Question

What is the difference between Scala traits Haskell type class and C++0x Concepts?

Like in this example below where Observer declare an abstract members receiveUpdate
Observer is in fact a “anonymous” types or Structural types.

package observer
trait Subject {
   type Observer = { def receiveUpdate(subject: Any) }
   private var observers = List[Observer]()
   def addObserver(observer:Observer) = observers ::= observer
   def notifyObservers = observers foreach (_.receiveUpdate(this))
}

Answer 1

• Concepts are not coming to C++0x, they’ve been removed in the last few versions of the draft spec.
• Type-classes where originally designed for bounded quantification of parametric polymorphism (generic constraints, “forall x such that x is an…”) and a mechanism to provide ad-hoc polymorphism for a totally type inferred language in a less ad-hoc manner.
• Concepts are also for bounded quantification of parametric polymorphism and provide concept overloading, which makes up for the lack of partial specializations of template functions in C++. They where originally designed to deal with template error message problem in C++.
• Traits are mechanism to mix-in behaviors without using multiple-inheritance.

So only two of them have something in common but not by much, that is Concepts and Type-classes. There already has been comparisons between the two in this paper: A comparison of C++ concepts and Haskell type classes