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  • How do I make a module in PLT Scheme?

    - by kunjaan
    I tried doing this: #lang scheme (module duck scheme/base (provide num-eggs quack) (define num-eggs 2) (define (quack n) (unless (zero? n) (printf "quack\n") (quack (sub1 n))))) But I get this error: module: illegal use (not at top-level) in: (module duck scheme/base (provide num-eggs quack) (define num-eggs 2) (define (quack n) (unless (zero? n) (printf "quack\n") (quack (sub1 n))))) what is the correct way?

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  • What are the most interesting equivalences arising from the Curry-Howard Isomorphism?

    - by Tom
    I came upon the Curry-Howard Isomorphism relatively late in my programming life, and perhaps this contributes to my being utterly fascinated by it. It implies that for every programming concept there exists a precise analogue in formal logic, and vice versa. Here's an "obvious" list of such analogies, off the top of my head: program/definition | proof type/declaration | proposition inhabited type | theorem function | implication function argument | hypothesis/antecedent function result | conclusion/consequent function application | modus ponens recursion | induction identity function | tautology non-terminating function | absurdity tuple | conjunction (and) disjoint union | exclusive disjunction (xor) parametric polymorphism | universal quantification So, to my question: what are some of the more interesting/obscure implications of this isomorphism? I'm no logician so I'm sure I've only scratched the surface with this list. For example, here are some programming notions for which I'm unaware of pithy names in logic: currying | "((a & b) => c) iff (a => (b => c))" scope | "known theory + hypotheses" And here are some logical concepts which I haven't quite pinned down in programming terms: primitive type? | axiom set of valid programs? | theory ? | disjunction (or)

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  • Bubble sort algorithm implementations (Haskell vs. C)

    - by kingping
    Hello. I have written 2 implementation of bubble sort algorithm in C and Haskell. Haskell implementation: module Main where main = do contents <- readFile "./data" print "Data loaded. Sorting.." let newcontents = bubblesort contents writeFile "./data_new_ghc" newcontents print "Sorting done" bubblesort list = sort list [] False rev = reverse -- separated. To see rev2 = reverse -- who calls the routine sort (x1:x2:xs) acc _ | x1 > x2 = sort (x1:xs) (x2:acc) True sort (x1:xs) acc flag = sort xs (x1:acc) flag sort [] acc True = sort (rev acc) [] False sort _ acc _ = rev2 acc I've compared these two implementations having run both on file with size of 20 KiB. C implementation took about a second, Haskell — about 1 min 10 sec. I have also profiled the Haskell application: Compile for profiling: C:\Temp ghc -prof -auto-all -O --make Main Profile: C:\Temp Main.exe +RTS -p and got these results. This is a pseudocode of the algorithm: procedure bubbleSort( A : list of sortable items ) defined as: do swapped := false for each i in 0 to length(A) - 2 inclusive do: if A[i] > A[i+1] then swap( A[i], A[i+1] ) swapped := true end if end for while swapped end procedure I wonder if it's possible to make Haskell implementation work faster without changing the algorithm (there's are actually a few tricks to make it work faster, but neither implementations have these optimizations)

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  • Why is the Clojure Hello World program so slow compared to Java and Python?

    - by viksit
    Hi all, I'm reading "Programming Clojure" and I was comparing some languages I use for some simple code. I noticed that the clojure implementations were the slowest in each case. For instance, Python - hello.py def hello_world(name): print "Hello, %s" % name hello_world("world") and result, $ time python hello.py Hello, world real 0m0.027s user 0m0.013s sys 0m0.014s Java - hello.java import java.io.*; public class hello { public static void hello_world(String name) { System.out.println("Hello, " + name); } public static void main(String[] args) { hello_world("world"); } } and result, $ time java hello Hello, world real 0m0.324s user 0m0.296s sys 0m0.065s and finally, Clojure - hellofun.clj (defn hello-world [username] (println (format "Hello, %s" username))) (hello-world "world") and results, $ time clj hellofun.clj Hello, world real 0m1.418s user 0m1.649s sys 0m0.154s Thats a whole, garangutan 1.4 seconds! Does anyone have pointers on what the cause of this could be? Is Clojure really that slow, or are there JVM tricks et al that need to be used in order to speed up execution? More importantly - isn't this huge difference in performance going to be an issue at some point? (I mean, lets say I was using Clojure for a production system - the gain I get in using lisp seems completely offset by the performance issues I can see here). The machine used here is a 2007 Macbook Pro running Snow Leopard, a 2.16Ghz Intel C2D and 2G DDR2 SDRAM. BTW, the clj script I'm using is from here and looks like, #!/bin/bash JAVA=/System/Library/Frameworks/JavaVM.framework/Versions/1.6/Home/bin/java CLJ_DIR=/opt/jars CLOJURE=$CLJ_DIR/clojure.jar CONTRIB=$CLJ_DIR/clojure-contrib.jar JLINE=$CLJ_DIR/jline-0.9.94.jar CP=$PWD:$CLOJURE:$JLINE:$CONTRIB # Add extra jars as specified by `.clojure` file if [ -f .clojure ] then CP=$CP:`cat .clojure` fi if [ -z "$1" ]; then $JAVA -server -cp $CP \ jline.ConsoleRunner clojure.lang.Repl else scriptname=$1 $JAVA -server -cp $CP clojure.main $scriptname -- $* fi

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  • Practical Scheme Programming

    - by Ixmatus
    It's been a few months since I've touched Scheme and decided to implement a command line income partitioner using Scheme. My initial implementation used plain recursion over the continuation, but I figured a continuation would be more appropriate to this type of program. I would appreciate it if anyone (more skilled with Scheme than I) could take a look at this and suggest improvements. I'm that the multiple (display... lines is an ideal opportunity to use a macro as well (I just haven't gotten to macros yet). (define (ab-income) (call/cc (lambda (cc) (let ((out (display "Income: ")) (income (string->number (read-line)))) (cond ((<= income 600) (display (format "Please enter an amount greater than $600.00~n~n")) (cc (ab-income))) (else (let ((bills (* (/ 30 100) income)) (taxes (* (/ 20 100) income)) (savings (* (/ 10 100) income)) (checking (* (/ 40 100) income))) (display (format "~nDeduct for bills:---------------------- $~a~n" (real->decimal-string bills 2))) (display (format "Deduct for taxes:---------------------- $~a~n" (real->decimal-string taxes 2))) (display (format "Deduct for savings:-------------------- $~a~n" (real->decimal-string savings 2))) (display (format "Remainder for checking:---------------- $~a~n" (real->decimal-string checking 2)))))))))) Invoking (ab-income) asks for input and if anything below 600 is provided it (from my understanding) returns (ab-income) at the current-continuation. My first implementation (as I said earlier) used plain-jane recursion. It wasn't bad at all either but I figured every return call to (ab-income) if the value was below 600 kept expanding the function. (please correct me if that apprehension is incorrect!)

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  • How do I profile in DrScheme?

    - by kunjaan
    How Do I profile my functions using DrScheme? (require profile) (define (factorial n) (cond ((= n 1) 1) (else (* n (factorial (- n 1)))))) (profile factorial) The above code returns Profiling results ----------------- Total cpu time observed: 0ms (out of 0ms) Number of samples taken: 0 (once every 0ms) ==================================== Caller Idx Total Self Name+srcLocal% ms(pct) ms(pct) Callee ==================================== > I tried: - (profile (factorial 100)) - (profile factorial) (factorial 100) But it gives me the same result. What am I doing wrong?

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  • Generics and Constrained Polymorphism versus Subtyping

    - by Rahul G
    Hullo all. In this (Warning: PDF) presentation on Haskell Type Classes, on slide #54, there's this question: Open Question: In a language with generics and constrained polymorphism, do you need subtyping too? My questions are: How do generics and constrained polymorphism make subtyping unnecessary? If generics and constrained polymorphism make subtyping unnecessary, why does Scala have subtyping?

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  • State Monad, why not a tuple?

    - by thr
    I've just wrapped my head around monads (at least I'd like to think I have) and more specifically the state monad, which some people that are way smarter then me figured out, so I'm probably way of with this question. Anyway, the state monad is usually implemented with a M<'a as something like this (F#): type State<'a, 'state> = State of ('state -> 'a * 'state) Now my question: Is there any reason why you couldn't use a tuple here? Other then the possible ambiguity between MonadA<'a, 'b> and MonadB<'a, 'b> which would both become the equivalent ('a * 'b) tuple. Edit: Added example for clarity type StateMonad() = member m.Return a = (fun s -> a, s) member m.Bind(x, f) = (fun s -> let a, s_ = x s in f a s_) let state = new StateMonad() let getState = (fun s -> s, s) let setState s = (fun _ -> (), s) let execute m s = m s |> fst

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  • Automate Testing on future only items business rules

    - by Titan
    I currently have a business object with a validation business rule, which is it can only be created for the future, tomorrow onwards, and I cannot create new items for today. I have a process, which runs the non-future business objects through some steps.. Because I have to set things up today, and test tomorrow, and when it fails, I can only create a new object tomorrow and test the following day. Are there any easy ways to automate this process in any testing frameworks? I think our testers are using the visual studio 2010 test manager. How do you guys manage situations like this? Cheers

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  • Confusion regarding laziness

    - by Abhinav Kaushik
    I have a function myLength = foldl (\ x _ -> x + 1) 0 which fails with stack overflow with input around 10^6 elements (myLength [1..1000000] fails). I believe that is due to the thunk build up since when I replace foldl with foldl', it works. So far so good. But now I have another function to reverse a list : myReverse = foldl (\ acc x -> x : acc) [] which uses the lazy version foldl (instead of foldl') When I do myLength . myReverse $ [1.1000000]. This time it works fine. I fail to understand why foldl works for the later case and not for former?

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  • seperating interface and implemention with normal functions

    - by ace
    this seems like it should be pretty simple, im probably leaving something simple out. this is the code im trying to run. it is 3 files, 2*cpp and 1*header. -------------lab6.h ifndef LAB6_H_INCLUDED define LAB6_H_INCLUDED int const arraySize = 10; int array1[arraySize]; int array2[arraySize]; void generateArray(int[], int ); void displayArray(int[], int[], int ); void reverseOrder(int [],int [], int); endif // LAB6_H_INCLUDED -----------------lab6.cpp include using std::cout; using std::endl; include using std::rand; using std::srand; include using std::time; include using std::setw; include "lab6.h" void generateArray(int array1[], int arraySize) { srand(time(0)); for (int i=0; i<10; i++) { array1[i]=(rand()%10); } } void displayArray(int array1[], int array2[], int arraySize) { cout<<endl<<"Array 1"<<endl; for (int i=0; i<arraySize; i++) { cout<<array1[i]<<", "; } cout<<endl<<"Array 2"<<endl; for (int i=0; i<arraySize; i++) { cout<<array2[i]<<", "; } } void reverseOrder(int array1[],int array2[], int arraySize) { for (int i=0, j=arraySize-1; i<arraySize;j--, i++) { array2[j] = array1[i]; } } ------------and finally main.cpp include "lab6.h" int main() { generateArray(array1, arraySize); reverseOrder(array1, array2, arraySize); displayArray(array1, array2, arraySize); return 0; }

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  • Pattern Matching in Scheme

    - by kunjaan
    How do I accept the following input? (list of 0 or more charcters and ends with 3) or (list of 1 or more characters 4 and 0 or more characters after 4) something like (match ( list 3)) -> #t (match ( list 1 2 3)) -> #t (match (list 1 2 3 4)) -> #t (match (list 1 2 3 4 5)) -> #t (match (list 4)) -> #f EDIT: THIS IS NOT MY HOMEWORK. I trying to write something like ELIZA from PAIP but I know only how to write a pattern that begins with a word.

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  • Are there any tools for performing static analysis of Scala code?

    - by Roman Kagan
    Are there any tools for performing static analysis of Scala code, similar to FindBugs and PMD for Java or Splint for C/C++? I know that FindBugs works on the bytecode produced by compiling Java, so I'm curious as to how it would work on Scala. Google searches (as of 27 October 2009) reveal very little. Google searches (as of 01 February 2010) reveal this question.

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  • Naming Suggestions For A Function Providing Method Chaining In A Different Way

    - by sid3k
    I've coded an experimental function which makes passed objects chainable by using high order functions. It's name is "chain" for now, and here is a usage example; chain("Hello World") (print) // evaluates print function by passing "Hello World" object. (console.log,"Optional","Parameters") (returnfrom) // returns "Hello World" It looks lispy but behaves very different since it's coded in a C based language, I don't know if there is a name for this idiom and I couldn't any name more suitable than "chain". Any ideas, suggestions?

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  • F# exercise help - Beginner

    - by bobjink
    I hope you can help me with these exercises I have been stuck with. I am not in particular looking for the answers. Tips that help me solve them myself are just as good :) 1. Write a function implode : char list - string so that implode s returns the characters concatenated into a string: let implode (cArray:char list) = List.foldBack (+) 0 cArray;; Error. I am thinking that i need to cast every char to strings before I do the above. I bet however, that there is a much simpler and better solution. 2. Write a function palindrome : string - bool, so that palindrome s returns true if the string s is a palindrome let isIdentical cArray1 cArray2 = (cArray1 = cArray2);; let palinDrome (word:string) = isIdentical(word.ToCharArray(), Array.rev(word.ToCharArray()));; I get the value: val palinDrome : string - (char [] * char [] - bool), which is not what I want. 3. Write a function combinePair : int list - int list so that combinePair xs returns the list with elements from xs combined into pairs. If xs contains an odd number of elements, then the last element is thrown away: I have no idea for this one.

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  • Explaining persistent data structures in simple terms

    - by Jason Baker
    I'm working on a library for Python that implements some persistent data structures (mainly as a learning exercise). However, I'm beginning to learn that explaining persistent data structures to people unfamiliar with them can be difficult. Can someone help me think of an easy (or at least the least complicated) way to describe persistent data structures to them? I've had a couple of people tell me that the documentation that I have is somewhat confusing. (And before anyone asks, no I don't mean persistent data structures as in persisted to the file system. Google persistent data structures if you're unclear on this.)

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  • Lamda functions in php

    - by Oden
    Hey, Im really interested in the way of using lamda functions. Does it make sense to use them in a high-level programming language? If yes, why? Is this really just a function embedded in a function, (Like this) or is there more behind?

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  • How can I use functools.partial on multiple methods on an object, and freeze parameters out of order

    - by Joseph Garvin
    I find functools.partial to be extremely useful, but I would like to be able to freeze arguments out of order (the argument you want to freeze is not always the first one) and I'd like to be able to apply it to several methods on a class at once, to make a proxy object that has the same methods as the underlying object except with some of its methods parameter being frozen (think of it as generalizing partial to apply to classes). I've managed to scrap together a version of functools.partial called 'bind' that lets me specify parameters out of order by passing them by keyword argument. That part works: >>> def foo(x, y): ... print x, y ... >>> bar = bind(foo, y=3) >>> bar(2) 2 3 But my proxy class does not work, and I'm not sure why: >>> class Foo(object): ... def bar(self, x, y): ... print x, y ... >>> a = Foo() >>> b = PureProxy(a, bar=bind(Foo.bar, y=3)) >>> b.bar(2) Traceback (most recent call last): File "<stdin>", line 1, in <module> TypeError: bar() takes exactly 3 arguments (2 given) I'm probably doing this all sorts of wrong because I'm just going by what I've pieced together from random documentation, blogs, and running dir() on all the pieces. Suggestions both on how to make this work and better ways to implement it would be appreciated ;) One detail I'm unsure about is how this should all interact with descriptors. Code follows. from types import MethodType class PureProxy(object): def __init__(self, underlying, **substitutions): self.underlying = underlying for name in substitutions: subst_attr = substitutions[name] if hasattr(subst_attr, "underlying"): setattr(self, name, MethodType(subst_attr, self, PureProxy)) def __getattribute__(self, name): return getattr(object.__getattribute__(self, "underlying"), name) def bind(f, *args, **kwargs): """ Lets you freeze arguments of a function be certain values. Unlike functools.partial, you can freeze arguments by name, which has the bonus of letting you freeze them out of order. args will be treated just like partial, but kwargs will properly take into account if you are specifying a regular argument by name. """ argspec = inspect.getargspec(f) argdict = copy(kwargs) if hasattr(f, "im_func"): f = f.im_func args_idx = 0 for arg in argspec.args: if args_idx >= len(args): break argdict[arg] = args[args_idx] args_idx += 1 num_plugged = args_idx def new_func(*inner_args, **inner_kwargs): args_idx = 0 for arg in argspec.args[num_plugged:]: if arg in argdict: continue if args_idx >= len(inner_args): # We can't raise an error here because some remaining arguments # may have been passed in by keyword. break argdict[arg] = inner_args[args_idx] args_idx += 1 f(**dict(argdict, **inner_kwargs)) new_func.underlying = f return new_func

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  • Examples of useful or non-trival dual interfaces

    - by Scott Weinstein
    Recently Erik Meijer and others have show how IObservable/IObserver is the dual of IEnumerable/IEnumerator. The fact that they are dual means that any operation on one interface is valid on the other, thus providing a theoretical foundation for the Reactive Extentions for .Net Do other dual interfaces exist? I'm interested in any example, not just .Net based.

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  • Learning Lisp - Why ?

    - by David
    I really feel that I should learn Lisp and there are plenty of good resources out there to help me do it. I'm not put off by the complicated syntax, but where in "traditional commercial programming" would I find places it would make sense to use it instead of a procedural language. Is there a commercial killer-app out there that's been written in Lisp ?

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  • Ruby on Rails testing: How can I test or at the very least see a form_for's error_messages_for?

    - by williamjones
    I'm working on creating a tests, and I can't figure out why the creation of a model from a form_for is failing in the test but works in real browsers. Is there a straightforward way for me to see what the problems are in the model creation? Even better would be, is there a straightforward way for me to test the error outputs that I access via error_messages_for? In that case, I'd like to also add in tests that make sure that malformed forms are outputting the correct errors.

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