Purity in Java & D

[Walter Bright]

Jonathan M Davis wrote:
> Having the compiler determine purity would be cool, but it runs into a few of 
> problems.

The most serious one I can think of is:

Suppose you are depending on a function being pure. If the compiler determines 
its purity, it *does not tell you* if it is impure. Your code just doesn't 
behave as you expect. If you happen to notice that it isn't being treated as 
pure, you won't know why. It could be the function itself or any of the 
functions in its transitive call tree.

Basically, anyone working on any function in the call tree could break purity 
for the whole shebang, and you'd have no idea this was happening.

With D, purity is part of the function signature. That means the compiler does 
not determine purity, the compiler *checks* purity. That's a fundamental 
difference. If it isn't pure, you'll immediately know it, and where. If you 
change a function from pure to impure, any function that calls it will need to 
be recompiled (or it will fail to link). There won't be any stealth changes 
subverting the system.