The "type" type dream

[Nick Sabalausky]

"Michel Fortin" <michel.fortin at michelf.com> wrote in message 
news:gghpq4$2nc6$1 at digitalmars.com...
> In the "Unification and extension of compile-time reflection" thread, 
> Jarrett Billingsley wrote:
>
>> static if(is(T : V[K], K, V))
>> {
>>    // V and K are defined here
>> }
>
> Since we're talking about unifying things...
>
> [enter dream mode]
>
> Imagine types as first-class values.
>
> static if (type V[type K] = T)
> {
> // V and K assigned from type T.
> }
>
> Basically, we're declaring variables V and K of type "type". The "type" 
> type simply holds a type. "type V[type K]" is a combined 
> declaration/assignment containing the name and relative disposition of the 
> two type variables to which we try to assign T. An impossible type 
> assignment would assign void to both, a correct assigment would assign the 
> corresponding non-void type for each variable in the specified position. 
> In the if statement, void types converts to false, non-void types converts 
> to true.
>
> Now you can do all the above as separate statements if you wish (should 
> make things easier to understand):
>
> type V; // V is void
> type K; // K is void
> V[K] = T; // compiler assign T to V and K according to the V[K] 
> disposition.
> if (V && K) ...
>
> Even better, type as function return type:
>
> type valueTypeOf(type T)
> {
> type V[type K] = T;
> return V;
> }
>
> static if (type V = valueTypeOf(T))
> {
> // V assigned from function valueTypeOf(T) (which returns a type).
> }
>
> valueTypeOf(T) a; // variable of the type returned by valueTypeOf(T).
>
> Here, valueTypeOf is a function taking a type as argument and returning 
> another type. Obviously, it's a function that can only be evaluated at 
> compile-time, but with it we can just forget about using templates and 
> declarative programming when we need to create variables of related types.
>
> [exit dream mode]
>
> Not sure what all this would entail, but if we could return types from 
> functions, would we still need templates at all? Couldn't we just create 
> any struct or class inside the function and return that? Unifying 
> templates and functions... hum, I'm probably still dreaming.
>
> -- 
> Michel Fortin
> michel.fortin at michelf.com
> http://michelf.com/
>

I love it!