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

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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