Hello, 

Down is useful because the function compare recognizes things wrapped in Down and reverses the usual order of comparison. 

What does that mean! I did not get the idea behind the Down type! I google it too but not clear results !

Can I have a more clear explanation for it, please? 

In the following giving examples:

compare (Down x) (Down y) = compare y x
> sortBy (comparing Down) [4, -2, -7, 5]
[5,4,-2,-7]
-- That’s the same as
> sortBy (\a b -> compare (Down a) (Down b)) [4, -2, -7, 5] [5,4,-2,-7]
-- Which is the same as
> sortBy (\a b -> compare b a) [4, -2, -7, 5]
[5,4,-2,-7] 

Since the all the three functions are the same !!

Then, we need to use the Down type?

Thank you 

Eman

Great looking forward to learning about that.

Thank you, professor Abbott