Question

Is it possible to define generic operators in Scala?

Scala lets me map arbitrary operators on functions, which is incredibly useful. It seems restrictive however, in a case where I might want the operators to change given the state of the application.

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To give an example: I have a table with users and a table with their respective relationships. Each relationship has a type, such as: “friends-with”, “works-with”, etc.
Based on my domain model, I would like the DSL to allow for: is(john friends-with mary). In this case, both john and mary would be of Object User, which would have a generic operator def <relationship> (a:User): Boolean = {...}.

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What I wanted to achieve was exactly what Dynamic would allow me to do (see answer). The description fits perfectly:

A marker trait that enables dynamic invocations. Instances x of this trait allow calls x.meth(args) for arbitrary method names meth and argument lists args. If a call is not natively supported by x, it is rewritten to x.applyDynamic(“meth”, args).

More information here: http://www.scala-lang.org/api/current/scala/Dynamic.html

Answer 1

Look at trait Dynamic, which will be available on Scala 2.10 (it’s experimental on Scala 2.9).

For example:

scala> :paste
// Entering paste mode (ctrl-D to finish)

case class User(name: String) extends Dynamic {
  def applyDynamic(relationship: String)(to: User) =
    Relation(relationship, this, to)
}
case class Relation(kind: String, from: User, to: User)

// Exiting paste mode, now interpreting.

defined class User
defined class Relation

scala> User("john") friendsWith User("mary")
res0: Relation = Relation(friendsWith,User(john),User(mary))