Good introduction to free theorems [closed]

I stumbled upon a nice idea of free theorems in functional language. However, the only resource I was able to find is Wadler's article "Theorems for Free". It's quite good but it definitely not a tutorial and hard for me to get through (I understood about half of it and it required for me to spend quite a lot of time). Can you recommend me another article or tutorial which is oriented towards a software developer familiar with functional programming instead of hard core functional language researcher?

Thanks.

http://www.iai.uni-bonn.de/~jv/free-slides.pdf

http://daniel.yokomizo.org/2011/12/understanding-higher-order-code-for.html

http://arxiv.org/pdf/1107.1203.pdf

(Also in typeclassopedia Section 3.3)

http://hackage.haskell.org/package/free-theorems-seq

http://hackage.haskell.org/package/free-theorems-counterexamples