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

I don’t think you even need to serialize it for that:
(define my-hash (make-hash))
(define out (open-output-file “dump.dat”))
(write my-hash out)
(close-output-port out)

… later
(define in (open-input-file “dump.dat”))
(define my-hash (read in))
(close-input-port in)

On Jul 28, 2014, at 11:59 PM, Roman Klochkov <kalimehtar at mail.ru> wrote:

> You may also export all hash to file and later read it.
> 
> http://docs.racket-lang.org/reference/serialization.html
> 
> (require racket/serialize)
> 
> (define my-hash (make-hash))
> 
> (define out (open-output-file "dump.dat"))
> (write (serialize my-hash) out)
> (close-output-port out)
> 
> ... later
> (define in (open-input-file "dump.dat"))
> (define my-hash (deserialize (read in)))
> (close-input-port in)
> 
> Mon, 28 Jul 2014 17:21:40 -0300 от Henry Lenzi <henry.lenzi at gmail.com>:
> 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 ("|")? 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.
> >>>
> >
> > ____________________
> > Racket Users list:
> > http://lists.racket-lang.org/users
> ____________________
>   Racket Users list:
>   http://lists.racket-lang.org/users
> 
> 
> -- 
> Roman Klochkov
> ____________________
>  Racket Users list:
>  http://lists.racket-lang.org/users

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