Explain Contramap

Question

Can someone explain contramap to me? What would this implementation look like? What would good examples of usage look like?

// contravariant funct         


        

Answer 1

If you look at the following Ordering.on method of the standard library:

def on[U](f: U => T): Ordering[U]

You'll see that on transforms an Ordering[T] into an Ordering[U] while taking a function from U to T. So the method on witnesses the fact that Ordering can be seen as a Contravariant functor with:

def contramap[A, B](f: B => A) = (fa: Ordering[A]) => fa.on(f)

I also saw the blog post from Tony and it helped me finally makes sense of this three year old answer from retronym to one of my question.