On the contrary, purity is needed because of laziness! https://www.microsoft.com/en-us/research/publication/wearing-hair-shirt-retrospective-haskell-2003/ http

On the contrary, purity is needed because of laziness! https://www.microsoft.com/en-us/research/publication/wearing-hair-shirt-retrospective-haskell-2003/ https://www.microsoft.com/en-us/research/publication/wearing... Not, mind you, merely the possibility of laziness; as you say, you can do that without monads or purity. But with pervasive laziness it becomes difficult to reason about the precise order in which thunks will be evaluated and be sure that (say) the warning will be printed before the missiles are fired instead of after. With purity it (mostly) doesn't matter the precise evaluation order, so (again, in the setting of pervasive laziness) we want that wherever we can have it! But at some point we need to actually fire those missiles and print those messages. The Monad interface for IO, in chaining together effectful computations with bind, makes the input of later computations depend on the output of earlier computations such that the regular data dependencies between the thunks ensure that things happen in the order we asked for.