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Scala supports first-class functions, which means you can express functions in function literal syntax, i.e., (x: Int)
=> x + 1, and that functions can be represented by objects, which are called function values. The following
expression creates a successor function for integers:
var inc = (x:Int) => x+1
Variable inc is now a function that can be used the usual way:
var x = inc(7)-1
It is also possible to define functions with multiple parameters as follows:
var mul = (x: Int, y: Int) => x*y
Variable mul is now a function that can be used the usual way:
println(mul(3, 4))
It is also possible to define functions with no parameter as follows:
var userDir = () => { System.getProperty("user.dir") }
Variable userDir is now a function that can be used the usual way:
println( userDir )
Partially Applied Functions
When you invoke a function, you're said to be applying the function to the arguments. If you pass all the expected
arguments, you have fully applied it. If you send only a few arguments, then you get back a partially applied
function. This gives you the convenience of binding some arguments and leaving the rest to be filled in later.
Following is a simple example to show the concept:
import java.util.Date
object Test {
def main(args: Array[String]) {
val date = new Date
log(date, "message1" )
log(date, "message2" )
log(date, "message3" )
}
}
def log(date: Date, message: String) = {
println(date + "----" + message)
}
Here, the log( ) method takes two parameters: date and message. We want to invoke the method multiple times,
with the same value for date but different values for message. We can eliminate the noise of passing the date to
each call by partially applying that argument to the log( ) method. To do so, we first bind a value to the date
parameter and leave the second parameter unbound by putting an underscore at its place. The result is a partially
applied function that we've stored in a variable. We can now invoke this new method with only the unbound
argument message as follows:
import java.util.Date
object Test {
def main(args: Array[String]) {
val logWithDateBound = log(new Date, _ : String)
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