Question

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What do <:<, <%<, and =:= mean in Scala 2.8, and where are they documented?

e.g. in this example, from scala-arm 1.0:

def toTraversable[B](implicit ev: R <:< TraversableOnce[B]): Traversable[B] = 
  new ManagedTraversable[B,R] {
    val resource = self
    override protected def internalForeach[U](resource: R, g : B => U) : Unit = 
      ev(resource).foreach(g) 
  }

Answer 1

<:< (and similar strange looking constructs) are defined in Predef.scala (source at scala-lang.org), which is probably the best resource for working out what they are.

In general, classes like that can be used to provide further bounds on the type parameters within the scope of a particular method.

The <:< in particular is used to require that R is a subtype of TraversableOnce[B].

The description from documentation is:

An instance of A <:< B witnesses that A is a subtype of B. Requiring
an implicit argument of the type A <:< B encodes the generalized
constraint A <: B.

To constrain any abstract type T that’s in scope in a method’s
argument list (not just the method’s own type parameters) simply add
an implicit argument of type T <:< U, where U is the required upper
bound; or for lower-bounds, use: L <:< T, where L is the required
lower bound.