scheme/dsssl question

[David Megginson <ak117@xxxxxxxxxxxxxxxxxxx>]

Norman Walsh writes:

 > Given that I've got
 > 
 > (define foo "this is some text")
 > 
 > in my stylesheet, is it possible to write a function that
 > will take "foo" (literally) as an argument and return 
 > "this is some text"?
 > 
 > For example:
 > 
 > (define (bar xyz) 
 >   ...something I can't quite wrap my head around in here...
 > )
 > 
 > such that
 > 
 > (bar "foo") returns "this is some text"?

No, there is not.  This would require the use of an 'eval' function
(requiring access to an interpreter), while DSSSL's
parent/sibling/cousin Scheme is designed to be compiled as well as
interpreted.

The key is to use the (assoc ...) function from the expression
language.  First, declare an association list of all the keys and
values that interest you (note the dotted pairs, a hard-core LISP
thing):

(define settings
  '(("foo" . "this is some text")
    ("hack" . "this is some more text")))

Next, declare a lookup function:

(define (lookup key)
  (let ((result (assoc key settings)))
    (if result
	(cdr result)
	#f)))

Now, you can use

  (lookup "foo")

with the expected results.

NOTE: Jade does not currently implement the (assoc ..) function, but
here's a scarcely-tested facsimile that you can use as a work-around:

(define (assoc key list)
  (cond ((null? list)
	 #f)
	((equal? (car (car list)) key)
	 (car list))
	(else
	 (assoc key (cdr list)))))


Good luck, and all the best,


David

-- 
David Megginson                 ak117@xxxxxxxxxxxxxxxxxxx
Microstar Software Ltd.         dmeggins@xxxxxxxxxxxxx
      http://home.sprynet.com/sprynet/dmeggins/


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