructor[F]

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    Implicit information
    This member is added by an implicit conversion from Constructor[F] to Ensuring[Constructor[F]] performed by method Ensuring in scala.Predef.
    Definition Classes
    Ensuring

  10. def ensuring(cond: Boolean): Constructor[F]

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    Implicit information
    This member is added by an implicit conversion from Constructor[F] to Ensuring[Constructor[F]] performed by method Ensuring in scala.Predef.
    Definition Classes
    Ensuring

  11. def formatted(fmtstr: String): String

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    Implicit information
    This member is added by an implicit conversion from Constructor[F] to StringFormat[Constructor[F]] performed by method StringFormat in scala.Predef.
    Definition Classes
    StringFormat
    Annotations
    @inline()

  12. def getClass(): Class[_ <: AnyVal]

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    Definition Classes
    AnyVal → Any

  13. final def isInstanceOf[T0]: Boolean

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    Definition Classes
    Any

  14. val newInstance: F

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  15. def toString(): String

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    Definition Classes
    Any

  16. def →[B](y: B): (Constructor[F], B)

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    Implicit information
    This member is added by an implicit conversion from Constructor[F] to ArrowAssoc[Constructor[F]] performed by method ArrowAssoc in scala.Predef.
    Definition Classes
    ArrowAssoc

Inherited from AnyVal

Inherited from Any

Inherited by implicit conversion any2stringadd from Constructor[F] to any2stringadd[Constructor[F]]

Inherited by implicit conversion StringFormat from Constructor[F] to StringFormat[Constructor[F]]

Inherited by implicit conversion Ensuring from Constructor[F] to Ensuring[Constructor[F]]

Inherited by implicit conversion ArrowAssoc from Constructor[F] to ArrowAssoc[Constructor[F]]

Ungrouped