[plt-scheme] Re: trying not to use eval - in a define-syntax

From: Richard C. Cobbe (cobbe at ccs.neu.edu)
Date: Wed Oct 30 08:30:27 EST 2002

Lo, on Tuesday, October 29, Jim Witte did write:

>   For list-related administrative tasks:
>   http://list.cs.brown.edu/mailman/listinfo/plt-scheme
> >> I want to take the value from a variable and use it as a procedure
> >> name,   and have it be evaluated.  I remember running across some
> >> advice in a text that using eval was *BAD*, but I am stumped on how
>    I want to make a 'foreach' syntax which would function like map, 
> except that it would (1) go in order (instead of arbitrary), (2) pass 
> in the reference for each element the function is currently processing, 
> and (3) be able to pass in an arbirary number of 'static' variables 
> that don't change from one invocation to the other:  So I could do 
> something like this silly example:

It's not clear to me why you need special support for part 3.  Why not
just use Scheme's support for lexical closures?

And, given that, I don't think you need any sort of syntactic support at 
all.  Why not just use a function?

Instead of

> (define add-index-and-constant
>    (lambda (el i a)
>      (+ el i a)))
> (foreach with-vars add-index-and-constant '(1 2 3 4) '(10))
> --> (10 13 15 17)

I'd probably write something like

(define add-index-and-constant
  (lambda (a)
    (lambda (elt idx)
      (+ elt idx a))))

(define map-lr/index
  (lambda (f lst)
    (let loop ((idx 0) (lst lst))
      (if (null? lst)
          ;; Force l->r eval order
          (let ([result (f (car lst) idx)])
            (cons result (loop (+ 1 idx) (cdr lst))))))))

(map-lr/index (add-index-and-constant 10) '(1 2 3 4))
 --> '(11 13 15 17)

No syntax involved.  (Making map-lr/index tail-recursive is left as an
exercise for the reader.)


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