This whole thing is a bit disingenuous, because the type signature argued over is an alternate signature which no one uses, because it provides custom concatenation of Traversables. All of the provided collections have reasonable implementations of concatenation, so this implies you're inventing your own collection.
Everyone uses the alternate form: flatMap [B] (f: (A) ⇒ Traversable[B]): CC[B]. Nevertheless, I'll attempt to explain the provided signature.
If you understand Java generics, you should hopefully be able to understand this response.
A, B, and That are type parameters/generics. So they can be filled in by any type. This flatMap method exists on a trait (think Java Interface) called Traversable[A]. Traversable is similar to Iterable in java.
(f: (A) ⇒ Traversable[B]) means that this function takes one argument called f. That argument is a closure or anonymous function. The closure itself takes one argument of type A (an element in the parent Traversable[A]), and returns a Traversable[B], where B is an arbitrary type.
Skipping ahead, flatMap returns an instance of That. But what's a That? It appears to be unspecified. This is where the "(implicit bf: CanBuildFrom[List[A], B, That])" makes it's appearance.
Implicit parameters are like default arguments. "def foo(i: Int = 12)" will use the i if provided, else it will default to 12. "def foo(implicit i: Int)" will take the i given, else it will grab the first available Int in the containing scope. If you, "val c = 11; def foo(implicit i: Int) = i * 2", calling foo(1) yields 2, but calling foo() yields 22.
The object bf is an object that provides an overridable custom definition of concatenation. So when you write your custom collection, you include an instance of CanBuildFrom[List[A], B, That] in the correct scope, and it gets used for the concatenation. CanBuildFrom's type parameters say that it exists in a List[A], takes a B, and returns a That.
So yeah, in your implementation of bf, you could make it a new MyCanBuildFrom[List[A], B, FunkyCollection[B]], and do funky concatenation.
Everyone uses the alternate form: flatMap [B] (f: (A) ⇒ Traversable[B]): CC[B]. Nevertheless, I'll attempt to explain the provided signature.
If you understand Java generics, you should hopefully be able to understand this response.
A, B, and That are type parameters/generics. So they can be filled in by any type. This flatMap method exists on a trait (think Java Interface) called Traversable[A]. Traversable is similar to Iterable in java.
(f: (A) ⇒ Traversable[B]) means that this function takes one argument called f. That argument is a closure or anonymous function. The closure itself takes one argument of type A (an element in the parent Traversable[A]), and returns a Traversable[B], where B is an arbitrary type.
Skipping ahead, flatMap returns an instance of That. But what's a That? It appears to be unspecified. This is where the "(implicit bf: CanBuildFrom[List[A], B, That])" makes it's appearance.
Implicit parameters are like default arguments. "def foo(i: Int = 12)" will use the i if provided, else it will default to 12. "def foo(implicit i: Int)" will take the i given, else it will grab the first available Int in the containing scope. If you, "val c = 11; def foo(implicit i: Int) = i * 2", calling foo(1) yields 2, but calling foo() yields 22.
The object bf is an object that provides an overridable custom definition of concatenation. So when you write your custom collection, you include an instance of CanBuildFrom[List[A], B, That] in the correct scope, and it gets used for the concatenation. CanBuildFrom's type parameters say that it exists in a List[A], takes a B, and returns a That.
So yeah, in your implementation of bf, you could make it a new MyCanBuildFrom[List[A], B, FunkyCollection[B]], and do funky concatenation.
Hope that helps.