Proposal: Relax rules for 'pure'

[Michel Fortin]

On 2010-09-23 16:24:20 -0400, Tomek Sowiński <just at ask.me> said:

> Michel Fortin napisał:
> 
>> The interesting thing with this change is that you can now call
>> mutators functions on the local variables inside the pure function,
>> because those can be made pure. You can't even iterate over a range
>> inside a pure function without this!
>> 
>> pure int test() {
>>     int result;
>>     auto r = iota(0, 10);
>>     while (!r.empty) {
>>         result += r;
>>         r.popFront(); // can't be pure by current rules!
>>     }
>>     return result;
>> }
> 
> It's because popFront() can't be made pure. It could be if there was 
> @tail immutable in the
> language -- pure functions would mark their arguments with @tail 
> immutable instead of
> immutable. That allows popFront() to advance the range.

I know popFront() can't be made pure by the current rules. The proposal 
that started this thread is to relax 'pure' so that popFront() can be 
made pure. I was simply pointing a use case for that.

And to tell the truth, I have no idea where a tail-immutable qualifier 
would be helpful in this situation... could you elaborate?


-- 
Michel Fortin
michel.fortin at michelf.com
http://michelf.com/