Packages

t

s_mach.codetools

IsDistinctTypeAlias

trait IsDistinctTypeAlias[A] extends AnyRef

Marker trait used to mark a type alias as being a distinct type alias (DTA). A DTA is an alternative to the Scala value-class (http://docs.scala-lang.org/overviews/core/value-classes.html) that never needs to box (or unbox) since type aliases are eliminated in byte code.

A DTA is declared with a unique empty trait "tag" to make the type alias distinct in the type system. Since the DTA is a distinct type from its alias, implicit generics (e.g. type-classes) of the DTA can be resolved separately from the aliased type. Implicits resolving a normal type-alias will always resolve to an instance for the aliased type.

Example:

type Age1 = Int
implicitly[Format[Age1] ] // resolves to Format[Int] b/c Age1 isn't distinct
trait AgeTag // empty "tag" trait
type Age = Int with AgeTag
implicitly[Format[Age] ] // resolves to Format[Age] b/c Age is distinct

Unlike value-classes, which are elided by the compiler under most circumstances, DTAs are always elided at runtime. Value-classes used in any kind of generic, such as List or Option, cannot be elided by the compiler and boxing/unboxing penalties are incurred.

This means if a conversion from a generic of the value-class type to the underlying type is required (or vice-versa), a map operation must be used to box or unbox the value-class. This incurs a O(n) penalty simply to cast between the value-class and the underlying type. Because DTAs are type aliases for their underlying type the compiler treats a generic of a DTA as a generic of the underlying type without any impact on runtime.

While covariant generics will automatically cast from a generic of the DTA to its underlying type, casting from the underlying type to the DTA is not supported directly. Instead an implicit generic converter must be imported:

Example generic converter:

implicit def ma_to_mv[M[_],V < : IsDistinctTypeAlias[A],A](
  ma: M[A]
) : M[V] = ma.asInstanceOf[M[V] ]

Note: this converter is declared in codetools package.scala

To declare a DTA, first create an empty trait tag. Next, declare a type alias for the aliased type but that also extends the tag and the marker trait IsDistinctTypeAlias. Finally, declare an implicit def constructor method that mimics the apply method of a companion object.

Example:

trait AgeTag
type Age = Int with AgeTag with IsDistinctTypeAlias[Int]
implicit def Age(i: Int) = i.asInstanceOf[Age]

Note1: The IsDistinctTypeAlias marker trait self-documents the DTA and is used by the implicit def converters to prevent ambiguous implicit resolution errors. Note2: While it is possible to declare an object for the DTA, it will not be treated as a companion object for the DTA by the compiler (implicit generics in the object will not resolve implicitly like normal companion objects)

A

type aliased by the DTA

Self Type
IsDistinctTypeAlias[A] with A
Linear Supertypes
AnyRef, Any
Ordering
  1. Alphabetic
  2. By Inheritance
Inherited
  1. IsDistinctTypeAlias
  2. AnyRef
  3. Any
  1. Hide All
  2. Show All
Visibility
  1. Public
  2. All

Value Members

  1. final def !=(arg0: Any): Boolean
    Definition Classes
    AnyRef → Any
  2. final def ##(): Int
    Definition Classes
    AnyRef → Any
  3. final def ==(arg0: Any): Boolean
    Definition Classes
    AnyRef → Any
  4. final def asInstanceOf[T0]: T0
    Definition Classes
    Any
  5. def clone(): AnyRef
    Attributes
    protected[java.lang]
    Definition Classes
    AnyRef
    Annotations
    @throws( ... )
  6. final def eq(arg0: AnyRef): Boolean
    Definition Classes
    AnyRef
  7. def equals(arg0: Any): Boolean
    Definition Classes
    AnyRef → Any
  8. def finalize(): Unit
    Attributes
    protected[java.lang]
    Definition Classes
    AnyRef
    Annotations
    @throws( classOf[java.lang.Throwable] )
  9. final def getClass(): Class[_]
    Definition Classes
    AnyRef → Any
  10. def hashCode(): Int
    Definition Classes
    AnyRef → Any
  11. final def isInstanceOf[T0]: Boolean
    Definition Classes
    Any
  12. final def ne(arg0: AnyRef): Boolean
    Definition Classes
    AnyRef
  13. final def notify(): Unit
    Definition Classes
    AnyRef
  14. final def notifyAll(): Unit
    Definition Classes
    AnyRef
  15. final def synchronized[T0](arg0: ⇒ T0): T0
    Definition Classes
    AnyRef
  16. def toString(): String
    Definition Classes
    AnyRef → Any
  17. final def wait(): Unit
    Definition Classes
    AnyRef
    Annotations
    @throws( ... )
  18. final def wait(arg0: Long, arg1: Int): Unit
    Definition Classes
    AnyRef
    Annotations
    @throws( ... )
  19. final def wait(arg0: Long): Unit
    Definition Classes
    AnyRef
    Annotations
    @throws( ... )

Inherited from AnyRef

Inherited from Any

Ungrouped