eval/macro help needed.

From: Harvey J. Stein <hjstein_at_MATH.HUJI.AC.IL>
Date: Thu, 13 Apr 1995 16:54:33 +0300

I'm having a little trouble doing indirection through symbols and am
hoping someone on the list can help me.

The setting is I wrote a table-widget for STk. It basically consists
of a matrix of entry widgets, each of which is tied to an STk
variable. The table-widget manages a matrix of data. Scrolling is
accomplished by resetting all the variables to a different submatrix
of the data.

I was originally doing something like:

(for-each (lambda (x y) (eval `(set! ,x ,y)))
        variable-list
        value-list)

where variable-list was something like (|G11| |G12| ...) and
value-list is just the list of values to use.

The problem is that this is slow, and I traced it down to the (eval
`set! ,x ,y) being slow. I discovered that defining a macro for this
speeded things up by a factor of 10! But, the macro didn't work
inside of a function!

Here's a simple example of the problem I was having:

STk> (define foo 'a)
#[undefined]
STk> (define a 3)
#[undefined]
STk> a
3
STk> (eval `(set! ,foo 6))
#[undefined]
STk> foo
a
STk> a
6

Thus, if foo's value is a symbol, I can set the value of this symbol
(to 6 for example) with the expression:

   (eval `(set! ,foo 6))

However, this is slow. On the other hand, the following is much
faster (by about a factor of 10!):

   (define-macro (superset! a b) `(set! ,(eval a) ,b))

The above macro also works:

STk> foo
a
STk> a
6
STk> (define-macro (superset! a b) `(set! ,(eval a) ,b))
#[undefined]
STk> (superset! foo 19)
#[undefined]
STk> foo
a
STk> a
19


My problem is that the above doesn't work inside a function:

(define (bad-guy x y) (superset! x y))

STk> (define (bad-guy x y) (superset! x y))
#[undefined]
STk> foo
a
STk> a
19
STk> (bad-guy foo 3)

*** Error:
    unbound variable: x
Current eval stack:
__________________
  0 x
  1 (eval a)
  2 (quasiquote (set! (unquote (eval a)) (unquote b)))
  3 (%replace params (apply (lambda (a b) (quasiquote (set! (unquote (eval a)) (unquote b)))) (cdr params)))
  4 (superset! x y)


Anyone have any idea of how to do this?

Thanks,
Received on Thu Apr 13 1995 - 15:55:38 CEST