Compile time initialization of AA

[Patrick Schluter]

On Friday, 23 March 2018 at 22:43:47 UTC, Xavier Bigand wrote:
> I am trying to initialize an global immutable associative array 
> of structs, but it doesn't compile.
> I am getting the following error message : "Error: not an 
> associative array initializer".
>
> As I really need to store my data for a compile time purpose if 
> we can't do that with AA, I'll use arrays instead.
>
> Here is my code :
> struct EntryPoint
> {
>     string  moduleName;
>     string  functionName;
>     bool    beforeForwarding = false;
> }
>
> immutable EntryPoint[string]  entryPoints = [
>     "wglDescribePixelFormat": 
> {moduleName:"opengl32.forward_initialization", 
> functionName:"wglDescribePixelFormat"}
> ];

Another solution, radically different is to not use an AA but a 
simple array.
Instead of indexing on a string you could simply index on an enum 
type. As your array is compile time constant, the dynamic nature 
of AA is not really used and indexing on a enum value is faster 
and simpler anyway.
The trick here is to generate the enum type at compile time. This 
can be achieved by a string mixin built at compile time.

Here an example that I used to generate an enum from the values 
from another enum. In your case you can look where the 
"wglDescribePixelFormat" are defined and using imports or string 
building code.

mixin({
   string code = "enum LANBIT : ulong { "~
                         "init = 0,";      /* We set the dummy 
init value to 0 */
   foreach(lanCode; __traits(allMembers, LANIDX)) {
     static if(lanCode == "IN")
       code ~= "INVALID = LANIDX2LANID(LANIDX."~lanCode~"),";
     else
       code ~= lanCode~"= LANIDX2LANID(LANIDX."~lanCode~"),";
   }
   code ~= "
     ALL    =  INVALID-1,                     /**< All bits except 
the LANID_INVALID */
     OFFICIAL= 
BG|CS|DA|DE|EL|EN|ES|ET|FI|FR|GA|HR|HU|IT|LT|LV|MT|NL|PL|PT|RO|SK|SL|SV,  /**< Official languages of the EU */
     COOFFICIAL=CA|GL|EU|GD|CY             /**< Co-official 
languages of the EU */
   ";
   return code ~ "}";
}());


TL/DR
defining constant compile time AA is an oxymoron. AA are by 
nature dynamic runtime creatures. if the indexes are compile 
time, a normal array with fixed indexes is enough.