[plt-scheme] Programatically Constructed Symbol Used As An Identifier???

Thanks for nice explanation of top-level-forms , example2 seems to clear it.

> In short: 1) you rarely want to use `eval' for yourself, 2) you even more
> rarely want to recommend it as a solution to someone else's problem,
> especially if you don't fully understand it, and 3) you never want to
> recommend it to a novice.

Got the point , i will be careful next time :)

Veer


On Sat, Jun 27, 2009 at 6:54 PM, Dave Herman<dherman at ccs.neu.edu> wrote:
>> Your's and jos example is correct way to do what OP asked and
>> I would have used the similar method as your's. I just observed that
>> using eval also seems to work  :)  .
>
> The rule of thumb is: Don't use `eval' unless you really know what you're
> doing. And even then, you quite likely don't know what you're doing. :)
>
>> I really don't know what top-level-form is , can you give an example
>> of top-level-forms and not top-level-forms. I assume it is list of
>> definitions
>> or simply all forms which are not nested ,right or wrong?
>
> Ordinarily, you don't need to know the difference between top-level
> variables and lexically nested variables, but `eval' is not able to "see"
> lexically nested variables, so it's one place where you do need to know the
> difference. Roughly, a top-level binding is one that's defined at the
> outermost scope of a module body or at the outermost scope of the
> interactions window.
>
> Here are some examples that demonstrate Grant's point:
>
> *** EXAMPLE 1 ***
>
> ;; definitions window
> #lang scheme
> (define (adder x y) ;; at top-level
>  (+ x y))
>
> (define (example1)
>  ((eval 'adder) 3 4))
>
> ;; interactions window
>> (example1)
> 7
>
>
> *** EXAMPLE 2 ***
>
> ;; definitions window
> #lang scheme
> (define (example2)
>  (define (adder x y) ;; not at top-level
>    (+ x y))
>  ((eval 'adder) 3 4))
>
> ;; interactions window
>> (example2)
> reference to an identifier before its definition: adder
>
>
> *** EXAMPLE 3 ***
>
> ;; definitions window
> #lang scheme
> (define (example3)
>  (define (adder x y) ;; not at top-level
>    (+ x y))
>  ((eval 'adder) 3 4))
>
> ;; interactions window
>> (example3)
> reference to an identifier before its definition: adder
>> (define (adder x y) ;; at top-level
>    (* x y))
>> (example3)
> 12
>
> In short: 1) you rarely want to use `eval' for yourself, 2) you even more
> rarely want to recommend it as a solution to someone else's problem,
> especially if you don't fully understand it, and 3) you never want to
> recommend it to a novice.
>
> Dave
>
>