KeyType, ValueType traits for hashes

[Mantis]

24.01.2012 8:43, Andrej Mitrovic пишет:
> But I did implement them poorly, this is better:
>
> import std.traits;
>
> template KeyType(AA)
>      if (isAssociativeArray!AA)
> {
>      static if (is(AA V : V[K], K))
>      {
>          alias K KeyType;
>      }
> }
>
> template ValueType(AA)
>      if (isAssociativeArray!AA)
> {
>      static if (is(AA V : V[U], U))
>      {
>          alias V ValueType;
>      }
> }
>
Might I suggest that it be implemented this way:

template KeyType(AA)
{
     static if (is(AA V : V[K], K))
     {
         alias K KeyType;
     }
     else static assert( 0, "Not associative array: " ~ AA.stringof );
}


Of course, most likely that user already did type check, but if not, 
this will give less cryptic error:

Error: static assert "Not associative array: int"
instantiated from here: KeyType!(int)

, instead of:

Error: template instance KeyType!(int) KeyType!(int) does not match 
template declaration KeyType(AA) if (isAssociativeArray!(AA))