Passing scala.math.Integral as implicit parameter

Question

I have read the answer to my question about scala.math.Integral but I do not understand what happens when Integral[T] is passed as an implicit parameter. (I think I understand the implicit parameters concept in general).

Let's consider this function

import scala.math._

def foo[T](t: T)(implicit integral: Integral[T]) { println(integral) }

Now I call foo in REPL:

scala> foo(0)  
scala.math.Numeric$IntIsIntegral$@581ea2
scala> foo(0L)
scala.math.Numeric$LongIsIntegral$@17fe89

How does the integral argument become scala.math.Numeric$IntIsIntegral and scala.math.Numeric$LongIsIntegral ?

Answer 1

The short answer is that Scala finds IntIsIntegral and LongIsIntegral inside the object Numeric, which is the companion object of the class Numeric, which is a super class of Integral.

Read on for the long answer.

Types of Implicits

Implicits in Scala refers to either a value that can be passed "automatically", so to speak, or a conversion from one type to another that is made automatically.

Implicit Conversion

Speaking very briefly about the latter type, if one calls a method m on an object o of a class C, and that class does not support method m, then Scala will look for an implicit conversion from C to something that does support m. A simple example would be the method map on String:

"abc".map(_.toInt)

String does not support the method map, but StringOps does, and there's an implicit conversion from String to StringOps available (see implicit def augmentString on Predef).

Implicit Parameters

The other kind of implicit is the implicit parameter. These are passed to method calls like any other parameter, but the compiler tries to fill them in automatically. If it can't, it will complain. One can pass these parameters explicitly, which is how one uses breakOut, for example (see question about breakOut, on a day you are feeling up for a challenge).

In this case, one has to declare the need for an implicit, such as the foo method declaration:

def foo[T](t: T)(implicit integral: Integral[T]) {println(integral)}

View Bounds

There's one situation where an implicit is both an implicit conversion and an implicit parameter. For example:

def getIndex[T, CC](seq: CC, value: T)(implicit conv: CC => Seq[T]) = seq.indexOf(value)

getIndex("abc", 'a')

The method getIndex can receive any object, as long as there is an implicit conversion available from its class to Seq[T]. Because of that, I can pass a String to getIndex, and it will work.

Behind the scenes, the compile changes seq.IndexOf(value) to conv(seq).indexOf(value).

This is so useful that there is a syntactic sugar to write them. Using this syntactic sugar, getIndex can be defined like this:

def getIndex[T, CC <% Seq[T]](seq: CC, value: T) = seq.indexOf(value)

This syntactic sugar is described as a view bound, akin to an upper bound (CC <: Seq[Int]) or a lower bound (T >: Null).

Please be aware that view bounds are deprecated from 2.11, you should avoid them.

Context Bounds

Another common pattern in implicit parameters is the type class pattern. This pattern enables the provision of common interfaces to classes which did not declare them. It can both serve as a bridge pattern -- gaining separation of concerns -- and as an adapter pattern.

The Integral class you mentioned is a classic example of type class pattern. Another example on Scala's standard library is Ordering. There's a library that makes heavy use of this pattern, called Scalaz.

This is an example of its use:

def sum[T](list: List[T])(implicit integral: Integral[T]): T = {
    import integral._   // get the implicits in question into scope
    list.foldLeft(integral.zero)(_ + _)
}

There is also a syntactic sugar for it, called a context bound, which is made less useful by the need to refer to the implicit. A straight conversion of that method looks like this:

def sum[T : Integral](list: List[T]): T = {
    val integral = implicitly[Integral[T]]
    import integral._   // get the implicits in question into scope
    list.foldLeft(integral.zero)(_ + _)
}

Context bounds are more useful when you just need to pass them to other methods that use them. For example, the method sorted on Seq needs an implicit Ordering. To create a method reverseSort, one could write:

def reverseSort[T : Ordering](seq: Seq[T]) = seq.reverse.sorted

Because Ordering[T] was implicitly passed to reverseSort, it can then pass it implicitly to sorted.

Where do Implicits Come From?

When the compiler sees the need for an implicit, either because you are calling a method which does not exist on the object's class, or because you are calling a method that requires an implicit parameter, it will search for an implicit that will fit the need.

This search obey certain rules that define which implicits are visible and which are not. The following table showing where the compiler will search for implicits was taken from an excellent presentation about implicits by Josh Suereth, which I heartily recommend to anyone wanting to improve their Scala knowledge.

1. First look in current scope
   1. Implicits defined in current scope
   2. Explicit imports
   3. wildcard imports
   4. Same scope in other files
2. Now look at associated types in
   1. Companion objects of a type
   2. Companion objects of type parameters types
   3. Outer objects for nested types
   4. Other dimensions

Let's give examples for them.

Implicits Defined in Current Scope

implicit val n: Int = 5
def add(x: Int)(implicit y: Int) = x + y
add(5) // takes n from the current scope

Explicit Imports

import scala.collection.JavaConversions.mapAsScalaMap
def env = System.getenv() // Java map
val term = env("TERM")    // implicit conversion from Java Map to Scala Map

Wildcard Imports

def sum[T : Integral](list: List[T]): T = {
    val integral = implicitly[Integral[T]]
    import integral._   // get the implicits in question into scope
    list.foldLeft(integral.zero)(_ + _)
}

Same Scope in Other Files

This is like the first example, but assuming the implicit definition is in a different file than its usage. See also how package objects might be used in to bring in implicits.

Companion Objects of a Type

There are two object companions of note here. First, the object companion of the "source" type is looked into. For instance, inside the object Option there is an implicit conversion to Iterable, so one can call Iterable methods on Option, or pass Option to something expecting an Iterable. For example:

for {
    x <- List(1, 2, 3)
    y <- Some('x')
} yield, (x, y)

That expression is translated by the compile into

List(1, 2, 3).flatMap(x => Some('x').map(y => (x, y)))

However, List.flatMap expects a TraversableOnce, which Option is not. The compiler then looks inside Option's object companion and finds the conversion to Iterable, which is a TraversableOnce, making this expression correct.

Second, the companion object of the expected type:

List(1, 2, 3).sorted

The method sorted takes an implicit Ordering. In this case, it looks inside the object Ordering, companion to the class Ordering, and finds an implicit Ordering[Int] there.

Note that companion objects of super classes are also looked into. For example:

class A(val n: Int)
object A { 
    implicit def str(a: A) = "A: %d" format a.n
}
class B(val x: Int, y: Int) extends A(y)
val b = new B(5, 2)
val s: String = b  // s == "A: 2"

This is how Scala found the implicit Numeric[Int] and Numeric[Long] in your question, by the way, as they are found inside Numeric, not Integral.

Companion Objects of Type Parameters Types

This is required to make the type class pattern really work. Consider Ordering, for instance... it comes with some implicits in its companion object, but you can't add stuff to it. So how can you make an Ordering for your own class that is automatically found?

Let's start with the implementation:

class A(val n: Int)
object A {
    implicit val ord = new Ordering[A] {
        def compare(x: A, y: A) = implicitly[Ordering[Int]].compare(x.n, y.n)
    }
}

So, consider what happens when you call

List(new A(5), new A(2)).sorted

As we saw, the method sorted expects an Ordering[A] (actually, it expects an Ordering[B], where B >: A). There isn't any such thing inside Ordering, and there is no "source" type on which to look. Obviously, it is finding it inside A, which is a type parameter of Ordering.

This is also how various collection methods expecting CanBuildFrom work: the implicits are found inside companion objects to the type parameters of CanBuildFrom.

Outer Objects for Nested Types

I haven't actually seen examples of this. I'd be grateful if someone could share one. The principle is simple:

class A(val n: Int) {
  class B(val m: Int) { require(m < n) }
}
object A {
  implicit def bToString(b: A#B) = "B: %d" format b.m
}
val a = new A(5)
val b = new a.B(3)
val s: String = b  // s == "B: 3"

Other Dimensions

I'm pretty sure this was a joke. I hope. :-)

EDIT

Related questions of interest:

• Context and view bounds
• Chaining implicits