Expanding the horizons of D purity

[Denis Shelomovskij]

01.11.2013 21:12, H. S. Teoh пишет:
> On Fri, Nov 01, 2013 at 08:59:29PM +0400, Denis Shelomovskij wrote:
>> 01.11.2013 0:05, H. S. Teoh пишет:
> [...]
>>> So, in a nutshell, my proposal is:
>>>
>>> - Functions that, besides invoking a delegate parameter, are pure,
>>>   should be allowed to be marked as pure.
>>>
>>> - Template functions that, besides invoking a delegate parameter,
>>>   perform no impure operations should be inferred as pure.
>>>
>>> - A function that takes a delegate parameter cannot be strongly pure
>>>   (but can be weakly pure), unless the delegate itself is pure.
>>>   (Rationale: the delegate parameter potentially involves arbitrary
>>>   references to the outside world, and thus cannot be strongly pure.)
>>>
>>>
>>> T
>>>
>>
>> The code you like to make working looks good but I'm against the
>> language change. I'd say the issue is nested pure functions aren't
>> allowed to access outer function variables. Filed as Issue 11412.
> [...]
>
> No, that's not good enough. What if you need to pass an impure delegate
> to toString()? Sure, std.format can be made to use only pure delegates,
> but you can't predict other code that needs to use toString. Forcing
> toString to take only pure delegates makes it unusable with output
> ranges that need to perform impure operations. With my proposal,
> toString will work with *both* pure and impure delegates -- basically,
> the pure qualifier becomes a dependent purity ("purity of this function
> depends on the delegate parameter, body of function is pure besides the
> invocation of the delegate").
>
>
> T
>

I'd say `@safe`, `pure`, and `nothrow` are in the same boat here and the 
solution should work for them all. Probably we will need a delegate 
attributes wildcard. Something like this:
---
// a single wildcard for all:
void f(void delegate() @attr_wildcard del) @attr_wildcard;
---
or better:
---
// `f` is `@safe`, it also `pure` and `nothrow` if `del` is:
void f(void delegate() @safe pure? nothrow? del) @safe pure? nothrow?;
---

Also, do you disagree with the proposal in Issue 11412 [1]?

[1] http://d.puremagic.com/issues/show_bug.cgi?id=11412

-- 
Денис В. Шеломовский
Denis V. Shelomovskij