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• Describe the Damas-Milner type inference in a way that a CS101 student can understand

  - by user128807

  Hindley-Milner is a type system that is the basis of the type systems of many well known functional programming languages. Damas-Milner is an algorithm that infers (deduces?) types in a Hindley-Milner type system. Wikipedia gives a description of the algorithm which, as far as I can tell, amounts to a single word: "unification." Is that all there is to it? If so, that means that the interesting part is the type system itself not the type inference system. If Damas-Milner is more than unification, I would like a description of Damas-Milner that includes a simple example and, ideally, some code. Also, this algorithm is often said to do type inference. Is it really an inference system? I thought it was only deducing the types. Related questions: What is Hindley Miller? Type inference to unification problem

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• Producer/consumer in Grails?

  - by Mulone

  Hi all, I'm trying to implement a Consumer/Producer app in Grails, after several unsuccessful attempts at implementing concurrent threads. Basically I want to store all the events coming from a clients (through separate AJAX calls) in a single queue and then process such a queue in a linear way as soon as new events are added. This looks like a Producer/Consumer problem: http://en.wikipedia.org/wiki/Producer-consumer_problem How can I implement this in Grails (maybe with a timer or even better by generating an event 'process queue')? Basically I'd like to a have a singleton service waiting for new events in the queue and processing them linearly (even if the queue is loaded by several concurrent processes). Any hints? Cheers!

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• How do I override Currency symbols in Java?

  - by edgydruid

  I'm trying to print prices in Turkish Liras (ISO 4217 currency code TRY) with Java. When I do Currency curr = Currency.getInstance("TRY"); Locale trLocale = new Locale("tr", "TR"); System.out.println(curr.getSymbol(trLocale)); the output is: "YTL". However, the currency symbol for Turkish Lira has recently changed from "YTL" to "TL" (as can be seen on the Wikipedia page for Turkish Lira). Formatting with NumberFormat gives a similar result. I really don't want to write yet another Currency class, especially when Java has one built-in. Is there a way to override Java's default currency symbol for TRY to "TL"?

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• How to calculate order (big O) for more complex algorithms (ie quicksort)

  - by bangoker

  I know there are quite a bunch of questions about big O notation, I have already checked Plain english explanation of Big O , Big O, how do you calculate/approximate it?, and Big O Notation Homework--Code Fragment Algorithm Analysis?, to name a few. I know by "intuition" how to calculate it for n, n^2, n! and so, however I am completely lost on how to calculate it for algorithms that are log n , n log n, n log log n and so. What I mean is, I know that Quick Sort is n log n (on average).. but, why? Same thing for merge/comb, etc. Could anybody explain me in a not to math-y way how do you calculate this? The main reason is that Im about to have a big interview and I'm pretty sure they'll ask for this kind of stuff. I have researched for a few days now, and everybody seem to have either an explanation of why bubble sort is n^2 or the (for me) unreadable explanation a la wikipedia Thanks!

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• Design a GUI browser to view a tree

  - by iamrohitbanga

  I have a large tree. I want to be able to visualize it using a GUI tool. I want the ability to pan and zoom the tree image so that i can focus on part of the tree. Is there an existing tool to achieve this? If not i would like to write a small tool for myself to be able to do this. what is the simplest way of doing this? what computer language should i use? the image should look something like http://upload.wikimedia.org/wikipedia/commons/d/df/Binary_tree.png I should be able to zoom and pan the image.

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• Amarok on Ubuntu 9.10 rapidly skipping tracks

  - by danwoods

  Hello all. Subject really says it all. When I try to click on any track, Amarok rapidly moves through the playlist, pulling external information (wikipedia etc) for each song, but not playing it. I feel like it might be a problem with my sound card or driver or something, but all other media players work fine. I can play sounds from within Amarok when I test different sound devices (they all play). I removed the program and re-installed it and still no luck. Any ideas?

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• What's the future of std::valarray look like?

  - by andand

  Up until fairly recently I hadn't been keeping up with the C++0x deliberations. As I try to become more familiar with it the issues being worked, I've come across sites like this which seems to be advocating for deprecating or removing std::valarray since most people are using Blitz++ in any event. I guess I'm probably one of the few people out there who uses std::valarray (and yes I know the class has a sordid past, a tarnished present, and a questionable future). For me they work, and perhaps more important, they're part of the standard (for now any way). Aside from the one site above, I've been able to find very little on what is actually happening with std::valarray in the new standard, and was hoping that somebody on SO might be able to provide some insight and / or references where Google, Wikipedia and even the C++ Standards Committee Web Site have so far failed me. Thanks.

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• Does the traditional use of the controller in MVC lead to a violation of the Single Responsibility P

  - by Byron Sommardahl

  Wikipedia describes the Single Responsibility Principle this way: The Single Responsibility Principle states that every object should have a single responsibility, and that responsibility should be entirely encapsulated by the class. All its services should be narrowly aligned with that responsibility. The traditional use of the controller in MVC seems to lead a programmer towards a violation of this principle. Take a simple guest book controller and view. The controller might have two methods/actions: 1) Index() and 2) Submit(). The Index() displays the form. The Submit() processes it. Do these two methods represent two distinct responsibilities? If so, how does Single Responsibility come in to play?

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• WSDL for other protocols different than SOAP?

  - by SDReyes

  From wikipedia: The Web Services Description Language (WSDL, pronounced 'wiz-d?l') is an XML-based language that provides a model for describing Web services. But in most cases, I found that WSDL is used in conjunction with SOAP. but as far as I know, if the WSDL is a language to describe web services. there should be possible to describe another kind of web services different from SOAP. is this possible? any examples? Thanks!

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• Import and Export for CSV are both broken in Mathematica

  - by dreeves

  Consider the following 2 by 2 array: x = {{"a b c", "1,2,3"}, {"i \"comma-heart\" you", "i \",heart\" u, too"}} If we Export that to CSV and then Import it again we don't get the same thing back: Import[Export["tmp.csv", d]] Looking at tmp.csv it's clear that the Export didn't work, since the quotes are not escaped properly. According to the RFC which I presume is summarized correctly on Wikipedia's entry on CSV, the right way to export the above array is as follows: a b c, "1,2,3" "i ""heart"" you", "i "",heart"" u, too" Importing the above does not yield the original array either. So Import is broken as well. I've reported these bugs to [email protected] but I'm wondering if others have workarounds in the meantime. One workaround is to just use TSV instead of CSV. I tested the above with TSV and it seems to work (even with tabs embedded in the entries of the array).

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• webui for mercurial (hg)

  - by Extreme

  i'm looking for a good webui tool for mercurial. i've found this link Mercurial with WebUI but what i'm looking for is something that has the ability to do check-in and check-out per file like the PVCS. any suggestions? thanks Edit: Here's the link for PVCS. http://en.wikipedia.org/wiki/PVCS It's been around 4 years since I last used PVCS and a lot has probably changed but here's as far as I remember. It has a web application wherein a developer can check out a single file. If this file is checked-out, it will be locked to this developer and no other developer can check it out. Then can download the source but they won't be able to check it in unless the original developer (that checked-out the file) checks it in.

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• Static libraries, dynamic libraries, DLLs, entry points, headers ... how to get out of this alive?

  - by tunnuz

  Hello, I recently had to program C++ under Windows for an University project, and I'm pretty confused about static and dynamic libraries system, what the compiler needs, what the linker needs, how to build a library ... is there any good document about this out there? I'm pretty confused about the *nix library system as well (so, dylibs, the ar tool, how to compile them ...), can you point a review document about the current library techniques on the various architectures? Note: due to my poor knowledge this message could contain wrong concepts, feel free to edit it. Thank you Feel free to add more reference, I will add them to the summary. References Since most of you posted *nix or Windows specific references I will summarize here the best ones, I will mark as accepted answer the Wikipedia one, because is a good start point (and has references inside too) to get introduced to this stuff. Program Library Howto (Unix) Dynamic-Link Libraries (from MSDN) (Windows) DLL Information (StackOverflow) (Windows) Programming in C (Unix) An Overview of Compiling and Linking (Windows)

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• Is it worth it to learn an esoteric programming language?

  - by Thomas Owens

  Wikipedia: An esoteric programming language (sometimes shortened to esolang) is a programming language designed as a test of the boundaries of computer programming language design, as a proof of concept, or as a joke. There is usually no intention of the language being adopted for real-world programming. Such languages are often popular among hackers and hobbyists. This use of esoteric is meant to distinguish these languages from more popular programming languages. Some more popular languages may appear esoteric (in the usual sense of the word) to some, and though these could arguably be called "esoteric programming languages" too, this is not what is meant. I think it might be worth it, just to learn a new language and go through the process, although only if you don't have anything else to do (like a real project or learning a new real language). But what does the community think? Is there some value in these languages?

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• How to split a space separated file?

  - by simplesimon

  Hi I am trying to import this: http://en.wikipedia.org/wiki/List_of_countries_by_continent_%28data_file%29 which is of the format like: AS AF AFG 004 Afghanistan, Islamic Republic of EU AX ALA 248 Åland Islands EU AL ALB 008 Albania, Republic of AF DZ DZA 012 Algeria, People's Democratic Republic of OC AS ASM 016 American Samoa EU AD AND 020 Andorra, Principality of AF AO AGO 024 Angola, Republic of NA AI AIA 660 Anguilla if i do <? explode(" ",$data"); ?> that works fine apart from countries with more than 1 word. how can i split it so i get the first 4 bits of data (the chars/ints) and the 5th bit of data being whatever remains? this is in php thank you

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• using jquery to disable a series of radio buttons on demand.

  - by holden

  I'm trying to accomplish something similar to Wikipedia's History history page, dynamically disabling radio buttons in a series. Ie... if #4 in group two is selected, then 1-4 of group one are disabled, etc. I know how to disable them individually or as a group, but I'm not sure how to do it in a series of say 1-4: Individually: $("#version_history input[id^=versions_2_3]:radio").attr('disabled',true); or group: $("#version_history input[id^=versions_2]:radio").attr('disabled',true); The inputs are named versions_1_X and versions_2_X, X being some number. 1..2..3.. etc.

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• HowTo make a marching ants border in Morphic?

  - by Helene Bilbo

  I am looking for a marching ants border or line in Morphic: Wikipedia describes a possbile algorithm: The easiest way to achieve this animation is by drawing the selection using a pen pattern that contains diagonal lines. If the selection outline is only one pixel thick, the slices out of the pattern will then look like a dashed line, and the animation can easily be achieved by simply shifting the pattern one pixel sideways and redrawing the outline. As there is probably none readymade, has anybody a hint for me how to create such a border or line? (The marching ants pattern as a GIF animation is licensed under CC by Uli Kusterer)

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• Recursive vs. Iterative algorithms

  - by teehoo

  I'm implementing the Euclidian algorithm for finding the GCD (Greatest Common Divisor) of two integers. Two sample implementations are given: Recursive and Iterative. http://en.wikipedia.org/wiki/Euclidean_algorithm#Implementations My Question: In school I remember my professors talking about recursive functions like they were all the rage, but I have one doubt. Compared to an iterative version don't recursive algorithms take up more stack space and therefore much more memory? Also, because calling a function requires uses some overhead for initialization, aren't recursive algorithms more slower than their iterative counterpart?

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• Why do C compilers prepend underscores to external names?

  - by Michael Burr

  I've been working in C for so long that the fact that compilers typically add an underscore to the start of an extern is just understood... However, another SO question today got me wondering about the real reason why the underscore is added. A wikipedia article claims that a reason is: It was common practice for C compilers to prepend a leading underscore to all external scope program identifiers to avert clashes with contributions from runtime language support I think there's at least a kernel of truth to this, but also it seems to no really answer the question, since if the underscore is added to all externs it won't help much with preventing clashes. Does anyone have good information on the rationale for the leading underscore? Is the added underscore part of the reason that the Unix creat() system call doesn't end with an 'e'? I've heard that early linkers on some platforms had a limit of 6 characters for names. If that's the case, then prepending an underscore to external names would seem to be a downright crazy idea (now I only have 5 characters to play with...).

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• Real world example of Unification in First Order Logic?

  - by Sebi

  I know this is only part of a programming question, but at the moment, I'm doing a little bit of logic programming. One thing I still don't understand correctly is Unification in First Order Logic. I read the Wikipedia article and it is more or less clear that the purpose is searching a term that unifies two sentences... There are also examples in this article but I just don't get the point why this should be useful. Can anyone give an example with real world objects instead of A, B, C,, etc.? I hope this will help me to understand. Thanks

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• PostScript versus PDF as an output format

  - by Brecht Machiels

  I'm currently writing a typesetting application and I'm using PSG as the backend for producing postscript files. I'm now wondering whether that choice makes sense. It seems the ReportLab Toolkit offers all the features PSG offers, and more. ReportLab outputs PDF however. Advantages PDF offers: transparancy better support for character encodings (Unicode, for example) ability to embed TrueType and even OpenType fonts hyperlinks and bookmarks Is there any reason to use Postscript instead of directly outputting to PDF? While Postscript is a full programming language as opposed to PDF, as a basic output format for documents, that doesn't seem to offer any advantage. I assume a PDF can be readily converted to PostScript for printing? Some useful links: Wikipedia: PDF Adobe: PostScript vs. PDF

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• Trouble with copying dictionaries and using deepcopy on an SQLAlchemy ORM object

  - by Az

  Hi there, I'm doing a Simulated Annealing algorithm to optimise a given allocation of students and projects. This is language-agnostic pseudocode from Wikipedia: s ? s0; e ? E(s) // Initial state, energy. sbest ? s; ebest ? e // Initial "best" solution k ? 0 // Energy evaluation count. while k < kmax and e > emax // While time left & not good enough: snew ? neighbour(s) // Pick some neighbour. enew ? E(snew) // Compute its energy. if enew < ebest then // Is this a new best? sbest ? snew; ebest ? enew // Save 'new neighbour' to 'best found'. if P(e, enew, temp(k/kmax)) > random() then // Should we move to it? s ? snew; e ? enew // Yes, change state. k ? k + 1 // One more evaluation done return sbest // Return the best solution found. The following is an adaptation of the technique. My supervisor said the idea is fine in theory. First I pick up some allocation (i.e. an entire dictionary of students and their allocated projects, including the ranks for the projects) from entire set of randomised allocations, copy it and pass it to my function. Let's call this allocation aOld (it is a dictionary). aOld has a weight related to it called wOld. The weighting is described below. The function does the following: Let this allocation, aOld be the best_node From all the students, pick a random number of students and stick in a list Strip (DEALLOCATE) them of their projects ++ reflect the changes for projects (allocated parameter is now False) and lecturers (free up slots if one or more of their projects are no longer allocated) Randomise that list Try assigning (REALLOCATE) everyone in that list projects again Calculate the weight (add up ranks, rank 1 = 1, rank 2 = 2... and no project rank = 101) For this new allocation aNew, if the weight wNew is smaller than the allocation weight wOld I picked up at the beginning, then this is the best_node (as defined by the Simulated Annealing algorithm above). Apply the algorithm to aNew and continue. If wOld < wNew, then apply the algorithm to aOld again and continue. The allocations/data-points are expressed as "nodes" such that a node = (weight, allocation_dict, projects_dict, lecturers_dict) Right now, I can only perform this algorithm once, but I'll need to try for a number N (denoted by kmax in the Wikipedia snippet) and make sure I always have with me, the previous node and the best_node. So that I don't modify my original dictionaries (which I might want to reset to), I've done a shallow copy of the dictionaries. From what I've read in the docs, it seems that it only copies the references and since my dictionaries contain objects, changing the copied dictionary ends up changing the objects anyway. So I tried to use copy.deepcopy().These dictionaries refer to objects that have been mapped with SQLA. Questions: I've been given some solutions to the problems faced but due to my über green-ness with using Python, they all sound rather cryptic to me. Deepcopy isn't playing nicely with SQLA. I've been told thatdeepcopy on ORM objects probably has issues that prevent it from working as you'd expect. Apparently I'd be better off "building copy constructors, i.e. def copy(self): return FooBar(....)." Can someone please explain what that means? I checked and found out that deepcopy has issues because SQLAlchemy places extra information on your objects, i.e. an _sa_instance_state attribute, that I wouldn't want in the copy but is necessary for the object to have. I've been told: "There are ways to manually blow away the old _sa_instance_state and put a new one on the object, but the most straightforward is to make a new object with __init__() and set up the attributes that are significant, instead of doing a full deep copy." What exactly does that mean? Do I create a new, unmapped class similar to the old, mapped one? An alternate solution is that I'd have to "implement __deepcopy__() on your objects and ensure that a new _sa_instance_state is set up, there are functions in sqlalchemy.orm.attributes which can help with that." Once again this is beyond me so could someone kindly explain what it means? A more general question: given the above information are there any suggestions on how I can maintain the information/state for the best_node (which must always persist through my while loop) and the previous_node, if my actual objects (referenced by the dictionaries, therefore the nodes) are changing due to the deallocation/reallocation taking place? That is, without using copy?

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• The unmentioned parts of COBOL's history

  - by be nice to me.

  I'm very curious about old programming languages, especially COBOL, and as Wikipedia couldn't really tell me much about this topic, I decided to ask it here: Was COBOL the first programming language really being used in financial, stock and banking systems? Where exactly was COBOL used? Was it used more frequently than Fortran or BASIC, for example? I don't know if you lived at that time, but how did people react to the rising COBOL? Did they expect it to be the future? When has COBOL actually stopped being used to create new, big systems? Are you sure that there are still important legacy apps written in COBOL out there? I can't believe that somehow.

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• How to choose the right web application framework?

  - by thenextwebguy

  http://en.wikipedia.org/wiki/Comparison_of_web_application_frameworks Since we are ambitiously aiming to be big, scalability is important, and so are globalization features. Since we are starting out without funding, price/performance and cost of licences/hardware is important. We definitely want to bring AJAX well present in the web interface. But apart from these, there's no further criteria I can come up with. I'm most experienced with C#/ASP.net, PHP and Java, in that order, but don't turn down other languages (Ruby, Python, Scala, etc.). How can we determine from the jungle of frameworks the one that suits best our goal? What other questions should we be asking ourselves? Reference material: articles, book recommendations, websites, etc.?

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• How to keep history of record updates in MySQL?

  - by Proxium

  I have to create a code in PHP that will allow me to keep the history of record updates in MySQL database so I can find by date an old revision. Here is the example of what I actualy want to achive: http://en.wikipedia.org/w/index.php?title=Tunisia&action=history The data are mostly Numbers that we record about the company for generating reports and to extract indices. I plan to use codeigniter for it's simplicity and I'm looking for idea about a framework or an opensource project that use the same approche to keep history of modifications in the database.

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• Custom certificate as proof of transaction

  - by Andy

  I'm developing a site where a user conducts a given transaction and once completed, the user is issued with a 'secure certificate'. The certificate serves as proof of the transaction and the user is able to upload the certificate at a later stage, to view the details of the transaction. At the moment I'm using a custom XML document with encrypted fields. It works perfect, but I would like a standardized approach, such as an X.509 certificate. I'm no encryption expert, but from what I gather, X.509 is more geared towards SSL issued by a CA. Is it possible to create your own valid valid CRT file? As a test, I created a CRT file with the example provided on WikiPedia. However, when I open the file in Windows I get this warning: Invalid Public Key Security Object File - This file is invalid as the following: Security Certificate. Not having much luck here, so time to ask the experts. What direction should I be heading in? Any guidance would be greatly appreciated.

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