How do "pure" member functions work?

[Don]

Simen Kjaeraas wrote:
> On Mon, 22 Aug 2011 22:19:50 +0200, Don <nospam at nospam.com> wrote:
> 
>> BTW: The whole "weak pure"/"strong pure" naming was just something I 
>> came up with, to convince Walter to relax the purity rules. I'd rather 
>> those names disappeared, they aren't very helpful.
> 
> The concepts are useful, but better names might be worth it. But what
> short word eloquently conveys 'accesses no mutable global state'? :p

@nostatic  ?
stateless ?

> What we call strongly pure is what in other languages is simply called
> 'pure', and that is likely the word that should be used for it.


> Weakly pure is a somewhat different beast, and the 'best' solution would
> likely be for it to be the default (But as we all know, this would
> require changing the language too much. Perhaps in D3...). Noglobal
> might be the best we have. My favorite thus far is 'conditionally pure'.
> It conveys that the function is pure in certain circumstances, and not
> in others. However, it might be somewhat diluted by the addition of pure
> inference in newer versions of DMD - that definitely is conditionally
> pure.
> 
> Const pure is not a concept I'm particularly familiar with. Is this the
> special case of calling a conditionally pure function with only
> const/immutable parameters, with arguments that are immutable in the
> calling context, and that it in those cases can be considered
> strongly pure?

No, it's where the signature contains const parameters, rather than 
immutable ones.
Two calls to a const-pure function, with the same parameters, may give 
different results. You'd need to do a deep inspection of the parameters,
to see if they changed.

Consider:

x = foo(y);
x = foo(y);
where foo is const-pure.
Perhaps y contains a pointer to x. In that case, foo could depend on x.
Or, there might be a mutable pointer to y somewhere, and x might have an 
opAssign which modifies y.
In each case, the second y is different to the first one.

But, if foo is immutable-pure, it will return the same value both times, 
so one of the calls can be optimized away.