[racket] Use of map and eval to evaluate symbol in namespace

Thanks for your answers.

- Henry Lenzi

On Sun, Jul 27, 2014 at 2:36 AM, Matthew Butterick <mb at mbtype.com> wrote:
> The REPL automatically uses the current namespace, which is why you can
> casually use eval there without specifying a namespace. [1]
>
> But if you're using eval in the definitions window, you need to give it a
> namespace, otherwise you'll get "unbound identifier" errors.
>
> In this case, the namespace you want is the one you're already in. So rather
> than using (make-base-namespace), which will give you an new namespace [2],
> you need a namespace anchor. To make an anchor, put this in your
> definitions:
>
> (define-namespace-anchor a)
>
>
> And then to use the anchor, change your `map` to this:
>
> (map (λ(str) (eval str (namespace-anchor->namespace a)))
>     '((eval *med-name*) line *quant-str* pl) )
>
>
> And it works.
>
> PS I assume you have a reason for preferring eval to, say, a hash.
>
>
> [1]
> http://docs.racket-lang.org/guide/eval.html?q=namespaces#%28part._namespaces%29
>
> [2]
> http://docs.racket-lang.org/reference/Namespaces.html?q=make-base-namespace&q=namespaces#%28def._%28%28lib._racket%2Fprivate%2Fbase..rkt%29._make-base-namespace%29%29
>
>
>
> On Jul 26, 2014, at 8:10 PM, Henry Lenzi <henry.lenzi at gmail.com> wrote:
>
> #lang racket
>
> #|
>
> Dear Racketeers -
>
> I would like to create a little function
> that would write a line of a medical recipe.
>
> I would pass to "read" a shorthand form,
> and the program would expand the text. Like so:
>
> When I answer "hctz25" and "30", it would print:
> "Hydrochlorothiazide 25mg ----------- 30 pills".
>
> But I'm facing problems related to:
> - use of "map" with "eval" in the definition area (namespace issues...)
> See below.
> |#
>
>
> (require scheme/base) ; Is this necessary?
> (define this-namespace (make-base-namespace))
>
> (define hctz25 "Hydroclorothiazide 25mg")
> (define  line "-----------")
> (define pl "pills")
>
> (define *med-name* 'NIL) ; *med-name* has global scope - this is on purpose
> (define *med-name-str* "") ; *med-name-str* has global scope
>
> (define (ask-med-name)
>  (print "Medication") (newline)
>  (set! *med-name* (read))
>  (set! *med-name-str* (symbol->string *med-name*)))
>
> (define *quant* 0) (define *quant-str* " ")
>
> (define (ask-quant)
>  (print "Quantity") (newline)
>  (and
>   (set! *quant* (read)) (unless (number? *quant*) (error "It's not a
> number!")))
>  (set! *quant-str* (number->string *quant*)))
>
> (define (ask)
>  (ask-med-name)
>  (ask-quant))
> (ask)
>
> #|
> Where's the problem?
> In the REPL, I can do this:
>
> (map eval '((eval *med-name*) line *quant-str* pl))
>
> '("Hydrochlorothiazide 25mg" "-----------" "30" "pills")
>
> or this:
>
> (string-join (map eval '((eval *med-name*) line *quant-str* pl)))
>
>
> "Hydrochlorothiazide 25mg ----------- 30 pills"
>
>
> But, because of the namespace issues associated with "eval", I'm not
> able to do it in the definition area.
>
> I have no clue as to what the syntax should be.
>
> Thanks for any help.
>
> Henry Lenzi
>
> |#
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