[Dbix-class] Explicit ASTs (ping nate)

[Darren Duncan]

At 5:31 PM -0700 9/2/06, David E. Wheeler wrote:
>On Sep 2, 2006, at 13:53, Matt S Trout wrote:
>
>>     DWIM AST -> explicit AST -> SQL
>
>Agreed. This way, the AST does not have to be specific to any one 
>back end (PostgreSQL, SQLite, MySQL, LDAP, a object iterator, etc.).
>
>This is also what I was thinking, as I've been trying to work out a 
>DWIM query mini language that's just Perl and lexes itself into an 
>AST that can then be passed to some other module to actually do
>something with.

 From my experience with working on Set::Relation and the Rosetta DBMS 
(and the now defunct SQL::Routine), if you *really* want to have an 
explicit AST that says exactly what you mean, is expressive enough 
for 99+% of any real-world uses, and is very portable, you 
essentially have to define a whole turing complete language, 
including the basics (which is what I am doing).

It also helps to, on one hand, abstract away any differences in the 
syntax for invoking built-in operators (eg, '=', 'AND', 'LIKE') and 
user-defined operators (stored functions / procedures), so that 
syntax-wise, user-defined operators are simply extensions to the 
language.  But at the same time, it is very useful to have the 
concept of name-spaces (loosely analagous to SQL schemas and/or 
packages).  Have a pre-defined namespace for built-ins, eg, System::, 
and another for user-defined functions, eg, User::, and use these 
everywhere in the AST.  Then almost all things in the AST are simply 
expressions involving function calls, each of which takes zero or 
more arguments and returns a value.  All parts of a SELECT statement 
can also be composed in this format.  I suggest making the arguments 
named as well, so they are better self-documenting.  Note that the 
names you use in the built-ins under System:: don't have to look the 
same as the SQL analogs, especially when different DBMSs aren't the 
same as each other in those regards.

You'll also want to explicitly define a type system, and you'll want 
at least the Boolean type.

I'll write more on this later if it is helpful.

-- Darren Duncan