On Jul 28, 2014, at 4:21 PM, Henry Lenzi <henry.le...@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 <n...@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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