Template with template?

[Chris]

On Thursday, 20 March 2014 at 17:49:52 UTC, John Colvin wrote:
> 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

The elements are all of type Trait. However, Type itself might be
of different types. That's why it is not possible? I've come
across this restriction before when using templates, which is a
big disappointment because it restricts the "templatization" /
generalization of data structures somewhat.