DIP 50 - AST macros

[John Colvin]

On Tuesday, 12 November 2013 at 15:21:16 UTC, Ellery Newcomer 
wrote:
> On 11/12/2013 06:38 AM, John Colvin wrote:
>> On Tuesday, 12 November 2013 at 13:50:49 UTC, Jacob Carlborg 
>> wrote:
>>> auto person = Person.where(e => e.name == "John");
>>>
>>> Translates to:
>>>
>>> select * from person where name = 'John'
>>>
>>
>> for those of us entirely unfamiliar with linq, what is this 
>> supposed to
>> do? Select people with name "John" from a collection of 
>> people, like in
>> sql? It seems trivial to do this using filter, or am I missing
>> something...?
>
> linq provides an interface to query any collection, but what is 
> interesting in this case is
>
> Person.where(e => e.name == "John")
>
> invokes an engine that builds the necessary sql to issue an 
> equivalent query to your sql database and assembles the results 
> into a range of Person. And it can do this for any arbitrary 
> predicate (well, almost. It doesn't handle function calls to 
> arbitrary code too well).
>
> the .NET framework can do this because it exposes an api for 
> querying, building, and compiling asts.
>
> D cannot do this because it doesn't. (and I have tried to make 
> it work)

oh, I see. Would AST macros really be enough to make this work in 
D? "Arbitrary code" is a huge feature space in D, including much 
that doesn't map well to anything outside of a relatively 
low-level language, let alone SQL.
I can see it quickly becoming a nightmare that would be worse 
than just issuing the predicate as an sql string or some generic 
equivalent.