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Scalera - Monoids aren't démodé and thursday are the new fridays
object TupleSum extends App {
// Monoids aren't démodé and thursday are the new fridays
val l = List(
1 -> 1,
2 -> 3,
3 -> 3)
//println(l.sum)
//Could not find implicit value for parameter num: Numeric[(Int, Int)]
/*
* Ok, let's build manually our sum
*/
val sum = ((0,0) /: l){
case ((sum_1,sum_2), (_1, _2)) => (sum_1 + _1, sum_2 + _2)
}
println(sum)
/*
* Generic stuff please...
* We need a Numeric[(A, B)]
*
* First, let's get rid of some out-of-bounds methods
*/
trait LeanNumeric[T] extends Numeric[T] {
override def fromInt(x: Int): T = ???
override def toInt(x: T): Int = ???
override def minus(x: T, y: T): T = ???
override def times(x: T, y: T): T = ???
override def negate(x: T): T = ???
override def toLong(x: T): Long = ???
override def toFloat(x: T): Float = ???
override def toDouble(x: T): Double = ???
override def compare(x: T, y: T): Int = ???
}
/*
* And now, let's define our Numeric
*/
implicit def numeric[A, B](
implicit nA: Numeric[A],
nB: Numeric[B]): Numeric[(A, B)] = {
new LeanNumeric[(A, B)]{
override def zero = (nA.zero, nB.zero)
override def plus(x: (A, B), y: (A, B)): (A, B) = {
val (a1, b1) = x
val (a2, b2) = y
(nA.plus(a1, a2), nB.plus(b1, b2))
}
}
}
println(l.sum) //It works
/*
* But let's take a deeper look... which features does a monoid owe?
*
* Semigroup ( a1 + a2) and Zero
*
* Those are some of the properties of a Numeric[T]!
*
* Let's build our monoid
*/
{
import scalaz._
implicit object IntMonoid extends Monoid[Int]{
override def zero: Int = 0
override def append(f1: Int, f2: => Int): Int = f1 + f2
}
implicit def tupleMonoid[A,B](
implicit mA: Monoid[A],
mB: Monoid[B]): Monoid[(A,B)] = {
new Monoid[(A, B)] {
override def zero: (A, B) = (mA.zero, mB.zero)
override def append(f1: (A, B), f2: => (A, B)): (A, B) = {
val (a1, b1) = f1
lazy val (a2, b2) = f2
(mA.append(a1,a2), mB.append(b1, b2))
}
}
}
implicit def numeric[T](
implicit m: Monoid[T]): Numeric[T] = {
new LeanNumeric[T]{
override def zero = m.zero
override def plus(x: T, y: T): T = m.append(x, y)
}
}
println(l.sum)
}
}
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