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My understanding, which may be incorrect, is that the major reason that lawfulness matters in Haskell is the laziness makes it so that unlawful monads won't just do "something that violates the laws", an abstract, mathematical consideration that maybe you care about, maybe you don't, but that the combination of the laziness and the aggressively optimizing compiler means that result will be very unpredictable, and slight and seemingly isomorphic source changes can result in unpredictable results. In Python, if I write an iterator on something pretending to be a list, and when it sees strings it doesn't just return an uppercased string but actually modifies the contents of the list to be uppercased, that's stupid, but at least since it's a strict language that isn't interleaved with IO and all the other stuff flying around in Haskell it will be consistently stupid. It isn't going to blow up or behave differently if I accidentally flip an "a + b" into a "b + a" somewhere. It's bad, but Haskell has a whole different level of bad if you screw with it and don't play within the sandbox. There is a definite "I'm being more pragmatic here than the average Haskell programmer" effect going on here. I... how to put this... "won't blink" is too strong, but... if I need to violate a law, if I need to write something like the stupid iterator above, I am in fact willing to. I have the decency to feel bad about it, and there will be extensive and probably bitingly sarcastic comments attached to it, but I'll do it. (Generally only when I don't control one end of the source code, though. If I have full control I never do anything that stupid.) But in Haskell it's a particularly bad idea, mostly because of the laziness and its interaction with other things. And, heh, in a world where the struggle to explain what monads even are to people, monad combinators aren't even on my horizon. |