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Also, my practical-oriented blog posts on the topic in .net (referenced in my own answer on that page):

That first blog post of yours was great, FYI - exactly what I was looking for.

A simpler example would be say amplifying a signal functor. In Haskell,

let amplify x f = fmap (x *) f

amplify 2 [1,2,3]

>> [2,4,6]

cos pi

>> 1.0

let doublecos = amplify 2 cos

doublecos pi

>> 2.0

You can't do anything that operates so generically in a language without HKTs.

All that said, I don't find them all that useful in day-to-day work.