trait
IsTraversableLike[Repr] extends AnyRef
Type Members
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abstract
type
A
Concrete Value Members
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-
final
def
!=(arg0: Any): Boolean
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final
def
##(): Int
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def
+(other: String): String
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-
-
final
def
==(arg0: Any): Boolean
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final
def
asInstanceOf[T0]: T0
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def
clone(): AnyRef
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-
-
-
-
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def
equals(arg0: Any): Boolean
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def
finalize(): Unit
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def
formatted(fmtstr: String): String
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final
def
getClass(): Class[_]
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def
hashCode(): Int
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final
def
isInstanceOf[T0]: Boolean
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final
def
notify(): Unit
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final
def
notifyAll(): Unit
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final
def
synchronized[T0](arg0: ⇒ T0): T0
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def
toString(): String
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final
def
wait(): Unit
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final
def
wait(arg0: Long, arg1: Int): Unit
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final
def
wait(arg0: Long): Unit
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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 onString
s (String
s do not extendGenTraversableLike
, but can be converted toGenTraversableLike
)IsTraversable
provides two members:A
, which represents the element type of the targetGenTraversableLike[A, Repr]
conversion
, which provides a way to convert between the type we wish to add extension methods to,Repr
, andGenTraversableLike[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 methodmapReduce
to any type that extends or can be converted toGenTraversableLike
. In our example, this includesString
.Here, we begin by creating a class
ExtensionMethods
which contains ourmapReduce
extension method. Note thatExtensionMethods
takes a constructor argumentcoll
of typeGenTraversableLike[A, Repr]
, whereA
represents the element type andRepr
represents (typically) the collection type. The implementation ofmapReduce
itself is straightforward.The interesting bit is the implicit conversion
withExtensions
, which returns an instance ofExtensionMethods
. This implicit conversion can only be applied if there is an implicit valuetraversable
of typeIsTraversableLike[Repr]
in scope. SinceIsTraversableLike
provides value memberconversion
, which gives us a way to convert between whatever type we wish to add an extension method to (in this case,Repr
) andGenTraversableLike[A, Repr]
, we can now convertcoll
from typeRepr
toGenTraversableLike[A, Repr]
. This allows us to create an instance of theExtensionMethods
class, which we pass our newGenTraversableLike[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 conversionwithExtensions
is generic, it will be applied as long as an implicit value of typeIsTraversableLike[Repr]
can be found. Given thatIsTraversableLike
contains implicit members that return values of typeIsTraversableLike
, this requirement is typically satisfied, and the chain of interactions described in the previous paragraph is set into action. (See theIsTraversableLike
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 TypesOne must simply provide an implicit value of type
IsTraversableLike
specific to the new type, or an implicit conversion which returns an instance ofIsTraversableLike
specific to the new type.Below is an example of an implementation of the
IsTraversableLike
trait where theRepr
type isString
.2.10