Domain Specific Languages in D; was: C++, D: Dinosaurs?

[Aarti_pl]

Julio César Carrascal Urquijo pisze:
> Hello Aarti_pl,
> 
>> 3. Impossibility to statically control meta language contructs.
>>
>> Sql has its own syntax rules and currently they can not be enforced on
>> compile time.
>>
>> E.g. in select statement you would expect to have OrderBy after Where,
>> not other way.
> 
> Instead of returning the full query object you could return a "Fragment" 
> query. Something like this:
> 
> class WhereFragment
> {
>    this(Query q) {}
>    WhereFragment And(Expression e) {}
>    OrderByFragment OrderBy(Column[] columns) {}
> }

Yes, something like this would be possible. But it would start to be 
very complicated, unmanageable and not so useful soon. Implementing it 
in such a way for every DSL will be very hard. Also there are cases in 
normal programs where it would be useful to have such checks.

Currently I use simple runtime checks (somewhat simplified, pseudocodish 
and not tested code below):

class SelectStatement {
   string[] next;
   this() {
     next = ["Where", "From"]; //All possible options after construction
   }

   public SelectStatement Where(SqlExpression exp) {
     if ("Where" !in next) throw Exception("Syntax error");

     ...

     next = ["OrderBy"];
   }
}

Probably it should be possible to implement something similar in 
compiler to make checks on compile time. If some variable has assigned 
object of such a class compiler should trace this variable and check if 
methods are called properly (in order, that they *are* called etc.)

I don't know how easy / difficult is to implement something like this in 
compiler, but I think it may be difficult.

> 
>> Currently I can enforce order of clauses only on
>> runtime, although it should be probably doable to enforce it on
>> compile time. Currently it is also not possible to enforce that e.g.
>> some method must be called:
>>
>> Query query = Select(id).From(table);
>>
>> Imagine that you want to enforce that in above query From() must be
>> always called (Select is an object, From is method on this object).
> 
> You should probably do as LINQ to SQL and start the query with the FROM 
> statement instead. That way you can cascade the type and help 
> auto-complete in IDEs. 

Well, I don't see problem here with my approach... SQL has a lot of 
options after SELECT keyword and before WHERE. How can I use them using 
only From?

> But yes, I don't think it's currently possible to 
> enforce a method to be called.

BR
Marcin Kuszczak
(aarti_pl)