Implicits in Scala

Scala provides implicit conversions and parameters. Implicit conversion to an expected type is the first place the compiler uses implicits. For example, the following works:

scala> val d: Double = 2
d: Double = 2.0

Example 1.65

This works because of the following implicit method definition in the Int companion object (it was part of Predef prior to 2.10.x):

implicit def int2double(x: Int): Double = x.toDouble

Example 1.66

Another application of implicit conversion is the receiver of a method call. For example, let’s define a Rational class:

scala> class Rational(n: Int, d: Int) extends Ordered[Rational] {
     |
     |   require(d != 0)
     |   private val g = gcd(n.abs, d.abs)
     |   private def gcd(a: Int, b: Int): Int = if (b == 0) a else gcd(b, a % b)
     |   val numer = n / g
     |   val denom = d / g
     |   def this(n: Int) = this(n, 1)
     |   def +(that: Rational) = new Rational(
     |   this.numer * that.numer + this.denom * that.denom,
     |   this.denom * that.denom
     |   )
     |   def compare(that: Rational) = (this.numer * that.numer - this.denom * that.denom)
     |   override def toString = if (denom == 1) numer.toString else s"$numer/$denom"
     | }
defined class Rational

Example 1.67

Then declare a variable of the Rational type:

scala> val r1 = new Rational(1)
r1: Rational = 1
scala> 1 + r1
<console>:14: error: overloaded method value + with alternatives:
  (x: Double)Double <and>
  (x: Float)Float <and>
  (x: Long)Long <and>
  (x: Int)Int <and>
  (x: Char)Int <and>
  (x: Short)Int <and>
  (x: Byte)Int <and>
  (x: String)String
cannot be applied to (Rational)
       1 + r1
         ^

Example 1.68

If we try to add r1 to 1, we will get a compile-time error. The reason is the + method in Int does not support an argument of type Rational. In order to make it work, we can create an implicit conversion from Int to Rational:

scala> implicit def intToRational(n: Int): Rational = new Rational(n)
intToRational: (n: Int)Rational
scala> val r1 = new Rational(1)
r1: Rational = 1
scala> 1 + r1
res11: Rational = 2

Example 1.69