What is pure and what is not pure

[Fawzi Mohamed]

I think that your pot is very well done, and from it it is also clear 
why for pure functions something like the transitive invariant is 
needed: the pure function should not be able to see changes in its 
arguments, otherwise it is not anymore pure.

And purity is not just nice for the compiler optimizations, but also 
because it makes testing easier, and it guarantees the absence of 
indeterminism, something that when one programs in parallel is *very* 
nice to have.

As I pointed out in my earlier post (where I was such a newbie the I 
even mispelled newbie...) I think that const is a nice thing, and 
actually to have purity, one can relax the const by allowing suspended 
of still undefined values (if done in the correct way), and then one 
whould also have laziness.
These extension can be added later on the top of const and in a very 
controlled way by just having two new types of const pointers (that 
would be given by the language) that implement it.