What are implicit objects?

Question

I was reading about type classes where implicit objects were mentioned:

object Math {   trait NumberLike[T] {     def plus(x: T, y: T): T     def divide(x: T, y: Int): T     def minus(x: T, y: T): T   }   object NumberLike {     implicit object NumberLikeDouble extends NumberLike[Double] {       def plus(x: Double, y: Double): Double = x + y       def divide(x: Double, y: Int): Double = x / y       def minus(x: Double, y: Double): Double = x - y     }     implicit object NumberLikeInt extends NumberLike[Int] {       def plus(x: Int, y: Int): Int = x + y       def divide(x: Int, y: Int): Int = x / y       def minus(x: Int, y: Int): Int = x - y     }   } } 

What are they? Where are they described? I only found definition of implicit classes on the web but not of implicit objects.

Answer 1

In Scala, objects and values are treated mostly the same. An implicit object can be thought of as a value which is found in the process of looking up an implicit of its type.

In your example, if one implicitly looks for a NumberLike type class with type parameter Double or Int, one will find NumberLikeDouble and NumberLikeInt.

implicit object NumberLikeDouble extends NumberLike[Double] 

is thus roughly the same as

implicit val NumberLikeDouble: NumberLike[Double] = new NumberLike[Double] { ...} 

or

implicit def NumberLikeDouble: NumberLike[Double] = new NumberLike[Double] { ...} 

Like a val, there is only a single value of that type and instantiation is not needed.

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A simple use case:

import Math.NumberLike  def sum[A](x: A, y: A)(implicit nl: NumberLike[A]) = nl.plus(x, y)  sum(4, 5)   // finds NumberLikeInt