1. Implicits Inspected and
Explained
ScalaDays 2016 New York
Tim Soethout
http://blog.timmybankers.nl

2. Implicits Inspected and
Explained
ScalaDays 2016 New York
Tim Soethout
Outline: Introduction - Implicits - Resolving - Type Classes - Wrap up
http://blog.timmybankers.nl

3. About myself
Tim Soethout
Functional programmer at heart
Scala/FP evangelist/trainer inside ING Bank
PhD Candidate

5. What?
Use values without explicit reference
OO: is a + has a
Implicits add: is viewable as a
Loose Coupling, Tight Cohesion

6. Examples
Akka
Futures
trait ScalaActorRef
def !(message: Any)(implicit sender: ActorRef = Actor.noSender): Unit = ...
}
actorRef ! SomeMessage("text")
object Future {
def apply[T](body: =>T)(implicit executor: ExecutionContext): Future[T] = ...
}
Future {
doExpensiveComputation()
}

7. Examples (2)
Collections
trait TraversableOnce[+A] {
def sum[B >: A](implicit num: Numeric[B]): B = ...
}
List(1,2,3).sum
res0: Int = 6

8. Examples (3)
Finagle
@implicitNotFound("Builder is not fully configured: Cluster: ${HasCluster}, Codec: ${HasCodec},
HostConnectionLimit: ${HasHostConnectionLimit}")
private[builder] trait ClientConfigEvidence[HasCluster, HasCodec, HasHostConnectionLimit]
class ClientBuilder[Req, Rep, HasCluster, HasCodec, HasHostConnectionLimit] private[finagle](...) {
def build()(
implicit THE_BUILDER_IS_NOT_FULLY_SPECIFIED_SEE_ClientBuilder_DOCUMENTATION:
ClientConfigEvidence[HasCluster, HasCodec, HasHostConnectionLimit]
): Service[Req, Rep] = ...
}
val builder: ClientBuilder[Request, Response, Yes, Yes, Nothing] =
ClientBuilder()
.codec(Http())
.hosts("twitter.com:80")
builder.build()
Error:(24, 15) Builder is not fully configured: Cluster: com.twitter.finagle.builder.ClientConfig.Yes,
Codec: com.twitter.finagle.builder.ClientConfig.Yes, HostConnectionLimit: Nothing
builder.build()
^

9. Implicits enable
DSLs
Type evidence
Reduce verbosity
Type classes
Dependency Injection at Compile time
Extending libraries

10. But beware
Resolution rules can be difficult
Automatic conversions
Do not overuse

11. Demo
Implicit conversions (a.k.a. Implicit views)
Implicit parameters
Implicit classes
Implicit declarations
implicit def a2B(a : A) : B = ...
def method(implicit x : Int) = ...
implicit class X(y: Int)
implicit val x = ...

70. Scoping
Odersky Explains
Lookup precedence:
1. By name only, without any prefix
2. In "implicit scope":
companion/package object of
the source type
its parameters + supertype and supertraits

71. Demo
Scoping and resolving

87. Typeclass uses
Ad-hoc polymorphism
Extension of libraries
trait Numeric[T] extends Ordering[T] {
def plus(x: T, y: T): T
def minus(x: T, y: T): T
def times(x: T, y: T): T
...
}

88. Demo
Typeclass for JSON Serialisation
Naive with subtyping
Typeclass + improvements

121. Recap
Implicits are powerful
Be careful with conversions
Implicit precedence: first look local, then in companion/package object
Typeclasses to extend libraries

122. Recap
Implicits are powerful
Be careful with conversions
Implicit precedence: first look local, then in companion/package object
Typeclasses to extend libraries
Questions?

123. References
/
Scala documentation:
Book:
Blog
, special thanks to @Ichoran and @som-snytt
, for the demo slides
Slides Code
Java Converters
Finding Implicits
Scala In Depth
Effective Scala
All Things Runnable
Scala Gitter channel
REPLesent