On Thu, 2008-12-11 at 16:08 +0100, Maciek Pasternacki wrote:

Phil zaries@global.co.za writes:

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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=5776b2b42dde27...

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.

Regards, Maciej.

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.

On Thu, 2008-12-11 at 17:03 +0100, Maciek Pasternacki wrote:

Phil zaries@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=5776b2b42dde27...

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.

And all the behaviorists chorused AH_HAAAAA!!

Here is what how I understand this:

1. ` creates a list

2. Where the reader finds a , it evalutes what follows before the list is build, thus the result/value of what follows the , is passed into the list.

;Example not using a , CL-USER> `(1 2 3 4 (format nil "5")) (1 2 3 4 (FORMAT NIL "5"))

;Example using a , CL-USER> `(1 2 3 4 ,(format nil "5")) (1 2 3 4 "5")

;The following is the equavilant of the preceding but it uses list and not the shorthand ` CL-USER> (list 1 3 4 (format nil "5")) (1 3 4 "5")

3. The difference between using ` in a macro and a function is the timing of when the list is build and thus the time that stuff after a , is evaluated.

In a macro the list is build at compile time and the evaluation of stuff after a , is also done at compile time.

CL-USER> (defmacro test-macro () `(+ 1 2 3 4 ,(parse-integer "5"))) TEST-MACRO

;Simulate at compile time CL-USER> (macroexpand '(test-macro)) (+ 1 2 3 4 5) T ;When run CL-USER> (test-macro) 15