Operator overloading

[aarti_pl]

Don pisze:
> aarti_pl wrote:
>> Andrei Alexandrescu pisze:
>>  > We're trying to make that work. D is due for an operator overhaul.
>>  >
>>  > Andrei
>>
>> Is there any chance that we get possibility to overload "raw 
>> operators", like in C++? I think that they may coexist with currently 
>> defined operator overloads with simple semantic rules, which will not 
>> allow them to work together at the same time.
>>
>> I know that they may obfuscate code, but they are extremely useful 
>> when defining in D domain specific languages. For example I try to 
>> make SQL expressions work as D expressions:
> 
>>
>> string query = "SELECT * FROM a WHERE id=5;";
>> -->
>> Query query = Select(a).Where(Equals(id, 5));
>>
>> * "id" is D object in second expression
>>
>> This translation is quite awful and unreadable. It would be so much 
>> better to get:
>>
>> Query query = Select(a).Where(id == 5);
> 
> I think that's still awful. I prefer:
> Query query = mixin(SQL("SELECT * FROM a WHERE id=5"));
 >

Thanks for example - it talks to me more than 1000 words :-)

Your proposal is indeed interesting. (And I think it is what Dennis 
Luehring was reffering to).

But I have to say it has some drawbacks. Let me enumerate them:

1. Extending such a queries on runtime would be rather difficult IMHO, 
and will not look very nice. With my solution you can do something like 
this:
---
Column cols = [person.name, person.surname, person.nick];
for(i=0; i<rows; i++) {
   InsertStatement insert = Insert();
   foreach(Column col; cols) {
     insert ~= insert.insert(col, getText(i, col));
   }
   db.execute(insert);
}

* example not tested, but something like this is already possible in my 
framework.

Constructing queries dynamically is very useful for creating e.g. 
universal gui controls which could be just binded to database tables.

Another useful example is sending SQL expression (not the whole 
statement) as an argument to function.

To make it work nicely with mixins IMHO you will have to make this kind 
of SQL to look much uglier than in this simple case.

Do you have any proposals how it could look like with mixins?

2. I see also problem with IDE-s which will have to be changed to 
understand DLSs in mixins. I think that it is doable and even can be 
done quite nice, but currently there is *no* support for such a feature. 
It means that you will not have:

a. syntax highlighting
b. no IDE proposing your names of variables and methods
c. no refactoring

Currently in Eclipse I can rename database column names in my program 
with only single mouse click (Java version of my framework, which is the 
main branch currently). When SQLs will be in form of string mixins they 
will not change during refactoring. You will have to write special 
refactoring tools for your library to make it done.

3. Sometimes having more expressiveness in language can avoid creating 
new DSLs. I think it is a Good Thing (tm). In fact I took that way and 
decided to integrate SQL DSL language into mother language. That way 
programmers don't have to learn new syntax, but just use existing syntax 
of mother language to solve specific problems. Using DSL in mother 
language resolves then into just writing special kind of API, which can 
be used in standard way to achieve goal. D and most other languages from 
C family are almost perfect candidates to make something like this with SQL.

So while I can agree that original SQL looks much better than any kind 
of its imitation, I think that it is not necessary better for productivity.

Additionally I think that string mixins need some more features to make 
it possible to solve point 2. IDE will have to know what kind of DSL is 
in mixin. Maybe something like this:

mixin(sql.query, "SELECT ....");
mixin(sql.expression, "person.doctorid = doctor.id");
mixin(sql.statement, "INSERT ....");

...where sql.query, sql.expression, sql.statement are kind of markers 
usefull for IDE.

BR
Marcin Kuszczak
(aarti_pl)