pure-ifying my code

[Philippe Sigaud]

>> But `popFront()` changes an internal state with visible, external
>> effects: it changes `front` return value and in the end, it will
>> affect `empty` return value.
>> So no, I don't consider `popFront` to be pure.
>
> None of that matters for pure. _All_ that matters for pure is that the
> function does not access global, mutable state - so no mutable global or
> static variables. _Strong_ purity requires more, but you're not going to get
> that with a member function anyway, unless it's immutable, which is almost
> never the case.
>
> If you're trying to think about functional purity when dealing with pure,
> you're definitely going to misunderstand it, because functional purity really
> has very little to do with D's pure. At this point, pure is all about
> indicating that the function does not access anything mutable that you don't
> pass to it.

I know the difference between functional purity and D purity. It's
just I consider the range internal state to *be* something mutable
that I don't pass to the member.

I never considered that code like this:

auto a = range.front();
auto temp = a;
range.popFront(); // pure popFront()?
a = range.front();
assert(temp != a); // pure popFront() modified range.front() value!

could be authorized. Maybe that's because, until now, I only *read*
about D purity and never tried to use it in my code.


> I think that the typical approach at this point is to just drop purity for the
> moment, but if you want you really want it, you are indeed going to have to
> implement it yourself. But we'll get there with Phobos eventually. The primary
> holdup is some compiler improvements, and we'll get them. It's just a question
> of when.

I implemented it myself without problem (~ 10 LoC). The trouble really
came from `joiner` using an internal struct.
I wanted to keep purity, if only to get a feeling of the kind of
constraints it adds to my coding style. I regularly try to 'limit'
myself to CTFE-compatible code, since I often use my code at
compile-time. I would like to also learn to write pure code, as fas as
possible (and drop it when it's not reasonable, of course).