should chain be pure

[monarch_dodra]

On Tuesday, 15 October 2013 at 14:15:15 UTC, bearophile wrote:
> Daniel Davidson:
>
>> If you are agreeing that chain should be pure and it is just 
>> following all the calls and making all of them pure, until 
>> that happens by the professionals - is there a casting 
>> solution so I can fake a pure and move on?
>
> chain is a template, and in Phobos often templates are not 
> annotated with pure/nothrow, the compiler infers those 
> attributes.
>
> Regarding your code, perhaps you can put your call in an impure 
> delegate and than cast it, but D has no direct means to "cast" 
> purity, because it's highly unsafe and it's against the idea of 
> having purity in the language.
>
> So I suggest to replace the pure in your function/method tree 
> with /*pure*/, and later fix the code if/when chains becomes 
> pure.
>
> Bye,
> bearophile

@bearophile: The problem is actually with voldemort. Chain is 
implemented as:

auto chain(Arg...)(Args args)
{
     static struct Result //Non template struct
     {
         auto front(); //Non template function in a non-template 
struct.
     }
}

The problem is that the whole inference things stops at this 
level: the attributes of "front" are not infered, so chain is not 
pure simply because it isn't a template.

This could be simply solved by making Result a non voldermort 
"ChainResult" outside of the body of chain.

I'd do this, but Kenji had mentioned before that he thought the 
attributes should be inferred.

So for now, I didn't personally make the effort of doing anything 
to fix it myself. But if someone else where make the effort, I'd 
review and probably pull.