Authored by Win Myo Htet
Before we look into Abstract Value Members of List API, I assume that you have refreshed the List API scaladoc page so that the Search Section is back to default. There are two functions productArity and productElement under Abstract Value Members. In the search box, type "product" and change Ordering to By inheritance. We see that the two stated functions along with extra two functions are inherited from Product and there are also two other separate funtions, product from different classes. Product is, sort of, Tuple's cousin. I have covered a bit of Product in my blog: Learning Scala : "case class", twitter interview question? Please read up that blog to learn more about Product. I will still cover some of its usage here.
scala> val list=List(1,2,3,4,5,6,7,8,9,10)
list: List[Int] = List(1, 2, 3, 4, 5, 6, 7, 8, 9, 10)
scala> list.productArity
res0: Int = 2
scala> list.productElement(0)
res1: Any = 1
scala> list.productElement(1)
res2: Any = List(2, 3, 4, 5, 6, 7, 8, 9, 10)
scala> list.productPrefix
res3: java.lang.String = ::
final case class ::[B](private var hd: B, private[scala] var tl: List[B]) extends List[B] {
override def head : B = hd
override def tail : List[B] = tl
override def isEmpty: Boolean = false
.
.
.
That leaves us with two functions: product and product.
def product : A
def product [B >: A] (implicit num: Numeric[B]): B
[B >: A], and takes the implicit parameter.
scala> val numList=List(1,2,3)
numList: List[Int] = List(1, 2, 3)
scala> numList.product
res0: Int = 6
scala> numList.product()
<console>:9: error: not enough arguments for method product: (implicit num: Numeric[B])B.
Unspecified value parameter num.
numList.product()
^
scala> numList.sum
res2: Int = 6
scala> numList.sum()
<console>:9: error: not enough arguments for method sum: (implicit num: Numeric[B])B.
Unspecified value parameter num.
numList.sum()
^
scala> val stringList=List("one","two","three")
stringList: List[java.lang.String] = List(one, two, three)
scala> stringList.product
<console>:9: error: could not find implicit value for parameter num: Numeric[java.lang.String]
stringList.product
^
scala> stringList.sum
<console>:9: error: could not find implicit value for parameter num: Numeric[java.lang.String]
stringList.sum
^
scala> implicit object StringNumeric extends math.Numeric[String] {
| override def one=""
| override def zero=""
| def plus(x: String, y: String) =(x,y)match {case ("",y) =>y
| case (x,y) => x+" + "+y}
| def minus(x: String, y: String) = x+" - "+y
| def times(x: String, y: String)=(x,y)match {case ("",y) =>y
| case (x,y) => x+" * "+y}
| def negate(x: String): String ="-"+x
| def fromInt(x: Int) = x.toString
| def toInt(x: String) = -1
| def toLong(x: String) = toInt(x)
| def toFloat(x: String) = toInt(x)
| def toDouble(x: String) = toInt(x)
| def compare(x:String,y:String) = -1
| }
defined module StringNumeric
scala> stringList.product
res6: java.lang.String = one * two * three
scala> stringList.sum
res7: java.lang.String = one + two + three
Well, we have finished reading the Abstract Value Members section and 6 functions from the Concrete Value Members section : product, product, productIterator, productPrefix, sum and sum. Let's go take a look at those cryptic functions : ++ :+ :: :\ /:\ ...
Authored by Win Myo Htet
