scala.collection.generic

IsTraversableLike

trait IsTraversableLike[Repr] extends AnyRef

A trait which can be used to avoid code duplication when defining extension methods that should be applicable both to existing Scala collections (i.e., types extending GenTraversableLike) as well as other (potentially user-defined) types that could be converted to a Scala collection type. This trait makes it possible to treat Scala collections and types that can be implicitly converted to a collection type uniformly. For example, one can provide extension methods that work both on collection types and on Strings (Strings do not extend GenTraversableLike, but can be converted to GenTraversableLike)

IsTraversable provides two members:

  1. type member A, which represents the element type of the target GenTraversableLike[A, Repr]
  2. value member conversion, which provides a way to convert between the type we wish to add extension methods to, Repr, and GenTraversableLike[A, Repr].
Usage

One must provide IsTraversableLike as an implicit parameter type of an implicit conversion. Its usage is shown below. Our objective in the following example is to provide a generic extension method mapReduce to any type that extends or can be converted to GenTraversableLike. In our example, this includes String.

import scala.collection.GenTraversableLike
  import scala.collection.generic.IsTraversableLike

  class ExtensionMethods[A, Repr](coll: GenTraversableLike[A, Repr]) {
    def mapReduce[B](mapper: A => B)(reducer: (B, B) => B): B = {
      val iter = coll.toIterator
      var res = mapper(iter.next())
      while (iter.hasNext)
        res = reducer(res, mapper(iter.next()))
      res
    }
  }

  implicit def withExtensions[Repr](coll: Repr)(implicit traversable: IsTraversableLike[Repr]) =
    new ExtensionMethods(traversable.conversion(coll))

// See it in action!
List(1, 2, 3).mapReduce(_ * 2)(_ + _) // res0: Int = 12
"Yeah, well, you know, that's just, like, your opinion, man.".mapReduce(x => 1)(_ + _) // res1: Int = 59

Here, we begin by creating a class ExtensionMethods which contains our mapReduce extension method. Note that ExtensionMethods takes a constructor argument coll of type GenTraversableLike[A, Repr], where A represents the element type and Repr represents (typically) the collection type. The implementation of mapReduce itself is straightforward.

The interesting bit is the implicit conversion withExtensions, which returns an instance of ExtensionMethods. This implicit conversion can only be applied if there is an implicit value traversable of type IsTraversableLike[Repr] in scope. Since IsTraversableLike provides value member conversion, which gives us a way to convert between whatever type we wish to add an extension method to (in this case, Repr) and GenTraversableLike[A, Repr], we can now convert coll from type Repr to GenTraversableLike[A, Repr]. This allows us to create an instance of the ExtensionMethods class, which we pass our new GenTraversableLike[A, Repr] to.

When the mapReduce method is called on some type of which it is not a member, implicit search is triggered. Because implicit conversion withExtensions is generic, it will be applied as long as an implicit value of type IsTraversableLike[Repr] can be found. Given that IsTraversableLike contains implicit members that return values of type IsTraversableLike, this requirement is typically satisfied, and the chain of interactions described in the previous paragraph is set into action. (See the IsTraversableLike companion object, which contains a precise specification of the available implicits.)

Note: Currently, it's not possible to combine the implicit conversion and the class with the extension methods into an implicit class due to limitations of type inference.

Implementing IsTraversableLike for New Types

One must simply provide an implicit value of type IsTraversableLike specific to the new type, or an implicit conversion which returns an instance of IsTraversableLike specific to the new type.

Below is an example of an implementation of the IsTraversableLike trait where the Repr type is String.

implicit val stringRepr: IsTraversableLike[String] { type A = Char } =
new IsTraversableLike[String] {
  type A = Char
  val conversion = implicitly[String => GenTraversableLike[Char, String]]
}
Source
IsTraversableLike.scala
Since

2.10

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  1. IsTraversableLike
  2. AnyRef
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  1. by any2stringadd
  2. by any2stringfmt
  3. by any2ArrowAssoc
  4. by any2Ensuring
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Type Members

  1. abstract type A

    The type of elements we can traverse over.

Abstract Value Members

  1. abstract val conversion: (Repr) ⇒ GenTraversableLike[A, Repr]

    A conversion from the representation type Repr to a GenTraversableLike[A,Repr].

Concrete Value Members

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

    Definition Classes
    AnyRef
  2. final def !=(arg0: Any): Boolean

    Test two objects for inequality.

    Test two objects for inequality.

    returns

    true if !(this == that), false otherwise.

    Definition Classes
    Any
  3. final def ##(): Int

    Equivalent to x.hashCode except for boxed numeric types and null.

    Equivalent to x.hashCode except for boxed numeric types and null. For numerics, it returns a hash value which is consistent with value equality: if two value type instances compare as true, then ## will produce the same hash value for each of them. For null returns a hashcode where null.hashCode throws a NullPointerException.

    returns

    a hash value consistent with ==

    Definition Classes
    AnyRef → Any
  4. def +(other: String): String

    Implicit information
    This member is added by an implicit conversion from IsTraversableLike[Repr] to StringAdd performed by method any2stringadd in scala.Predef.
    Definition Classes
    StringAdd
  5. def ->[B](y: B): (IsTraversableLike[Repr], B)

    Implicit information
    This member is added by an implicit conversion from IsTraversableLike[Repr] to ArrowAssoc[IsTraversableLike[Repr]] performed by method any2ArrowAssoc in scala.Predef.
    Definition Classes
    ArrowAssoc
    Annotations
    @inline()
  6. final def ==(arg0: AnyRef): Boolean

    Definition Classes
    AnyRef
  7. final def ==(arg0: Any): Boolean

    Test two objects for equality.

    Test two objects for equality. The expression x == that is equivalent to if (x eq null) that eq null else x.equals(that).

    returns

    true if the receiver object is equivalent to the argument; false otherwise.

    Definition Classes
    Any
  8. final def asInstanceOf[T0]: T0

    Cast the receiver object to be of type T0.

    Cast the receiver object to be of type T0.

    Note that the success of a cast at runtime is modulo Scala's erasure semantics. Therefore the expression 1.asInstanceOf[String] will throw a ClassCastException at runtime, while the expression List(1).asInstanceOf[List[String]] will not. In the latter example, because the type argument is erased as part of compilation it is not possible to check whether the contents of the list are of the requested type.

    returns

    the receiver object.

    Definition Classes
    Any
    Exceptions thrown
    ClassCastException

    if the receiver object is not an instance of the erasure of type T0.

  9. def clone(): AnyRef

    Create a copy of the receiver object.

    Create a copy of the receiver object.

    The default implementation of the clone method is platform dependent.

    returns

    a copy of the receiver object.

    Attributes
    protected[java.lang]
    Definition Classes
    AnyRef
    Annotations
    @throws()
    Note

    not specified by SLS as a member of AnyRef

  10. def ensuring(cond: (IsTraversableLike[Repr]) ⇒ Boolean, msg: ⇒ Any): IsTraversableLike[Repr]

    Implicit information
    This member is added by an implicit conversion from IsTraversableLike[Repr] to Ensuring[IsTraversableLike[Repr]] performed by method any2Ensuring in scala.Predef.
    Definition Classes
    Ensuring
  11. def ensuring(cond: (IsTraversableLike[Repr]) ⇒ Boolean): IsTraversableLike[Repr]

    Implicit information
    This member is added by an implicit conversion from IsTraversableLike[Repr] to Ensuring[IsTraversableLike[Repr]] performed by method any2Ensuring in scala.Predef.
    Definition Classes
    Ensuring
  12. def ensuring(cond: Boolean, msg: ⇒ Any): IsTraversableLike[Repr]

    Implicit information
    This member is added by an implicit conversion from IsTraversableLike[Repr] to Ensuring[IsTraversableLike[Repr]] performed by method any2Ensuring in scala.Predef.
    Definition Classes
    Ensuring
  13. def ensuring(cond: Boolean): IsTraversableLike[Repr]

    Implicit information
    This member is added by an implicit conversion from IsTraversableLike[Repr] to Ensuring[IsTraversableLike[Repr]] performed by method any2Ensuring in scala.Predef.
    Definition Classes
    Ensuring
  14. final def eq(arg0: AnyRef): Boolean

    Tests whether the argument (arg0) is a reference to the receiver object (this).

    Tests whether the argument (arg0) is a reference to the receiver object (this).

    The eq method implements an equivalence relation on non-null instances of AnyRef, and has three additional properties:

    • It is consistent: for any non-null instances x and y of type AnyRef, multiple invocations of x.eq(y) consistently returns true or consistently returns false.
    • For any non-null instance x of type AnyRef, x.eq(null) and null.eq(x) returns false.
    • null.eq(null) returns true.

    When overriding the equals or hashCode methods, it is important to ensure that their behavior is consistent with reference equality. Therefore, if two objects are references to each other (o1 eq o2), they should be equal to each other (o1 == o2) and they should hash to the same value (o1.hashCode == o2.hashCode).

    returns

    true if the argument is a reference to the receiver object; false otherwise.

    Definition Classes
    AnyRef
  15. def equals(arg0: Any): Boolean

    The equality method for reference types.

    The equality method for reference types. Default implementation delegates to eq.

    See also equals in scala.Any.

    returns

    true if the receiver object is equivalent to the argument; false otherwise.

    Definition Classes
    AnyRef → Any
  16. def finalize(): Unit

    Called by the garbage collector on the receiver object when there are no more references to the object.

    Called by the garbage collector on the receiver object when there are no more references to the object.

    The details of when and if the finalize method is invoked, as well as the interaction between finalize and non-local returns and exceptions, are all platform dependent.

    Attributes
    protected[java.lang]
    Definition Classes
    AnyRef
    Annotations
    @throws()
    Note

    not specified by SLS as a member of AnyRef

  17. def formatted(fmtstr: String): String

    Returns string formatted according to given format string.

    Returns string formatted according to given format string. Format strings are as for String.format (@see java.lang.String.format).

    Implicit information
    This member is added by an implicit conversion from IsTraversableLike[Repr] to StringFormat performed by method any2stringfmt in scala.Predef.
    Definition Classes
    StringFormat
    Annotations
    @inline()
  18. final def getClass(): Class[_]

    A representation that corresponds to the dynamic class of the receiver object.

    A representation that corresponds to the dynamic class of the receiver object.

    The nature of the representation is platform dependent.

    returns

    a representation that corresponds to the dynamic class of the receiver object.

    Definition Classes
    AnyRef → Any
    Note

    not specified by SLS as a member of AnyRef

  19. def hashCode(): Int

    The hashCode method for reference types.

    The hashCode method for reference types. See hashCode in scala.Any.

    returns

    the hash code value for this object.

    Definition Classes
    AnyRef → Any
  20. final def isInstanceOf[T0]: Boolean

    Test whether the dynamic type of the receiver object is T0.

    Test whether the dynamic type of the receiver object is T0.

    Note that the result of the test is modulo Scala's erasure semantics. Therefore the expression 1.isInstanceOf[String] will return false, while the expression List(1).isInstanceOf[List[String]] will return true. In the latter example, because the type argument is erased as part of compilation it is not possible to check whether the contents of the list are of the specified type.

    returns

    true if the receiver object is an instance of erasure of type T0; false otherwise.

    Definition Classes
    Any
  21. final def ne(arg0: AnyRef): Boolean

    Equivalent to !(this eq that).

    Equivalent to !(this eq that).

    returns

    true if the argument is not a reference to the receiver object; false otherwise.

    Definition Classes
    AnyRef
  22. final def notify(): Unit

    Wakes up a single thread that is waiting on the receiver object's monitor.

    Wakes up a single thread that is waiting on the receiver object's monitor.

    Definition Classes
    AnyRef
    Note

    not specified by SLS as a member of AnyRef

  23. final def notifyAll(): Unit

    Wakes up all threads that are waiting on the receiver object's monitor.

    Wakes up all threads that are waiting on the receiver object's monitor.

    Definition Classes
    AnyRef
    Note

    not specified by SLS as a member of AnyRef

  24. final def synchronized[T0](arg0: ⇒ T0): T0

    Definition Classes
    AnyRef
  25. def toString(): String

    Creates a String representation of this object.

    Creates a String representation of this object. The default representation is platform dependent. On the java platform it is the concatenation of the class name, "@", and the object's hashcode in hexadecimal.

    returns

    a String representation of the object.

    Definition Classes
    AnyRef → Any
  26. final def wait(): Unit

    Definition Classes
    AnyRef
    Annotations
    @throws()
  27. final def wait(arg0: Long, arg1: Int): Unit

    Definition Classes
    AnyRef
    Annotations
    @throws()
  28. final def wait(arg0: Long): Unit

    Definition Classes
    AnyRef
    Annotations
    @throws()
  29. def [B](y: B): (IsTraversableLike[Repr], B)

    Implicit information
    This member is added by an implicit conversion from IsTraversableLike[Repr] to ArrowAssoc[IsTraversableLike[Repr]] performed by method any2ArrowAssoc in scala.Predef.
    Definition Classes
    ArrowAssoc

Shadowed Implict Value Members

  1. val self: Any

    Implicit information
    This member is added by an implicit conversion from IsTraversableLike[Repr] to StringAdd performed by method any2stringadd in scala.Predef.
    Shadowing
    This implicitly inherited member is ambiguous. One or more implicitly inherited members have similar signatures, so calling this member may produce an ambiguous implicit conversion compiler error.
    To access this member you can use a type ascription:
    (isTraversableLike: StringAdd).self
    Definition Classes
    StringAdd
  2. val self: Any

    Implicit information
    This member is added by an implicit conversion from IsTraversableLike[Repr] to StringFormat performed by method any2stringfmt in scala.Predef.
    Shadowing
    This implicitly inherited member is ambiguous. One or more implicitly inherited members have similar signatures, so calling this member may produce an ambiguous implicit conversion compiler error.
    To access this member you can use a type ascription:
    (isTraversableLike: StringFormat).self
    Definition Classes
    StringFormat

Deprecated Value Members

  1. def x: IsTraversableLike[Repr]

    Implicit information
    This member is added by an implicit conversion from IsTraversableLike[Repr] to ArrowAssoc[IsTraversableLike[Repr]] performed by method any2ArrowAssoc in scala.Predef.
    Shadowing
    This implicitly inherited member is ambiguous. One or more implicitly inherited members have similar signatures, so calling this member may produce an ambiguous implicit conversion compiler error.
    To access this member you can use a type ascription:
    (isTraversableLike: ArrowAssoc[IsTraversableLike[Repr]]).x
    Definition Classes
    ArrowAssoc
    Annotations
    @deprecated
    Deprecated

    (Since version 2.10.0) Use leftOfArrow instead

  2. def x: IsTraversableLike[Repr]

    Implicit information
    This member is added by an implicit conversion from IsTraversableLike[Repr] to Ensuring[IsTraversableLike[Repr]] performed by method any2Ensuring in scala.Predef.
    Shadowing
    This implicitly inherited member is ambiguous. One or more implicitly inherited members have similar signatures, so calling this member may produce an ambiguous implicit conversion compiler error.
    To access this member you can use a type ascription:
    (isTraversableLike: Ensuring[IsTraversableLike[Repr]]).x
    Definition Classes
    Ensuring
    Annotations
    @deprecated
    Deprecated

    (Since version 2.10.0) Use resultOfEnsuring instead

Inherited from AnyRef

Inherited from Any

Inherited by implicit conversion any2stringadd from IsTraversableLike[Repr] to StringAdd

Inherited by implicit conversion any2stringfmt from IsTraversableLike[Repr] to StringFormat

Inherited by implicit conversion any2ArrowAssoc from IsTraversableLike[Repr] to ArrowAssoc[IsTraversableLike[Repr]]

Inherited by implicit conversion any2Ensuring from IsTraversableLike[Repr] to Ensuring[IsTraversableLike[Repr]]

Ungrouped