Tour of Scala

Named Arguments


When calling methods, you can label the arguments with their parameter names like so:

def printName(first: String, last: String): Unit =
  println(s"$first $last")

printName("John", "Public")  // Prints "John Public"
printName(first = "John", last = "Public")  // Prints "John Public"
printName(last = "Public", first = "John")  // Prints "John Public"
printName("Elton", last = "John")  // Prints "Elton John"

This is useful when two parameters have the same type and the arguments could be accidentally swapped.

Notice that named arguments can be written in any order. However, once the arguments are not in parameter order, reading from left to right, then the rest of the arguments must be named.

In the following example, named arguments enable the middle parameter to be omitted. But in the error case, the first argument is out of order, so the second argument must be named.

def printFullName(first: String, middle: String = "Q.", last: String): Unit =
  println(s"$first $middle $last")

printFullName(first = "John", last = "Public")  // Prints "John Q. Public"
printFullName("John", last = "Public")  // Prints "John Q. Public"
printFullName("John", middle = "Quincy", "Public")  // Prints "John Quincy Public"
printFullName(last = "Public", first = "John")  // Prints "John Q. Public"
printFullName(last = "Public", "John")  // error: positional after named argument

Named arguments work with calls to Java methods, but only if the Java library in question was compiled with the -parameters flag.

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