Typesafe variadics in any position

[Jacob Carlborg]

On 2013-02-26 21:29, Steven Schveighoffer wrote:
> Having a conversation about the design of the new std.process, I
> lamented that typesafe variadics must be the last element in the list of
> parameters.
>
> But I would love to have a version that simply takes parameters as
> parameters.  In Tango's process class, it was a nice feature.  Stuff
> like this just worked:
>
> setArgs("prog.exe", "arg1", "arg2");
>
> setArgs("prog.exe arg1 arg2".split());
>
> All with one function.
>
> Because spawnProcess has optional parameters at the end, it makes it
> impossible to have this, since typesafe variadics requires that the
> variadic be the last part of the function parameters, and spawnProcess
> must take the optional stream and config parameters last.
>
> But why?  This seems perfectly plausible to me:
>
> spawnProcess(string progname, string[] args..., File _stdin = stdin,
> File _stdout = stdout, File _stderr = stderr, Config config = Config.none);
>
> There is no ambiguity with something like:
>
> spawnProcess("prog.exe", "arg1", "arg2", File("infile.txt"));
>
> A string does not implicitly convert to a File, and vice versa.
>
> Typesafe variadics are actually extremely simple to deal with (much
> simpler than C-style or D-style variadics), it's just passed as a simple
> D slice.  The compiler takes care of the nasty parts, and is entirely
> call-side.
>
> Basically, in order for this to work, a typesafe variadic parameter must
> have no way to implicitly convert to or from the type of the next
> parameters, up to the first non-variadic parameter.
>
> In other words:
>
> void foo(T[] arg1..., U[] arg2..., V arg3, W[] arg4...)
>
> This works if T, U and V cannot be implicitly converted to each other.
> W can implicitly convert to or from V, because V is positional (there
> MUST be one V argument in this call, and that matches arg3, so any args
> after that are assumed to be arg4)
>
> Would this work? Would it be something people want?
>
> -Steve

I like. But in the case of spawnProcess I would prefer named parameters:

spawnProcess("prog.exe", "arg1", "arg2", _stdin: File("infile.txt"));

-- 
/Jacob Carlborg