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Greg Michaelson: An introduction to functional programming through Lambda calculus (2011, Dover Publications)

I read this in an old hut in a mountain valley without internet connection and without computer by my side (even the electricity from solar panel was scarce). I was solving exercises in my notebook and I thought to myself that learning to program on a sheet of paper works just as good. I spend so much of my time behind computer screen illuminated by that blueish light that is bringing me insomnia, that I have to grab every opportunity I get to get away from it. Interestingly or ironically, I was digital detoxing while learning fundamentals of a group of programming languages.