Template with template?

[John Colvin]

On Thursday, 20 March 2014 at 16:40:50 UTC, Chris wrote:
> On Thursday, 20 March 2014 at 16:32:34 UTC, Vladimir Panteleev 
> wrote:
>> On Thursday, 20 March 2014 at 16:28:46 UTC, Chris wrote:
>>> How can I instantiate Person with Trait, i.e. a template with 
>>> a template?
>>>
>>> struct Trait(T0, T1) {
>>> T0 name;
>>> T1 value;
>>> T1[T0] map;
>>>
>>> this(T0 name, T1 value) {
>>>   this.name = name;
>>>   this.value = value;
>>>   map[name] = value;
>>> }
>>> }
>>>
>>> class Person(T) {
>>> T traits[];
>>>
>>> void addTrait(T trait) {
>>>   traits ~= trait;
>>> }
>>> }
>>>
>>>
>>> void main()
>>> {
>>> auto trait1 = Trait!(string, string)("Name", "John");
>>> auto trait2 = Trait!(string, int)("Age", 42);
>>> writefln(%s", trait1.map);
>>> writefln(%s", trait2.map);
>>> // above code compiles and works
>>> }
>>
>> Person!(Trait!(string, string)) person;
>>
>> -- or --
>>
>> alias MyTrait = Trait!(string, string);
>> Person!MyTrait person;
>>
>> Note that this approach won't let you have traits with 
>> different parameters within the same Person type.
>
> Yep, I've already tried this (sorry I should've mentioned it!). 
> But I don't want this restriction.

Arrays are homogeneous. All the elements must be of the same 
type. Different instantiations of templates are different types.

You could use an array of std.variant.Variant