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I don't know the story but I think it's dubious. We understand that evaluating an expression in a pure functional programming language is like using an old desk calculator: it's useful to us because the return value is of interest. SPJ muddies the waters in that video.

Heating the CPU could constitute in a side channel in the cryptographic sense, but it doesn't mean the computation isn't pure. I don't think there's a rigorous sense in which side channels are a form of computational side effect. By the way, see: conal.net/blog/posts/t...