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  • Write Scheme data structures so they can be eval-d back in, or alternative

    - by Jesse Millikan
    I'm writing an application (A juggling pattern animator) in PLT Scheme that accepts Scheme expressions as values for some fields. I'm attempting to write a small text editor that will let me "explode" expressions into expressions that can still be eval'd but contain the data as literals for manual tweaking. For example, (4hss->sexp "747") is a function call that generates a legitimate pattern. If I eval and print that, it becomes (((7 3) - - -) (- - (4 2) -) (- (7 2) - -) (- - - (7 1)) ((4 0) - - -) (- - (7 0) -) (- (7 2) - -) (- - - (4 3)) ((7 3) - - -) (- - (7 0) -) (- (4 1) - -) (- - - (7 1))) which can be "read" as a string, but will not "eval" the same as the function. For this statement, of course, what I need would be as simple as (quote (((7 3... but other examples are non-trivial. This one, for example, contains structs which print as vectors: pair-of-jugglers ; --> (#(struct:hand #(struct:position -0.35 2.0 1.0) #(struct:position -0.6 2.05 1.1) 1.832595714594046) #(struct:hand #(struct:position 0.35 2.0 1.0) #(struct:position 0.6 2.0500000000000003 1.1) 1.308996938995747) #(struct:hand #(struct:position 0.35 -2.0 1.0) #(struct:position 0.6 -2.05 1.1) -1.3089969389957472) #(struct:hand #(struct:position -0.35 -2.0 1.0) #(struct:position -0.6 -2.05 1.1) -1.8325957145940461)) I've thought of at least three possible solutions, none of which I like very much. Solution A is to write a recursive eval-able output function myself for a reasonably large subset of the values that I might be using. There (probably...) won't be any circular references by the nature of the data structures used, so that wouldn't be such a long job. The output would end up looking like `(((3 0) (... ; ex 1 `(,(make-hand (make-position ... ; ex 2 Or even worse if I could't figure out how to do it properly with quasiquoting. Solution B would be to write out everything as (read (open-input-string "(big-long-s-expression)")) which, technically, solves the problem I'm bringing up but is... ugly. Solution C might be a different approach of giving up eval and using only read for parsing input, or an uglier approach where the s-expression is used as directly data if eval fails, but those both seem unpleasant compared to using scheme values directly. Undiscovered Solution D would be a PLT Scheme option, function or library I haven't located that would match Solution A. Help me out before I start having bad recursion dreams again.

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  • How to bind "rest" variables to list of values in macro in Scheme

    - by Slartibartfast
    I want to make a helper macro for writing match-extensions. I have something like this: (define-match-expander my-expander (? (stx) (let* ([dat (cdr (syntax-e stx))] [var1 (car dat))] [var2 (cadr dat)]) ;transformer goes here ))) So I wanted a macro that will do this let binding. I've started with something like this: (define-syntax-rule (define-my-expander (id vars ...) body) (define-match-expander id (? (stx) (match-let ([(vars ...) (cdr (syntax-e stx))]) body)))) but match-let isn't defined in transformation time. First question would be is there any other way of doing this (making this expanders, I mean)? Maybe there is already something similar in plt-scheme that I'm not aware of, or I'm doing it wrong in some way. Regardless of answer on the first question, if I ever want to bound list of variables to list of values inside of a macro, how should I do it? EDIT: In combination with Eli's answer macro now looks like this: (define-syntax-rule (define-my-expander (id vars ...) body) (define-match-expander id (? (stx) (syntax-case stx () [(_ vars ...) body]))))

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  • Does Scheme work with Microsoft COM?

    - by Martin
    I'm new to Scheme -- the functional programming language and I like it a lot for its first-class/higher-order functions. However, my data comes from a COM source with an object-oriented API. I know Scheme and COM belong to different programming paradigms, but I'm wondering if there is any interface or a way for Scheme to connect to a COM source? Thanks.

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  • Scheme: what are the benefits of letrec?

    - by Ixmatus
    While reading "The Seasoned Schemer" I've begun to learn about letrec. I understand what it does (can be duplicated with a Y-Combinator) but the book is using it in lieu of recurring on the already defined function operating on arguments that remain static. An example of an old function using the defined function recurring on itself (nothing special): (define (substitute new old lat) (cond ((null? l) '()) ((eq? (car l) old) (cons new (substitute new old (cdr l)))) (else (cons (car l) (substitute new old (cdr l)))))) Now for an example of that same function but using letrec: (define (substitute new old lat) (letrec ((replace (lambda (l) (cond ((null? l) '()) ((eq? (car l) old) (cons new (replace (cdr l)))) (else (cons (car l) (replace (cdr l)))))))) (replace lat))) Aside from being slightly longer and more difficult to read I don't know why they are rewriting functions in the book to use letrec. Is there a speed enhancement when recurring over a static variable this way because you don't keep passing it?? Is this standard practice for functions with arguments that remain static but one argument that is reduced (such as recurring down the elements of a list)? Some input from more experienced Schemers/LISPers would help!

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  • Fetch elements from List in scheme

    - by fireball003
    Hi, How to go through a list or fetch element from a list in scheme? How can I name each element (like we do for variables in java) in a list? Thanks in advance. I want to compare every point in a list to another point. So, as we do in java or python- for(int i;i<list.size();i++){ if (list[i]> k){ //do something } } How can I do similar thing in scheme?

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  • Backtracking infinite loop

    - by Greenhorn
    This is Exercise 28.1.2 from HtDP. I've successfully implemented the neighbors function and all test cases pass. (define Graph (list (list 'A (list 'B 'E)) (list 'B (list 'E 'F)) (list 'C (list 'D)) (list 'D empty) (list 'E (list 'C 'F)) (list 'F (list 'D 'G)) (list 'G empty))) (define (first-line n alist) (cond [(symbol=? (first alist) n) alist] [else empty])) ;; returns empty if node is not in graph (define (neighbors n g) (cond [(empty? g) empty] [(cons? (first g)) (cond [(symbol=? (first (first g)) n) (first-line n (first g))] [else (neighbors n (rest g))])])) ; test cases (equal? (neighbors 'A Graph) (list 'A (list 'B 'E))) (equal? (neighbors 'B Graph) (list 'B (list 'E 'F))) (equal? (neighbors 'C Graph) (list 'C (list 'D))) (equal? (neighbors 'D Graph) (list 'D empty)) (equal? (neighbors 'E Graph) (list 'E (list 'C 'F))) (equal? (neighbors 'F Graph) (list 'F (list 'D 'G))) (equal? (neighbors 'G Graph) (list 'G empty)) (equal? (neighbors 'H Graph) empty) The problem comes when I copy-paste the code from Figure 77 of the text. It is supposed to determine whether a destination node is reachable from an origin node. However it appears that the code goes into an infinite loop except for the most trivial case where the origin and destination nodes are the same. ;; find-route : node node graph -> (listof node) or false ;; to create a path from origination to destination in G ;; if there is no path, the function produces false (define (find-route origination destination G) (cond [(symbol=? origination destination) (list destination)] [else (local ((define possible-route (find-route/list (neighbors origination G) destination G))) (cond [(boolean? possible-route) false] [else (cons origination possible-route)]))])) ;; find-route/list : (listof node) node graph -> (listof node) or false ;; to create a path from some node on lo-Os to D ;; if there is no path, the function produces false (define (find-route/list lo-Os D G) (cond [(empty? lo-Os) false] [else (local ((define possible-route (find-route (first lo-Os) D G))) (cond [(boolean? possible-route) (find-route/list (rest lo-Os) D G)] [else possible-route]))])) Does the problem lie in my code? Thank you.

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  • Why do all procedures have to be defined before the compiler sees them?

    - by incrediman
    For example, take a look at this code (from tspl4): (define proc1 (lambda (x y) (proc2 y x))) If I run this as my program in scheme... #!r6rs (import (rnrs)) (define proc1 (lambda (x y) (proc2 y x))) I get this error: expand: unbound identifier in module in: proc2 ...This code works fine though: #!r6rs (import (rnrs)) (define proc2 +) (define proc1 (lambda (x y) (proc2 y x))) (display (proc1 2 3)) ;output: 5

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  • How do I write Push and Pop in Scheme?

    - by kunjaan
    Right now I have (define (push x a-list) (set! a-list (cons a-list x))) (define (pop a-list) (let ((result (first a-list))) (set! a-list (rest a-list)) result)) But I get this result: Welcome to DrScheme, version 4.2 [3m]. Language: Module; memory limit: 256 megabytes. > (define my-list (list 1 2 3)) > (push 4 my-list) > my-list (1 2 3) > (pop my-list) 1 > my-list (1 2 3) What am I doing wrong? Is there a better way to write push so that the element is added at the end and pop so that the element gets deleted from the first?

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  • Writing an auto-memoizer in Scheme. Help with macro and a wrapper.

    - by kunjaan
    I am facing a couple of problems while writing an auto-memoizer in Scheme. I have a working memoizer function, which creats a hash table and checks if the value is already computed. If it has been computed before then it returns the value else it calls the function. (define (memoizer fun) (let ((a-table (make-hash))) (?(n) (define false-if-fail (?() #f)) (let ((return-val (hash-ref a-table n false-if-fail))) (if return-val return-val (begin (hash-set! a-table n (fun n)) (hash-ref a-table n))))))) Now I want to create a memoize-wrapper function like this: (define (memoize-wrapper function) (set! function (memoizer function))) And hopefully create a macro called def-memo which defines the function with the memoize-wrapper. eg. the macro could expand to (memoizer (define function-name arguments body ...) or something like that. So that I should be able to do : (def-memo (factorial n) (cond ((= n 1) 1) (else (* n (factorial (- n 1)))))) which should create a memoized version of the factorial instead of the normal slow one. My problem is that the The memoize-wrapper is not working properly, it doesnt call the memoized function but the original function. I have no idea how to write a define inside of the macro. How do I make sure that I can get variable lenght arguments and variable length body? How do I then define the function and wrap it around with the memoizer? Thanks a lot.

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  • How do you perform arithmetic calculations on symbols in Scheme/Lisp?

    - by kunjaan
    I need to perform calculations with a symbol. I need to convert the time which is of hh:mm form to the minutes passed. ;; (get-minutes symbol)->number ;; convert the time in hh:mm to minutes ;; (get-minutes 6:19)-> 6* 60 + 19 (define (get-minutes time) (let* ((a-time (string->list (symbol->string time))) (hour (first a-time)) (minutes (third a-time))) (+ (* hour 60) minutes))) This is an incorrect code, I get a character after all that conversion and cannot perform a correct calculation. Do you guys have any suggestions? I cant change the input type. Context: The input is a flight schedule so I cannot alter the data structure. ;; ---------------------------------------------------------------------- Edit: Figured out an ugly solution. Please suggest something better. (define (get-minutes time) (let* ((a-time (symbol->string time)) (hour (string->number (substring a-time 0 1))) (minutes (string->number (substring a-time 2 4)))) (+ (* hour 60) minutes)))

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  • Scheme Homework Assignment

    - by user1704677
    In a course I am taking we recently had to learn the programming language Scheme. I get all of the basics, which is pretty much all that we have gone though. I'm just having trouble learning to think in the different way that Scheme consists of. I was given an assignment and really do not even know how to start. I have sat here for a few hours trying to figure out how to even get started, but I'm kind of stumped. For the record, I'm not asking for the code to solve this problem, but more of some thoughts to get me on the right track. Anyway, here is the gist of the assignment... We are given a list of ten numbers that represent a voter's votes. The numbers are -1, 0 or 1. Then we are given a list of lists of Candidates, with a name and then ten numbers corresponding to that candidate's votes. These numbers are also -1 0 and 1. So for example. '(0 0 0 -1 -1 1 0 1 0 -1) '(Adams 0 1 -1 0 1 1 0 -1 -1 0 0) We are asked to implement a function called best_candidates that will take in a list of numbers (Voter) and a list of lists of Candidates. Then we have to compare the votes of the voter against the list of each candidate and return a list of names with the most common votes. So far, I've come up with a few things. I'm just confused on how I will check the values and retain the name of the voter? I guess I'm still stuck in thinking C/Java and it's making this very tough. Any suggestions to help get me started?

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  • Writing out to a file in scheme

    - by Ceelos
    The goal of this is to check if the character taken into account is a number or operand and then output it into a list which will be written out to a txt file. I'm wondering which process would be more efficient, whether to do it as I stated above (writing it to a list and then writing that list out into a file) or being writing out into a txt file right from the procedure. I'm new with scheme so I apologize if I am not using the correct terminology (define input '("3" "+" "4")) (define check (if (number? (car input)) (write this out to a list or directly to a file) (check the rest of file))) Another question I had in mind, how can I make it so that the check process is recursive? I know it's a lot of asking but I've getting a little frustrated with checking out the methods that I have found on other sites. I really appreciate the help!

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  • How I install drRacket?

    - by aseed
    i install drRacket in ubuntu. downloaded the source file from http://pre.racket-lang.org/installers/ and typed sudo ./racket-5.2.1-bin-i386-linux-ubuntu-karmic.sh after making it exe file so the file is in the usr/racket/bin/drracket and i want to run it from command line without going to its directory i cant make the drracket to run the .rkt files by the "open with" how can i make this two things ???

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  • C++11 support for higher-order list functions

    - by Giorgio
    Most functional programming languages (e.g. Common Lisp, Scheme / Racket, Clojure, Haskell, Scala, Ocaml, SML) support some common higher-order functions on lists, such as map, filter, takeWhile, dropWhile, foldl, foldr (see e.g. Common Lisp, Scheme / Racket, Clojure side-by-side reference sheet, the Haskell, Scala, OCaml, and the SML documentation.) Does C++11 have equivalent standard methods or functions on lists? For example, consider the following Haskell snippet: let xs = [1, 2, 3, 4, 5] let ys = map (\x -> x * x) xs How can I express the second expression in modern standard C++? std::list<int> xs = ... // Initialize the list in some way. std::list<int> ys = ??? // How to translate the Haskell expression? What about the other higher-order functions mentioned above? Can they be directly expressed in C++?

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  • need help with Java solution /newbie

    - by Racket
    Hi, I'm new to programming in general so i'm trying to be as specific as possible in this question. There's this book that i'm doing some exercises on. I managed to do more than half of what they say, but it's just one input that I have been struggling to find out. I'll write the question and thereafter my code, "Write an application that creates and prints a random phone number of the form XXX-XXX-XXXX. Include the dashes in the output. Do not let the first three digits contain an 8 or 9 (but don't be more restrictive than that), and make sure that the second set of three digits is not greater than 742. Hint: Think through the easiest way to construct the phone number. Each diigit does not have to be determined separately." OK, the highlighted sentence is what i'm looking at. Here's my code: import java.util.Random; public class PP33 { public static void main (String[] args) { Random rand = new Random(); int num1, num2, num3; num1 = rand.nextInt(900) + 100; num2 = rand.nextInt(643) + 100; num3 = rand.nextInt(9000) + 1000; System.out.println(num1+"-"+num2+"-"+num3); } } How am I suppose to do this? I'm on chapter 3 so we have not yet discussed if statements etcetera, but Aliases, String class, Packages, Import declaration, Random Class, Math Class, Formatting output (decimal- & numberFormat), Printf, Enumeration & Wrapper classes + autoboxing. So consider answer the question based only on these assumptions, please. The code doesn't have any errors. Thank you!

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  • need help with some basic java.

    - by Racket
    Hi, I'm doing the first chapter exercises on my Java book and I have been stuck for a problem for a while now. I'll print the question, prompt/read a double value representing a monetary amount. Then determine the fewest number of each bill and coin needed to represent that amount, starting with the highest (assume that a ten dollar bill is the maximum size needed). For example, if the value entered is 47,63 (forty-seven dollars and sixty-three cents), and the program should print the equivalent amount as: 4 ten dollar bills 1 five dollar bills 2 one dollar bills 2 quarters 1 dimes 0 nickels 3 pennies" etc. I'm doing an example exactly as they said in order to get an idea, as you will see in the code. Nevertheless, I managed to print 4 dollars, and I can't figure out how to get "1 five dollar", only 7 dollars (see code). Please, don't do the whole code for me. I just need some advice in regards to what I said. Thank you. import java.util.Scanner; public class PP29 { public static void main (String[] args) { Scanner sc = new Scanner (System.in); int amount; double value; double test1; double quarter; System.out.println("Enter \"double\" value: "); value = sc.nextDouble(); amount = (int) value / 10; // 47,63 / 10 = 4. int amount2 = (int) value % 10; // 47 - 40 = 7 quarter = value * 100; // 47,63 * 100 = 4736 int sum = (int) quarter % 100; // 4763 / 100 => 4763-4700 = 63. System.out.println(amount); System.out.println(amount2); } }

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  • Types in Lisp and Scheme

    - by user2054900
    I see now that Racket has types. At first glance it seems to be almost identical to Haskell typing. But is Lisp's CLOS covering some of the space Haskell types cover? Creating a very strict Haskell type and an object in any OO language seems vaguely similar. It's just that I've drunk some of the Haskell kool-aid and I'm totally paranoid that if I go down the Lisp road, I'll be screwed due to dynamic typing.

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  • Slow Motion Egg Destruction [Video]

    - by Jason Fitzpatrick
    We’ve said it before and we’ll say it again: everything is better in slow motion. In this instance it’s twenty two eggs made interesting by meeting their demise in a variety of ways. Of all the egg smashes in the video, we’re particularly fond of the tennis racket segment. Have a cool slow-mo video to share? Throw a link in the comments! Slo-As-A-Mofo-Sho – Egg Destruction [via Boing Boing] How To Delete, Move, or Rename Locked Files in Windows HTG Explains: Why Screen Savers Are No Longer Necessary 6 Ways Windows 8 Is More Secure Than Windows 7

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  • DrRacket icon doesnt work?

    - by Laurie
    I installed DrRacket from the Ubuntu Software Center. All went well and an icon appeared however nothing happened when I clicked the icon so I removed it. Then went to the Developer website and downloaded full-5.3.0.21-bin-x86_64-linux-debian-squeeze.sh. I installed this via Terminal with sudo apt-get install racket. The DrRacket icon came back in Dash Home but again clicking it nothing appears to happen. How do I start DrRacket? I am running Ubuntu 12.04 LTS dual boot on a 64 bit Dell

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  • Programming languages with extensible syntax

    - by Giorgio
    I have only a limited knowledge of Lisp (trying to learn a bit in my free time) but as far as I understand Lisp macros allow to introduce new language constructs and syntax by describing them in Lisp itself. This means that a new construct can be added as a library, without changing the Lisp compiler / interpreter. This approach is very different from that of other programming languages. E.g., if I wanted to extend Pascal with a new kind of loop or some particular idiom I would have to extend the syntax and semantics of the language and then implement that new feature in the compiler. Are there other programming languages outside the Lisp family (i.e. apart from Common Lisp, Scheme, Clojure (?), Racket (?), etc) that offer a similar possibility to extend the language within the language itself?

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