Scala is my first functional programming language. Recently I wrote a lot of codes in Scala to build a Data processing/analytics and Machine Learning application using Apache Spark.
Looking at source code like this:
val lineWithSaleStock = textFile.filter(line => line.contains("Sale Stock"))My first impression was: “Where the hell is line coming from?”. At previous
line, there is no definition of line variable/value at all.
I decided to look at
Spark API
and found the following filter method definition:
def filter(f: (T) ⇒ Boolean): RDD[T]
Return a new RDD containing only the elements that satisfy a predicate.It turns out that filter method took an argument of boolean function. So, this
part should be a boolean function then:
line => line.contains("Sale Stock")Ahh interesting, line is the input parameters and it returns a boolean value
from line.contains("Sale Stock") one. And that’s part is a
Scala Anonymous Function.
It seems, Scala is have a nice syntax to express the anonymous function. I can express previous function as:
val isContainsSaleStock = (s: String) => s.contains("Sale Stock")
val lineWithSaleStock = textFile.filter(isContainSaleStock)This post from Scala Doc provides good resources about Scala anonymous function.