Hi David, Thank you for your reply. Yes, I undestood that about macros. I'm trying to dynamically generate the variable definitions inside a let block from an alist, but the only way I manage to do that is if I type the alist manually. I want to achieve that with an alist stored in a variable. Simplified example:
> \version "2.19.80" > #(define-macro (custom-let alist . body) > `(let ,(map (lambda(pair) (list (car pair) (cdr pair))) alist) > . ,body)) > > #(custom-let ((my-key . "my-val") (another-key . "another-val")) > (pretty-print my-key) > (pretty-print another-key))%Outputs: > %"my-val" > %"another-val" > #(define my-alist '((my-key . "my-val") (another-key . "another-val"))) > > #(custom-let my-alist > (pretty-print my-key) > (pretty-print another-key))%No applicable method for #<<extended-generic> > map (0)> in call (map #<procedure #f (pair)> my-alist) > > This is why I wanted to be able to evaluate the contents of the variable inside the body of the macro. I thought of manually evaluating the argument, but if I understand correctly it is not possible to do it in the correct scope (only globally and not locally)? If so, then do you have a suggestion to solve the problem showed above? 2018-03-28 16:23 GMT-03:00 David Kastrup <d...@gnu.org>: > Stefano Troncaro <stefanotronc...@gmail.com> writes: > > > Hi everyone! > > > > I have a question about the following example: > > > >> \version "2.19.80" > >> #(define-macro (why-the-difference obj) > >> (display (format "~a , " obj)) > >> `(display (format "~a\n" ,obj))) > >> > >> #(why-the-difference (list 1 2 3))% => (list 1 2 3) , (1 2 3) > >> #(define var (list 1 2 3)) > >> > >> #(why-the-difference var)% => var , (1 2 3) > >> > >> So, I assume that the difference is because the macro has access to what > > is typed, and uses that to produce an expression that is later evaluated. > > So, in the first example, (list 1 2 3) was typed, so that's shown in the > > output before the comma. While in the second example, var was typed, so > the > > symbol var is shown instead. > > > > Is there a way to evaluate the symbol inside the body of the macro? > > Not reliably. It's a rather loosely defined point of time. > > > I tried the following to no avail: > > > >> \version "2.19.80" > >> #(use-modules (ice-9 r5rs)) > >> > >> #(define-macro (my-attempt obj) > >> (display (format "~a , " (eval 'obj (interaction-environment)))) > >> `(display (format "~a\n" ,obj))) > > Well, that's completely wrong. (interaction-environment) is a _global_ > environment. It does not have access to local variables/symbols like > obj. So you'd want to write > > (eval obj (interaction-environment)) if at all, and even then this will > only work for global variables. > > >> #(define var (list 1 2 3)) > >> > >> #(my-attempt var) > >> > >> This generates the error 'unbound variable: obj'. I don't understand why > > obj is not considered defined, when if I use obj I get the symbol var (as > > the first snippet showed). Anyways, I *can* evaluate var: > > > >> \version "2.19.80" > >> #(use-modules (ice-9 r5rs)) > >> > >> #(define-macro (my-attempt obj) > >> (display (format "~a , " (eval 'var (interaction-environment)))) > >> `(display (format "~a\n" ,obj))) > >> > >> #(define var (list 1 2 3)) > >> > >> #(my-attempt var)% => (1 2 3) , (1 2 3) > >> > >> Which makes sense. This achieves what I want but it is not useful > because > > I need to know the name of the variable before-hand, so it will not work > > dynamically. > > > > Does anyone know of a way around this? > > Apparently you don't understand what a macro does. A macro receives its > arguments _unevaluated_, and the result is later evaluated in the > closure where the macro is called. (eval obj (interaction-environment)) > is exactly equivalent to (eval 'var (interaction-environment)) here. > > -- > David Kastrup >
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