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

(read in) doesn't work.

This should be simple...I write
> (define out (open-output-file "Recipe.txt"))
> (print *med-name* out)
> (close-output-port out)

and what I see in the file is:
'hctz25

How can I import this from a file?
If I use (read in), I get:

''hctz25

(two quotes)

The issue here was that there was a definition
(define hctz25 "Hydrochlorothiazide 25mg)

and hctz25 thus evaluates to a string.

How can I get it back from a file, retaining the capacity to evaluate
it to its definition. That's sort of the issue.

TIA
- Henry




On Mon, Jul 28, 2014 at 6:05 PM, Alexander D. Knauth
<alexander at knauth.org> wrote:
>
> On Jul 28, 2014, at 4:21 PM, Henry Lenzi <henry.lenzi at gmail.com> wrote:
>
> Hi Neil --
>
> So how do you export hash keys as symbols to a file that can be read
> in again, not as string?
>
> Now, I haven't gotten around to reading the whole of Racket Scheme's
> documentation... Things are looking kind of hard.
>
> What I'm attempting to do is then read back the symbols defined, such
> as the one below:
>
> (define hctz25 "Hydrochlorothiazide 25mg")
>
> (close-input-port in)
> (define in (open-input-file "Recipe.txt"))
> (string->symbol (read-line in))
>
> '|'hctz25|
>
> But what I really want is the "hctz25" symbol that evaluates to a
> string. If I don't use string->symbol, I get the string "hctz25". And
> why the bars ("|”)?
>
>
> As an explanation of the bars:
> If you use read-line, does it return the string "hctz25", or the string
> "'hctz25" (with the quote as the first character)?
>  > (string->symbol "hctz25")
>  'hctz25
>  > (string->symbol "'hctz25")
>  '|'hctz25|
>
> I think you’d want to use read for that though, not read-line.
>
> You might be able to use something a bit like this:
> (define hash (make-hash))
> (match (read in)
>   [`(define ,sym ,str) (hash-set! hash sym str)])
> (hash-ref hash 'hctz25)
>
> Or something like this:
> (define hash
>   (for/hash ([def (in-port read in)])
>     (match-define `(define ,sym ,str) def)
>     (values sym str)))
> (hash-ref hash 'hctz25)
>
> I've read
>
> http://docs.racket-lang.org/reference/reader.html#%28part._parse-hashtable%29
>
> but it didn't help me much.
>
>
> Of course, the ultimate purpose would be to re-evaluate the imported
> symbol and reconstruct a medical recipe. The purpose of these
> baby-steps exercises is porting a medical recipe program I've written
> originally in Forth that allowed me to service 5.000 patients creating
> a little database of shorthand recipes that then expand into real
> medical recipes. I got hundreds of patients on renewable recipes for,
> say, hypertension. Hand writing is no fun. Typing them in Word is no
> fun. The hospital has is its own software, but it's is a load of
> baloney, extremely buggy, if you ask me, so I'm rolling my own again,
> except I want to print directly on the model paper our service uses,
> so I want graphics like Racket Scheme has (very good capabilities, as
> far as my needs are concerned).
>
> With Forth, it's very easy to design DSLs, because there's no syntax
> and you get a lot of advanced features for free. For instance, there's
> no need to write a parser for my little language. However, since Forth
> implementations fall short of dealing with images, graphics (unless
> you take the royal road to pain and learn to program for the Win32 API
> and how it works for a particular Forth vendor), I'm looking at Racket
> Scheme.
>
> TIA,
>
> Henry Lenzi
>
>
>
>
>
>
>
>
> On Mon, Jul 28, 2014 at 4:46 PM, Neil Van Dyke <neil at neilvandyke.org> wrote:
>
> I don't know the current state of the "eval" docs in the manual, but I think
> they should have a big warning at the very front, intended to scare away
> newbies.
>
> Remember that Racket is often used in conjunction with many different
> Scheme-based and other Lisp-based textbooks and courses. It seems that many
> CS instructors and textbook authors like to talk about ``EVAL'' (as an
> abstract operation) when talking about some models of evaluation, and "eval"
> (as an accessible language binding) to say, gosh, aren't dynamic languages
> interesting and powerful. So, we can't blame every fourth newbie for trying
> to use "eval" unnecessarily, in ways that make for bad software engineering.
>
> Given this reality of confusing instruction, I'm thinking that, as a
> reactive measure, "#lang paddle" will disable "eval" by default. Attempting
> to use "eval" will give you an error message, unless you have an assertion
> form like
> "(i-have-read-the-foo-document-and-understand-that-eval-is-usually-the-wrong-thing-but-honest-i-know-what-i-am-doing)".
>
> Cheers,
> Neil V.
>
> Vincent St-Amour wrote at 07/28/2014 02:21 PM:
>
> Maybe this should be linked to from the `eval' docs?
>
>
> http://blog.racket-lang.org/2011/10/on-eval-in-dynamic-languages-generally.html
>
> Vincent
>
>
> At Sun, 27 Jul 2014 16:16:52 -0400,
> Neil Van Dyke wrote:
>
>
> Maybe there should be a periodic public service announcement about not
> using "eval".  This time I will communicate in FAQ format:
>
> Q: How do I use eval?
> A: Don't use eval.
>
> Q: But don't so many academic books feature eval prominently, so doesn't
> that mean I should use try to eval?
> A: Those books use eval for pedagogic reasons, or because the author is
> enamored of some theoretical appeal of eval, or because the author wants
> to watch the world burn.  Don't use eval.
>
> Q: But, but, but, I am just starting to learn, and eval seems to do what
> I need.
> A: Eval is almost certainly not what you want.  Learn how to use the
> other basics effectively.  Don't use eval.
>
> Q: I now am very comfortable with the language, I am aware that I should
> avoid eval in almost all cases, and I can tell you why eval is actually
> the right thing in this highly unusual case.
> A: Cool, that's why eval is there.
>
> Neil V.
>
>
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