I started scala course on coursera, but I can't get one thing here:
trait Generator[+T] { self => // do this to be able to use self instead of this def generate: T def map[S](f: T => S): Generator[S] = new Generator[S] { def generate = f(self.generate) } } Why we are using map[S] not just map in function definition?
The [S] after map is a type parameter and makes it a so-called polymorphic method. In your example above, if you wrote the same def map, but without [S], the compiler wouldn't be able to tell what S is when encountering it in the remaining method definition. So the [S] makes the identifier S known to the compiler and puts it in the position to report typos as errors.
For example, assume a new method in which you accidentally wrote f: T => Floot rather than f: T => Float. What you want is the compiler to complain that Floot is an unknown identifier. You wouldn't want it to silently assume that Floot is some sort of type parameter.
maphas a type parameter,S.Generator[T]to anotherGenerator, not necessarely aGenerator[T], that's why you need to tell the compiler to the type you want to end up with.Generator[String]from aGenerator[Int], not justGenerator[Int]s can be returned this way. You have to refer to the new generic parameters somehow.Sand notU.