<html><head><meta http-equiv="Content-Type" content="text/html charset=windows-1252"></head><body style="word-wrap: break-word; -webkit-nbsp-mode: space; -webkit-line-break: after-white-space;"> <div><div>On Jul 29, 2014, at 11:42 PM, Henry Lenzi <<a href="mailto:henry.lenzi@gmail.com">henry.lenzi@gmail.com</a>> wrote:</div> <blockquote type="cite">(read in) doesn't work. This should be simple...I write <blockquote type="cite">(define out (open-output-file "Recipe.txt")) (print *med-name* out) </blockquote></blockquote>What you probably want here is write, not print. <blockquote type="cite"><blockquote type="cite">(close-output-port out) </blockquote></blockquote><div>Print is good for printing values like they could be written in a program, but it is not the opposite of read.</div><div>Write is the opposite of read. </div><div>In general, read usually puts an extra single quote in front of everything. </div><div>If you use print, then it will keep the quote, but if you use write then it will drop it. </div> <blockquote type="cite"> 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) </blockquote><div> </div><div>If the file contains this (no quotes in front of it): hctz25</div><div>Then using read-line on the file port will produce a string: "hctz25"</div><div>and using read on the port will produce a quoted symbol: 'hctz25</div><div>If the file contains this (with a quote in front of it): 'hctz25</div><div>Then using read-line on the file port will produce a string: "'hctz25"</div><div>and using read on the port will produce this (with two single quotes): ''hctz25</div><div>This: 'hctz25 is equivalent to this: (quote hctz25) and so</div><div>this: ''hctz25 is equivalent to this: '(quote hctz25) which is equivalent to (list 'quote 'hctz25)</div><div> </div><div>If you do this: (write '(define hctz25 "Hydrochlorothiazide 25mg") out), then the file will have this:</div><div>(define hctz25 "Hydrochlorothiazide 25mg")</div><div>And then if you use read on the file port, you get this back: '(define hctz25 "Hydrochlorothiazide 25mg")</div><div> </div><div> </div><blockquote type="cite"> 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 <<a href="mailto:alexander@knauth.org">alexander@knauth.org</a>> wrote: <blockquote type="cite"> On Jul 28, 2014, at 4:21 PM, Henry Lenzi <<a href="mailto:henry.lenzi@gmail.com">henry.lenzi@gmail.com</a>> 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)? <blockquote type="cite">(string->symbol "hctz25") </blockquote> 'hctz25 <blockquote type="cite">(string->symbol "'hctz25") </blockquote> '|'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 <a href="http://docs.racket-lang.org/reference/reader.html#%28part._parse-hashtable%29">http://docs.racket-lang.org/reference/reader.html#%28part._parse-hashtable%29</a> 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@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 </blockquote> ____________________ Racket Users list: <a href="http://lists.racket-lang.org/users">http://lists.racket-lang.org/users</a> </blockquote></div> </body></html>