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While Haskell's purity comes with a whole bunch of benefits, it makes us tackle some problems differently than we would in impure languages. Because of referential transparency, one value is as good as another in Haskell if it represents the same thing.

So if we have a tree full of fives (high-fives, maybe?) and we want to change one of them into a six, we have to have some way of knowing exactly which five in our tree we want to change. We have to know where it is in our tree. In impure languages, we could just note where in our memory the five is located and change that. But in Haskell, one five is as good as another, so we can't discriminate based on where in our memory they are. We also can't really change anything; when we say that we change a tree, we actually mean that we take a tree and return a new one that's similar to the original tree, but slightly different.



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