Re: What is your favorite D feature? [Call for snippets]

[Patrick Schluter via Digitalmars-d]

On Thursday, 22 June 2017 at 07:00:17 UTC, Seb wrote:

Thanks a lot, but I was looking for small snippets (< 10 LoC) 
than could be used to show the awesomeness of D at the roulette


enum E {IN = -1, V1, V2, V3, X1 }

mixin({
  string code = "enum EBIT : ulong { "~
                        "init = 0,";      /* We set the dummy 
init value to 0 */
  foreach(Code; __traits(allMembers, E)) {
    static if(Code == "IN")
      code ~= "INVALID = -1,";
    else
      code ~= Code~"= 1 << E."~Code~",";
  }
  code ~= "
    ALL    =  -1,
  return code ~ "}";
}());

Generates an enum which values depend on the values of another 
enum.

In C or other languages I would needed to write the 2nd enum 
myself

enum EBIT { INVALID = -1,
             V1= 1 << E.V1,
             V2= 1 << E.V2,
             V3= 1 << E.V3,
             X1= 1 << E.X1,
          }

For a 4 value enum it doesn't look such a big deal but in my real 
project coming from C, the first enum holds more than 50 regular 
values and 3 special values. When I have to add or remove a value 
I have to edit the first enum, but also the 2nd enum, 5 derived 
lookup tables and 2 functions using a big switch/case with the 
enum values.
In D I edit only the first enum, the rest is auto-generated.

As 2nd trick/nice thing here is a function geenrating a 
switch/case with the enums.

string E2thing(E val)
{
  switch(val) {
    foreach(e; __traits(allMembers, E))
    static if(e != "IN")
      mixin(`case E.`~e~`: return "`~e~`";`);
    default: return "UNDEFINED";
  }
}