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Scala Extractors Infix Sample with Rational Numbers

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Scala Extractors Infix Sample with Rational Numbers

The author demonstrates the use of Scala Extractors through a short exercise involving rational numbers.

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I keep coming back to the awesome introductory material on Scala put together by Daniel Westheide. One of the examples that he provides for extractors using an infix operation pattern is the Streams API -

val xs = 58 #:: 43 #:: 93 #:: Stream.empty
xs match {
case first #:: second #:: _ => first - second
case _ => -1
}

Where the extractor is defined this way:

object #:: {
def unapply[A](xs: Stream[A]): Option[(A, Stream[A])] =
if (xs.isEmpty) None
else Some((xs.head, xs.tail))
}

Given this I wanted to try an extractor on the Rational number samples from the Scala by Example book, this is how the Rational number looks:

class Rational(n: Int, d: Int) {
private def gcd(x: Int, y: Int): Int = {
if (x == 0) y
else if (x < 0) gcd(-x,y)
else if (y < 0) -gcd(x, -y)
else gcd(y % x, x)
}
private val g = gcd(n, d)

val numer: Int = n/g
val denom: Int = d/g

def +(that: Rational) =
new Rational(numer * that.denom + that.numer * denom,
denom * that.denom)

def -(that: Rational) =
new Rational(numer * that.denom - that.numer * denom,
denom * that.denom)

def *(that: Rational) =
new Rational(numer * that.numer, denom * that.denom)

def /(that: Rational) =
new Rational(numer * that.denom, denom * that.numer)


override def toString = "" + numer + "/" + denom + ""

def square = new Rational(numer*numer, denom*denom)
}

And I wanted an extractor which would behave the following way:

val r = new Rational(2, 3) 

r match {
case num / denom => num + "/" + denom
} 

This is absolutely feasible in Scala given the flexibility in the names of identifiers. Given this an extractor called "/" can be defined the following way: 

object / {
def unapply(r: Rational): Option[(Int, Int)] = Some(r.numer, r.denom)
}

And used for extracting the numerator and denominator of the rational number!

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Topics:
scala ,java ,streams api

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