Proposal: Relax rules for 'pure'

[Jesse Phillips]

Walter Bright Wrote:

> Steven Schveighoffer wrote:
> >> A pure function also cannot modify any data via its parameters. In 
> >> other words, its parameters must be transitively const.
> > 
> > Yes, a strongly pure function must have those traits.
> 
> No, that would be a weakly pure one.
> 
> > But, purity exists to allow for optimization.  A weakly pure function 
> > cannot be optimized anymore than a normal function, but a strongly pure 
> > can still be optimized even if it calls weakly-pure functions.
> > 
> > I'll give you an example (with a new keyword to help you understand the 
> > difference):
> > 
> > weaklypure void reverse(int[] x)
> > {
> >    for(int i = 0; i * 2 < x.length; i++)
> >        swap(x[i], x[$-1-i]);
> > }
> > 
> > pure int foo(const(int)[] x)
> > {
> >     auto x2 = x.dup;
> >     reverse(x2);
> >     // do some calculation on x2
> >     ...
> >     return calculation;
> > }
> > 
> > Hopefully you can see how foo still is pure, while being able to call 
> > reverse.  Essentially, weakly-pure functions can be used to build 
> > strongly-pure functions.
> 
> This isn't what I was talking about - you didn't modify the contents of the 
> array x[].

The problem is that you can't call reverse or sort unless the function is 'pure,' but the function can not be pure because it modifies its own parameters which means his example function cannot be marked pure even though it is.