Unpack Variadic Args?

[Steven Schveighoffer]

On 2/13/20 11:29 AM, Jeff wrote:
> On Thursday, 13 February 2020 at 08:06:52 UTC, Paul Backus wrote:
>> Variadic template arguments unpack automatically in D, so you don't 
>> need to do anything special here:
>>
>>     void g(Args...)(auto ref Args args) {
>>         import core.lifetime: forward; // like std::forward
>>         f(forward!args);
>>     }
>>
>> You can read more about variadic template arguments in this article:
>>
>> https://dlang.org/articles/ctarguments.html
> 
> That would result in the call:
> 
>      f( args[0], args[1], ... );
> 
> But the C++ version does the following:
> 
>      f( foo(args[0]), foo(args[1]), ... );
> 
> They are different.

the f(foo(args)...) syntax doesn't have a D equivalent.

I don't think it's possible to do without some form of mixin. While 
compile-time lists are available, they must be strictly made of things 
that are either CTFE expressions or symbols.

A possible mixin solution:

string doMixin(T...)(string formatspec)
{
    string result;
    import std.format: format;
    static foreach(i; 0 .. T.length)
       result ~= format(formatspec, __traits(identifier, T[i])) ~ ",";
    return result[0 .. $-1]; // trim last comma
}

void g(T...)(T args) {
    mixin("f(" ~ doMixin!args("foo(%s)") ~ ");");
}

-Steve