These are called generalized type constraints. They allow you, from within a type-parameterized class or trait, to further constrain one of its type parameters. Here’s an example: case class Foo[A](a:A) { // ‘A’ can be substituted with any type // getStringLength can only be used if this is a Foo[String] def getStringLength(implicit evidence: A =:= … Read more
The answer is found on the definition of map: def map[B, That](f : (A) => B)(implicit bf : CanBuildFrom[Repr, B, That]) : That Note that it has two parameters. The first is your function and the second is an implicit. If you do not provide that implicit, Scala will choose the most specific one available. … Read more
I hope it’s not a “suicide note”, but I can see your point. You hit on what is at the same time both a strength and a problem of Scala: its extensibility. This lets us implement most major functionality in libraries. In some other languages, sequences with something like map or collect would be built … Read more