[postmodern-devel] Building s-sql on the fly

[Maciek Pasternacki]

Phil <zaries at global.co.za> writes:

>> > I am looking for an example of how to create a s-sql query on the fly. I
>> > am sure I saw something like that along the line on some website but I
>> > cant find it again.
>> 
>> Take a look at http://repo.or.cz/w/cl-trane.git?a=blob;f=src/taxonomy.lisp;h=5776b2b42dde277f9478c0df621ffa9742106c06;hb=HEAD
>> 
>> Query building takes place e.g. on lines 274-304 and 365-394.  Second
>> example may be interesting: it's a way to generate CASE statement from
>> list of values (it may be generalized to receive alists, I just needed
>> the order), to effectively make an inline-dictionary without writing
>> the CASE statement by hand.
>
> Firstly thanx for the example.
>
> Secondly, this might be a stupid question but being new to lisp I would
> have expected those functions to be macro's you are using the `. Could
> you please explain the reasoning behind them being functions.

Let's start from macros.  Macros are just functions that are called
after reading in the code (in macroexpansion time, to be exact), that
take literal parts of code as arguments, and return other piece of
code code in form of list as a return value.

Let's take trivial macro:

(defmacro add-two (form)
  `(+ 2 ,form))

(add-two (* 5 foo)) gets a list '(* 5 foo) as its argument, and
macroexpands to (is equivalent to) (+ 2 (* 5 foo)).

And macro definition is equivalent to building the resulting list by hand:

(defmacro add-two (form)
  (list 'plus 2 form))

(actually, backtick would be more efficient, as ` is able to reuse
some conses, but these are details now).

I do the other way around: often I do need to construct a list from
template, e.g. '(1 2 (4 3) <something>).  I could write (list 1 2
(list 4 3) something), but I can use backtick as well: `(1 2 (3 4)
,something).  Try to evaluate backtick-and-comma forms in REPL, play
around and see yourself how it works.  Backtick is "quasi-quote": it
quotes everything that is not preceded by comma, or comma-at (,@), and
it's most widely used in macros (since these are usually templates -
but don't actually need to be), but it can be used to build
complicated nested lists from templates inside functions as well.

Hope this helps,
regards,
Maciej.

-- 
Maciej Pasternacki -><- http://www.pasternacki.net/ -><- http://www.3ofcoins.net/