[lmm]

Learning the term before the concept is putting the cart before the horse. But once you find yourself dealing with the fact that e.g. one can use a Source[Float] as a Source[Number] and not vice versa, whereas one can use a Sink[Number] as a Sink[Float] but not vice versa; one can use a Function[Number, Float] as a Function[Float, Float] or a Function[Number, Number] and either of those as a Function[Float, Number], but not in reverse, then it's useful to have words for these relationships so that we can talk about them.