How do I make a parameter the default value of another parameter in Scala?
Why can't I do the following:
def multiply(a: Int, b: Int = a) = a * b
or
case class Point(x: Int, y: Int = x)
Is there any other way to achieve the same? The reason I need this is because sometimes arguments that have different meanings are the exception rather than the rule. Take for example:
case class User(name: String, age: String, description: String, displayName: String = name, online: Boolean = false)
In 90% of cases, the display name and name should be the same, but in a few cases it should not have an edge. When using one parameter, the default value of the other will be very useful. Is there a way to do this? If not, why not?
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Yes. Parameters in a parameter list can refer to the values of arguments in previous parameter lists, in the same way as type inference can refer to types in previous parameter lists:
def multiply(a: Int)(b: Int = a) = a * b
case class Point(x: Int)(y: Int = x)
case class User(name: String, age: String, description: String)(displayName: String = name, online: Boolean = false)
must work.
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One way could be defining the case class as
case class User(name: String, age: String, description: String, displayName: String, online: Boolean = false) {
def this(name: String, age: String, description: String, online: Boolean = true) =
this(name, age, description, name, online)
}
Then you can create a case class like
val user = new User("User name", "5", "description")
//> user : User = User(User name,5,description,User name,true)
user.displayName
//> res0: String = User name
val userWithDisplayName = new User("User name", "5", "description", "displayName")
//> userWithDisplayName : User = User(User name,5,description,displayName,false)
You can also override the apply method in the companion object. This way you don't have to write a new one before creating the object.
scala> :paste
// Entering paste mode (ctrl-D to finish)
case class User(name: String, age: String, description: String, displayName: String, online: Boolean = false)
object User {
def apply(name: String, age: String, description: String, online: Boolean) =
new User(name, age, description, name, online)
}
// Exiting paste mode, now interpreting.
defined class User
defined module User
scala> User("User name", "5", "description", "displayName")
res0: User = User(User name,5,description,displayName,false)
scala> User("User name", "5", "description", true)
res1: User = User(User name,5,description,User name,true)
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