Proposal: Relax rules for 'pure'

[Walter Bright]

Jesse Phillips wrote:
> 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.


The function in the example does not modify its parameters.