What is the Scala identifier "implicitly"?

Question

I have seen a function named implicitly used in Scala examples. What is it, and how is it used?

Example here:

scala> sealed trait Foo[T] { def apply(list : List[T]) : Unit }; object Foo {
     |                         implicit def stringImpl = new Foo[String] {
     |                             def apply(list : List[String]) = println("String")
     |                         }
     |                         implicit def intImpl = new Foo[Int] {
     |                             def apply(list : List[Int]) =  println("Int")
     |                         }
     |                     } ; def foo[A : Foo](x : List[A]) = implicitly[Foo[A]].apply(x)
defined trait Foo
defined module Foo
foo: [A](x: List[A])(implicit evidence$1: Foo[A])Unit

scala> foo(1)
<console>:8: error: type mismatch;
 found   : Int(1)
 required: List[?]
       foo(1)
           ^
scala> foo(List(1,2,3))
Int
scala> foo(List("a","b","c"))
String
scala> foo(List(1.0))
<console>:8: error: could not find implicit value for evidence parameter of type
 Foo[Double]
       foo(List(1.0))
          ^

Note that we have to write implicitly[Foo[A]].apply(x) since the compiler thinks that implicitly[Foo[A]](x) means that we call implicitly with parameters.

Also see How to investigate objects/types/etc. from Scala REPL? and Where does Scala look for implicits?

Answer 1

Implicitly is avaliable in Scala 2.8 and is defined in Predef as:

def implicitly[T](implicit e: T): T = e

It is commonly used to check if an implicit value of type T is available and return it if such is the case.

Simple example from retronym's presentation:

scala> implicit val a = "test" // define an implicit value of type String
a: java.lang.String = test
scala> val b = implicitly[String] // search for an implicit value of type String and assign it to b
b: String = test
scala> val c = implicitly[Int] // search for an implicit value of type Int and assign it to c
<console>:6: error: could not find implicit value for parameter e: Int
       val c = implicitly[Int]
                         ^