Ola, you do not understand 'pure.'
To consider the design of pure, you must first consider that you cannot add functional purity to an imperative language. It cannot be done. What you can do instead is what D offers with a 'weak purity' guarantee. That global state is not modified indirectly, that side-effects do not occur in a function, except through the function's parameters, except for memory allocation. D allows you to come pretty close to strong purity when a function is marked pure, does not allocate memory inside it or through its arguments, and has only const or immutable arguments. The only way you can get a better purity guarantee than that is to use a functional programming language like Haskell.
