Is this function pure?

[Janice Caron]

On 9/19/07, Nathan Reed <nathaniel.reed at gmail.com> wrote:
> CTFE is
> basically like running an interpreter, right?

I presume that stands for Compile Time Function Execution and not
chlorotrifluoroethylene, right? :-)

My feeling is that Ingo is basically right, in that Walter needs to
assume that new can never fail - that it can successfully allocate
infinite amounts of memory without causing trouble. If that assumption
holds (...obviously it doesn't, but bear with me...) then new is
allowed. Otherwise the simplest things would be banned: a function
which does toString (e.g. that turns 42 into "42") would be banned; a
function which adds a passed-in element to a passed-in associative
array would be banned, and so on. I guess it just has to be allowed.

But I still have a problem with it, because such a program is only
"logically pure", not "physically pure". And I have to wonder, will be
be allowed to write our own "logically pure" functions and declare
them pure?

How will custom allocators for "new" fit into the picture? A custom
allocator must surely access global state? A custom allocator could be
written to be (logically) pure, or it could be decidedly impure (for
example by initialising the created variables with rand()).

Will purity be transitive and as strictly enforced as const? Will we
be able to cast away purity when we need to?

I guess we'll wait and see, but I would surely like to know.