Code shared from run.dlang.io.

main.d

import std.stdio;

auto inter(Args...)(Args args)
{
    import std.conv : to;
    
    string result = "";
    static foreach (argIndex, argType; Args)
    {
        static if (is(argType : string))
        {
            result ~= args[argIndex];
        }
        else
        {
            result ~= args[argIndex].to!string;
        }
    }
    return result;
}
template somefun()
{    
     import std.conv : to;

     static enum State
     {
             start = 0,             
             in_var = 1,
             in_literal = 2,
             choice = 3,
     }
    
     auto iterpolate(string s)()
     {            
         return mixin(feach!(s, s.length, State.start));
     }
     string feach(string s, size_t length, State state : State.start)()
     {         
         if (s[0] == '$')
         	return "inter(" ~ feach!(s[1 .. $], s.length - 1, State.in_var);
         else
            return "inter(\"" ~ feach!(s[0 .. $], s.length, State.in_literal); 
     }
    
    string feach(string s, size_t length, State state : State.choice)()
     {
         if (s[0] == '$')
         	return "," ~ feach!(s[1 .. $], s.length - 1, State.in_var);
         else
            return ",\"" ~ feach!(s[0 .. $], s.length, State.in_literal); 
     }
    
    string feach(string s, size_t length, State state : State.in_var)()
     {
         if (s[0] == ' ')
         	return feach!(s[0 .. $], s.length, State.choice);
         else
            return s[0] ~ feach!(s[1 .. $], s.length - 1, state); 
     }
    
    string feach(string s, size_t length, State state : State.in_literal)()
     {
         if (s[0] == '$')
         	return "\"" ~ feach!(s[0 .. $], s.length, State.choice);
         else
            return s[0] ~ feach!(s[1 .. $], s.length - 1, state); 
     }
    
    string feach(string s, size_t length : 0, State state : State.in_literal)()
     {
         return "\")";
     }
    
    string feach(string s, size_t length : 0, State state : State.in_var)()
     {
         return ")";
     }
}

enum enableInterpolate = "mixin somefun A; alias _ = A.iterpolate;";

void main()
{    
    mixin(enableInterpolate);
    int a = 5;
    string b = "BIG B";
    _!("Value of a is $a and value of b is $b").writeln;  
}