Re: Using eval with environments and R7RS modules

[Damien MATTEI <damien.mattei@oca.eu>]

Le 02/12/2020 à 15:58, Jamison Hope a écrit :
> On Mon, Nov 30, 2020 at 1:09 PM Damien MATTEI <damien.mattei@oca.eu> wrote:
>
>> strange that the SICP cover book put the couple EVAL / APPLY as
>> important notion, APPLY seems more usefull,
>
> FWIW, I would point out that SICP is *not* a book on how to program in
> Scheme. It uses Scheme, yes, but much like the pseudo code in CLRS, it’s
> just as an easy-to-understand syntax for presenting concepts about
> programming. We did have to write and run some programs in 6.001 on actual
> computers for homework, but most of the work was done on paper or a
> chalkboard. (There were no computers in the room for the final exam.)
>
> The pair of EVAL/APPLY presented the two stages of *Interpreting* a
> Computer Program using the environment model: after parsing the Structure
> (which for Lisp just means turning text with parentheses into a list of
> atoms and lists — I don’t recall SICP mentioning vector literals, but it’s
> been a while), the two steps to interpret the program — to decide what the
> program’s result is — are to evaluate the car of the list to see what
> function it represents, then evaluate each function argument, and then to
> apply the function to its arguments (each of those EVAL steps might entail
> recursively EVAL/APPLYing an inner function application). There’s a little
> more to it to handle special forms like IF which don’t evaluate every
> argument first, but that’s the essence of the model.

i was just trying some sort of coding, for evaluating a postfix expression :

i started with recusively EVALuating all but unfortnately it worked only 
at TOPLEVEL

and finally 3 page of wrong code with EVAL where replaced by a single 
special form:

;; (define x 7)

;; (postfix ((2 (3 2 +) *) x +)) -> 17

;; (let ((x 7)) (postfix ((2 (3 2 +) *) x +))) -> 17
(define-syntax postfix (syntax-rules ()

                ((_ (a ... op)) ((postfix op) (postfix a) ...))
                ((_ a) a) ;; MUST be here not before to avoid ambigious call
                ;; misenterpreting a = (a ... op)

                ))

> This was really a mental exercise, not a description of how a real physical
> computer does or should work. The chapter on register machines and
> compilation addresses the fact that in reality we don’t need to use EVAL to
> look up each variable at runtime, because we can precompute where that
> variable resides in the stack based on where it was defined (see the
> section called “Lexical Addressing”).
>
>
>>
>> Damien
>>
>> -JH