Proposal: Relax rules for 'pure'

[Steven Schveighoffer]

On Wed, 22 Sep 2010 15:36:34 -0400, Walter Bright  
<newshound2 at digitalmars.com> wrote:

> Don wrote:
>> Don wrote:
>>> The docs currently state that:
>>
>>> PROPOSAL:
>>> Drop the first requirement. Only one requirement is necessary:
>>>
>>> A pure function does not read or write any global mutable state.
>>>
>>  Wow. It seems that not one person who has responded so far has  
>> understood this proposal! I'll try again. Under this proposal:
>>  If you see a function which has mutable parameters, but is marked as  
>> 'pure', you can only conclude that it doesn't use global variables.  
>> That's not much use on it's own. Let's call this a 'weakly-pure'  
>> function.
>>  However, if you see a function maked as 'pure', which also has only  
>> immutable parameters, you have the same guarantee which 'pure' gives us  
>> as the moment. Let's call this a 'strongly-pure' function.
>>  The benefit of the relaxed rule is that a strongly-pure function can  
>> call a weakly-pure functions, while remaining strongly-pure.
>> This allows very many more functions to become strongly pure.
>>  The point of the proposal is *not* to provide the weak guarantee. It  
>> is to provide the strong guarantee in more situations.
>
> 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.

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.

-Steve