Proposal: Relax rules for 'pure'

[Jesse Phillips]

Steven Schveighoffer Wrote:

> I just thought of something -- we may be defining the common case.  For  
> example, D breaks from the default of shared globals in C because most of  
> the time, variables *aren't* shared.  When I first heard of shared, I  
> thought surely Walter was losing his mind.  Why would shared not be the  
> default, clearly as it's done in C!  But after having used the language, I  
> see that it would have been a huge issue if I had to mark everything as  
> unshared.
> 
> If we can define weakly pure functions this way, they most likely will be  
> way more common than unpure functions.  I know I avoid accessing global  
> variables in most of my functions.  Think about a range, almost all the  
> methods in a range can be weakly pure.  So that means you need to mark  
> every function as pure.
> 
> Would it not be less tedious to mark unpure functions instead of pure  
> functions?  Or am I just going too far with this?
> 
> OR, maybe we could say, mark strongly pure functions as pure, mark  
> functions that access global data as something else (global?) and weakly  
> pure functions just aren't marked.
> 
> -Steve

This is interesting, I'm not sure if either of these approaches could make it into D2, though a conservative approach was originally taken only because it was the most easy to prove.

I also find it interesting that D actually takes the 3-stage approach to several things. mutable, const, immutable; @safe, @trusted, @system. And this being unpure, contained, pure.

D recognizes the importance of these stages in other areas, so I feel it would be a miss if there is not a good reason the proposal doesn't work. Sadly what you learn from one of the stated areas (mutability, safety, purity) can't be applied to the other. But the pattern is there.