[arunc]

If a function is pure, it is automatically inferred by the compiler and it will be marked for purity (sorry for the lack of words). We don't have to annotate with pure attribute. Also D offers the right balance of functional programming that I can think of. Can you elaborate on what's missing from that point?

[WalterBright]

Being able to mark a function as pure means the compiler will check it for you - it's a guarantee. Inferring purity is useful, but it is not visible if it is actually pure or not.