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

Hello again-

I suppose this doesn't look like a case for eval when you first look into
it, but since I would like to export the shorthand symbols to a file, I
just might need eval.

- Henry.

Em domingo, 27 de julho de 2014, Pierpaolo Bernardi <olopierpa at gmail.com>
escreveu:

> On Sun, Jul 27, 2014 at 5:10 AM, Henry Lenzi <henry.lenzi at gmail.com
> <javascript:;>> 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.
>
> This is not a case where eval is useful (very few of such cases exist).
>
> Here's a hint:
>
> #lang racket
>
> (define abbreviations
>   (hash 'hctz25 "Hydroclorothiazide 25mg"
>         ;; more abbreviations here
>         ))
>
> (define (print-a-line med quant)
>   (printf "~a ---- ~a pills~n"
>        (hash-ref abbreviations med "uh?")
>        quant))
>
> then:
>
> > (print-a-line 'hctz25 30)
> Hydroclorothiazide 25mg ---- 30 pills
>
>
> I hope this will get you started
>
> P.
>
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