What is the Scala identifier "implicitly"?

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?

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]
                         ^

Here are a few reasons to use the delightfully simple method implicitly.

To understand/troubleshoot Implicit Views

An Implicit View can be triggered when the prefix of a selection (consider for example, the.prefix.selection(args) does not contain a member selection that is applicable to args (even after trying to convert args with Implicit Views). In this case, the compiler looks for implicit members, locally defined in the current or enclosing scopes, inherited, or imported, that are either Functions from the type of that the.prefix to a type with selection defined, or equivalent implicit methods.

scala> 1.min(2) // Int doesn't have min defined, where did that come from?                                   
res21: Int = 1

scala> implicitly[Int => { def min(i: Int): Any }]
res22: (Int) => AnyRef{def min(i: Int): Any} = <function1>

scala> res22(1) // 
res23: AnyRef{def min(i: Int): Int} = 1

scala> .getClass
res24: java.lang.Class[_] = class scala.runtime.RichInt

Implicit Views can also be triggered when an expression does not conform to the Expected Type, as below:

scala> 1: scala.runtime.RichInt
res25: scala.runtime.RichInt = 1

Here the compiler looks for this function:

scala> implicitly[Int => scala.runtime.RichInt]
res26: (Int) => scala.runtime.RichInt = <function1>

Accessing an Implicit Parameter Introduced by a Context Bound

Implicit parameters are arguably a more important feature of Scala than Implicit Views. They support the type class pattern. The standard library uses this in a few places -- see scala.Ordering and how it is used in SeqLike#sorted. Implicit Parameters are also used to pass Array manifests, and CanBuildFrom instances.

Scala 2.8 allows a shorthand syntax for implicit parameters, called Context Bounds. Briefly, a method with a type parameter A that requires an implicit parameter of type M[A]:

def foo[A](implicit ma: M[A])

can be rewritten as:

def foo[A: M]

But what's the point of passing the implicit parameter but not naming it? How can this be useful when implementing the method foo?

Often, the implicit parameter need not be referred to directly, it will be tunneled through as an implicit argument to another method that is called. If it is needed, you can still retain the terse method signature with the Context Bound, and call implicitly to materialize the value:

def foo[A: M] = {
   val ma = implicitly[M[A]]
}

Passing a subset of implicit parameters explicitly

Suppose you are calling a method that pretty prints a person, using a type class based approach:

trait Show[T] { def show(t: T): String }
object Show {
  implicit def IntShow: Show[Int] = new Show[Int] { def show(i: Int) = i.toString }
  implicit def StringShow: Show[String] = new Show[String] { def show(s: String) = s }

  def ShoutyStringShow: Show[String] = new Show[String] { def show(s: String) = s.toUpperCase }
}

case class Person(name: String, age: Int)
object Person {
  implicit def PersonShow(implicit si: Show[Int], ss: Show[String]): Show[Person] = new Show[Person] {
    def show(p: Person) = "Person(name=" + ss.show(p.name) + ", age=" + si.show(p.age) + ")"
  }
}

val p = Person("bob", 25)
implicitly[Show[Person]].show(p)

What if we want to change the way that the name is output? We can explicitly call PersonShow, explicitly pass an alternative Show[String], but we want the compiler to pass the Show[Int].

Person.PersonShow(si = implicitly, ss = Show.ShoutyStringShow).show(p)

Starting Scala 3 implicitly has been replaced with improved summon which has the advantage of being able to return a more precise type than asked for

> The summon method corresponds to implicitly in Scala 2. It is > precisely the same as the the method in Shapeless. The difference > between summon (or the) and implicitly is that summon can return a > more precise type than the type that was asked for.

For example given the following type

trait F[In]:
  type Out
  def f(v: Int): Out

given F[Int] with 
  type Out = String
  def f(v: Int): String = v.toString

implicitly method would summon a term with erased type member Out

scala> implicitly[F[Int]]
val res5: F[Int] = given_F_Int$@7d0e5fbb

scala> implicitly[res5.Out =:= String]
1 |implicitly[res5.Out =:= String]
  |                               ^
  |                               Cannot prove that res5.Out =:= String.

scala> val x: res5.Out = ""
1 |val x: res5.Out = ""
  |                  ^^
  |                  Found:    ("" : String)
  |                  Required: res5.Out

In order to recover the type member we would have to refer to it explicitly which defeats the purpose of having the type member instead of type parameter

scala> implicitly[F[Int] { type Out = String }]
val res6: F[Int]{Out = String} = given_F_Int$@7d0e5fbb

scala> implicitly[res6.Out =:= String]
val res7: res6.Out =:= String = generalized constraint

However summon defined as

def summon[T](using inline x: T): x.type = x

does not suffer from this problem

scala> summon[F[Int]]
val res8: given_F_Int.type = given_F_Int$@7d0e5fbb

scala> summon[res8.Out =:= String]
val res9: String =:= String = generalized constraint

scala> val x: res8.Out = ""
val x: res8.Out = ""

where we see type member type Out = String did not get erased even though we only asked for F[Int] and not F[Int] { type Out = String }. This can prove particularly relevant when chaining dependently typed functions:

> The type summoned by implicitly has no Out type member. For this > reason, we should avoid implicitly when working with dependently typed > functions.

A "teach you to fish" answer is to use the alphabetic member index currently available in the Scaladoc nightlies. The letters (and the #, for non-alphabetic names) at the top of the package / class pane are links to the index for member names beginning with that letter (across all classes). If you choose I, e.g., you'll find the implicitly entry with one occurrence, in Predef, which you can visit from the link there.