Interpolated strings

[Martin Tschierschke via Digitalmars-d]

On Wednesday, 19 April 2017 at 17:51:05 UTC, Nick Sabalausky 
(Abscissa) wrote:
> On 04/17/2017 03:41 PM, Jonas Drewsen wrote:
[...]
>>>    exho!"The number ${num} doubled is ${num * 2}!"
> Also, it only works if you're just sending the string to 
> writeln. It doesn't help the general case :(
you can define:

   auto mixinter(string x)(){return mixin(interp!x);}

(in the same scope as the vars used inside x)
  And use:

   string text = mixinter!"${name} and this are app ${age*365*24} 
hours!";


But can someone explain me why this works:

import scriptlike;
void main()
{

   auto name = userInput!string("Please enter your name");
   auto age = userInput!int("And your age");

   // The proposed notation for scriplike string interpolation

   writeln(mixin(interp!"${name} you are app. ${age*365} days 
old"));

   // with exho definition
   auto exho(string x)(){return mixin("writeln("~interp!x~")");}

   exho!"${name} and this are app ${age*365*24} hours!";
}


and this not?

import scriptlike;

// with exho definition outside the scope of the used vars

auto exho(string x)(){return mixin("writeln("~interp!x~")");}

void main()
{

   auto name = userInput!string("Please enter your name");
   auto age = userInput!int("And your age");

   writeln(mixin(interp!"${name} you are app. ${age*365} days 
old"));

   exho!"${name} and this are app ${age*365*24} hours!";
}

Can it be possible to allow a function to be defined outside the 
scope of use to return a "mixin object"?, than everything can be 
done in a lib outside, no need to add parsing complexity to the 
language?