Compile-time AAs

[Don]

Jeremie Pelletier wrote:
> Don Wrote:
> 
>> bearophile wrote:
>>> Don has recently said that adding dynamic arrays at compile-time looks easy. I'd also like to have compile-time associative arrays. So you can fill them inside a CT function (the compiler may try to do the same for AAs created inside a static this(){}), and save some run time.
>>> Even if such compile-time AAs have to be immutable at run-time they can be useful anyway.
>>> A smarter implementation of such CT AAs may even use a perfet hashing, to make them quite fast.
>> Indeed. I think perfect hashing is one of the primary appeals of a 
>> compile-time AA.
>>
>> BTW, you can use AAs inside CTFE already. There's probably missing 
>> functionality, though -- create a Bugzilla test case for anything you 
>> find. The primary thing which is missing is that you can't use an AA 
>> literal to populate a runtime AA (this is a backend issue, not a CTFE 
>> limitation). You can get the AA keys and values as arrays, though, so 
>> you could populate the AA yourself.
> 
> Just out of curiosity, how are compile time AAs implemented? The D runtime already handles the creation and lookups of these arrays, so let's suppose I changed my runtime to have a completely different AA implementation, what would happen when you mix both compile time and runtime AAs?
The funny thing is, there are AA literals, but they can only be used at 
compile time. "An AssocArrayLiteral cannot be used to statically 
initialize anything." (expression.html in the spec).
A compile-time AA is just a pointer to an AA literal. It's really 
peculiar, because that's the only time they can be used. So there's 
pretty much zero interaction between compile-time and run-time AAs!