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:         


        

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