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• How to implement a network protocol?

  - by gotch4

  Here is a generic question. I'm not in search of the best answer, I'd just like you to express your favourite practices. I want to implement a network protocol in Java (but this is a rather general question, I faced the same issues in C++), this is not the first time, as I have done this before. But I think I am missing a good way to implement it. In fact usually it's all about exchanging text messages and some byte buffers between hosts, storing the status and wait until the next message comes. The problem is that I usually end up with a bunch of switch and more or less complex if statements that react to different statuses / messages. The whole thing usually gets complicated and hard to mantain. Not to mention that sometimes what comes out has some "blind spot", I mean statuses of the protocol that have not been covered and that behave in a unpredictable way. I tried to write down some state machine classes, that take care of checking start and end statuses for each action in more or less smart ways. This makes programming the protocol very complicated as I have to write lines and lines of code to cover every possible situation. What I'd like is something like a good pattern, or a best practice that is used in programming complex protocols, easy to mantain and to extend and very readable. What are your suggestions?

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• How to maintain form state after Post-Redirect-Get in ASP.net?

  - by Ian Boyd

  Imagine a page with a form input: Search Criteria: crackers                   From: [email protected]           To: [email protected]       Subject: How to maintain form state with PRG? Message: Imagine a page with form input:                         Send After the user clicks Send, the server will instruct to client to Redirect, as part of the Post-Redirect-Get pattern. POST /mail/u/compose HTTP/1.1 303 See Other Location: http://stackoverflow.com/mail/u/compose And the client will issue a GET of the new page. The problem is that some elements of the existing form are lost: Search Criteria:                    It gets worse when there are a few drop-downs, and checkboxes. How can i maintain form state in using Post-Redirect-Get in ASP.net, given that the viewstate is then non-existent. Bonus Reading ASP.NET: How to redirect, prefilling form data?

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• Help regarding database and logic layer for my ASP.NET MVC application

  - by Ismail S

  I'm going to start a new project which is going to be small initially but may grow to big over the years. I'm strongly convinced that I'm going to use ASP.NET MVC with jQuery for UI. I want to go for MySQL as database for some reasons but worried on few things. I've a good years of experience working on SQL Server databases and on one project I've had a bad experience creating and managing stored procedures on MySQL database. I'm totally new to Linq but I see that it is easier to use once you are familiar with it. First thing is that accessing data should be easy. So I thought I should use MySQL to Linq but somewhere I read that it is not directly supported but MySQL .NET connector adds support for EntityFramework. I don't know what are the pros and cons of it. I would love if I can implement repository pattern. Will it be possible if I use Entity Framework? I'm not clear on how I should go about all this or I should just forget every thing and directly use SQL to Linq on SQL Server. I'm also concerned about the performance. Someone told me that if we use Entity framework it fetches lot of data and then filter it. Is that right?

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• Save Xml in an Excel cell value causes ComException

  - by mas_oz2k1

  I am trying to save an object (Class1) as string in a cell value. My issue is that from time to time I have a ComException: HRESULT: 0x8007000E (E_OUTOFMEMORY) (It is kind of random but I have not identified any particular pattern yet) when I write the value into a cell. Any ideas will be welcome For illustration purposes: Let Class1 be the class to be converted to an Xml string. (Notice that I removed the xml declaration at the start of the string to avoid having the preamble present- non printable character) <Class1 xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance" xmlns:xsd="http://www.w3.org/2001/XMLSchema" <ElementID HL690375</ElementID </Class1" Class1 myClass = new Class1(); this class is converted to a string s. s= ConvertObjectToXmlString(myClass); then s is assigned to a cell Range r = Application.ActiveCell; r.Value2 = s; Note: (1) If the string is too big, I limit it to 32000 chars and split the string in chunks of 32000 chars and save the chunks in multiple cells. (2) I do not to quote the string before adding to a cell. Do I need to? If so how it can be done? (3) All object contents are English. (4) C# code sample will be great but VB.net code is OK.

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• Where to prompt for required file location at start of Win Forms application

  - by Murph

  I have an application that uses a file to store its data. I store the location of the file in the app settings so have two tests at startup: Do I have a setting for the file and Does the file (if I have a setting) exist If I fail either test I want to prompt the user for the file location - the mechanics of the are not the problem, I can read and write the app settings, fire off dialogs and otherwise request the data. If the user refuses to choose a file (or at least a file location) I want to exit the app. My problem is where to do this i.e. at what point in the flow of code. In an ideal world you start the app, show a splash screen, load the main form and run from there... I'm looking for a general pattern that allows me to slot the test for parameters into the right place so that I can prompt the user for whatever (and allowing that I have to worry about the fact that my splash screen is currently topmost for my app). I appreciate that this is a bit vague so will update this with code as we go along.

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• What is the best Design/Way to keep user connected ?

  - by Fasih Hansmukh

  Am working on a POC for self learning in which I want to keep my user connected in LIVE pattern. For example, A game in which 4 user can play at a time , here I need to keep this user connected to my game . M not good at Socket type of programming and love to do that in Services way.What i wana know is 'What is the best way of doing this'. According to my initial Brain Storming, I have decided that I will use SilverLight(In Browser Or Out of Browser) as Front end [I have no issue in that]. I m more concern in back end. Either I make an handler or make a WCF service or use full duplex service and use pooling mechanism for that. As a random thought I come up with a Timer type logic that will fire every after 10 seconds at clients end and get status like Is it now Its turn to roll a dice Home many user left (in case if some of them left) What are connected user status in game like there score/points ect and update game view according to this at his end Kindly place your best answers here that will help me to learn this. Regards and thanks in Advance EDIT: Starting Bounty as i need more feedback. FH

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• Dependency injection in constructors

  - by andre

  Hello everyone. I'm starting a new project and setting up the base to work on. A few questions have risen and I'll probably be asking quite a few in here, hopefully I'll find some answers. First step is to handle dependencies for objects. I've decided to go with the dependency injection design pattern, to which I'm somewhat new, to handle all of this for the application. When actually coding it I came across a problem. If a class has multiple dependencies and you want to pass on multiple dependencies via the constructor (so that they cannot be changed after you instantiate the object). How do you do it without passing an array of dependencies, using call_user_func_array(), eval() or Reflection? This is what i'm looking for: <?php class DI { public function getClass($classname) { if(!$this->pool[$classname]) { # Load dependencies $deps = $this->loadDependencies($classname); # Here is where the magic should happen $instance = new $classname($dep1, $dep2, $dep3); # Add to pool $this->pool[$classname] = $instance; return $instance; } else { return $this->pool[$classname]; } } } Again, I would like to avoid the most costly methods to call the class. Any other suggestions?

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• Regarding the ViewModel

  - by mizipzor

  Im struggling to understand the ViewModel part of the MVVM pattern. My current approach is to have a class, with no logic whatsoever (important), except that it implements INotifyPropertyChanged. The class is just a collection of properties, a struct if you like, describing an as small part of the data as possible. I consider this my Model. Most of the WPF code I write are settings dialogs that configure said Model. The code-behind of the dialog exposes a property which returns an instance of the Model. In the XAML code I bind to subproperties of that property, thereby binding directly to the Model's properties. Which works quite well since it implements the INotifyPropertyChanged. I consider this settings dialog the View. However, I havent really been able to figure out what in all this is the ViewModel. The articles Ive read suggests that the ViewModel should tie the View and the Model together, providing the logic the Model lacks but is still to complex to go directly into the View. Is this correct? Would, in my example, the code-behind of the settings dialog be considered the ViewModel? I just feel a bit lost and would like my peers to debunk some of my assumptions. Am I completely off track here?

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• Java abstract visitor - guarantueed to succeed? If so, why?

  - by disown

  I was dealing with hibernate, trying to figure out the run-time class behind proxied instances by using the visitor pattern. I then came up with an AbstractVisitable approach, but I wonder if it will always produce correct results. Consider the following code: interface Visitable { public void accept(Visitor v); } interface Visitor { public void visit(Visitable visitorHost); } abstract class AbstractVisitable implements Visitable { @Override public void accept(Visitor v) { v.visit(this); } } class ConcreteVisitable extends AbstractVisitable { public static void main(String[] args) { final Visitable visitable = new ConcreteVisitable(); final Visitable proxyVisitable = (Visitable) Proxy.newProxyInstance( Thread.currentThread().getContextClassLoader(), new Class<?>[] { Visitable.class }, new InvocationHandler() { @Override public Object invoke(Object proxy, Method method, Object[] args) throws Throwable { return method.invoke(visitable, args); } }); proxyVisitable.accept(new Visitor() { @Override public void visit(Visitable visitorHost) { System.out.println(visitorHost.getClass()); } }); } } This makes a ConcreteVisitable which inherits the accept method from AbstractVisitable. In c++, I would consider this risky, since this in AbstractVisitable could be referencing to AbstractVisitable::this, and not ConcreteVisitable::this. I was worried that the code under certain circumstances would print class AbstractVisible. Yet the code above outputs class ConcreteVisitable, even though I hid the real type behind a dynamic proxy (the most difficult case I could come up with). Is the abstract visitor approach above guaranteed to work, or are there some pitfalls with this approach? What guarantees are given in Java with respect to the this pointer?

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• On the search for my next great .Net Read

  - by user127954

  Just got done with "The art of unit testing". It was a great read and i think everyone should go buy a copy. With that said i think the next book I'm like to read would be a architecture / Design type book that would focus heavily on building your objects / software in such a way that it would be: Low Coupling High Cohesion Easily Maintainable / Extended Easy to test Easy to Navigate / Debug The above characteristcs are the most important ones but also maybe it would also include (but not necessary) designing for: Performance - Don't want to design a system at at the end find out its dog slow :) Scalability - Again don't want to design something at the end find out it won't scale. I'd also prefer (but not necessary again): Something newer - Architectural principles seem to gradually evolve / improve over time and id like something with current thinking. .Net as illustrating language - like i said above its not mandatory but since its what i use every day id prefer it to be in .net. Doesn't really matter if its in vb.net or c# Some of the topics that would be talked about its how to minimize dependencies and using interfaces throughout your solution rather than concrete classes. Maybe it would constract /compare some of the newest design principles like DDD, Repository Pattern, Ect... I already have "Clean Code" (don't know if its this type of book or not) and "Working effectively with legacy code" on my radar but id like to read a book based upon the topic i talked about above first. Is there such a book?

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• How to programmatically create a node in Drupal 8?

  - by chapka

  I'm designing a new module in Drupal 8. It's a long-term project that won't be going public for a few months at least, so I'm using it as a way to figure out what's new. In this module, I want to be able to programmatically create nodes. In Drupal 7, I would do this by creating the object, then calling "node_submit" and "node_save". These functions no longer exist in Drupal 8. Instead, according to the documentation, "Modules and scripts may programmatically submit nodes using the usual form API pattern." I'm at a loss. What does this mean? I've used Form API to create forms in Drupal 7, but I don't get what the docs are saying here. What I'm looking to do is programmatically create at least one and possibly multiple new nodes, based on information not taken directly from a user-presented form. I need to be able to: 1) Specify the content type 2) Specify the URL path 3) Set any other necessary variables that would previously have been handled by the now-obsolete node_object_prepare() 4) Commit the new node object I would prefer to be able to do this in an independent, highly abstracted function not tied to a specific block or form. So what am I missing?

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• re.sub emptying list

  - by jmau5

  def process_dialect_translation_rules(): # Read in lines from the text file specified in sys.argv[1], stripping away # excess whitespace and discarding comments (lines that start with '##'). f_lines = [line.strip() for line in open(sys.argv[1], 'r').readlines()] f_lines = filter(lambda line: not re.match(r'##', line), f_lines) # Remove any occurances of the pattern '\s*<=>\s*'. This leaves us with a # list of lists. Each 2nd level list has two elements: the value to be # translated from and the value to be translated to. Use the sub function # from the re module to get rid of those pesky asterisks. f_lines = [re.split(r'\s*<=>\s*', line) for line in f_lines] f_lines = [re.sub(r'"', '', elem) for elem in line for line in f_lines] This function should take the lines from a file and perform some operations on the lines, such as removing any lines that begin with ##. Another operation that I wish to perform is to remove the quotation marks around the words in the line. However, when the final line of this script runs, f_lines becomes an empty lines. What happened? Requested lines of original file: ## English-Geek Reversible Translation File #1 ## (Moderate Geek) ## Created by Todd WAreham, October 2009 "TV show" <=> "STAR TREK" "food" <=> "pizza" "drink" <=> "Red Bull" "computer" <=> "TRS 80" "girlfriend" <=> "significant other"

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• Linking a template class using another template class (error LNK2001)

  - by Luís Guilherme

  I implemented the "Strategy" design pattern using an Abstract template class, and two subclasses. Goes like this: template <class T> class Neighbourhood { public: virtual void alter(std::vector<T>& array, int i1, int i2) = 0; }; and template <class T> class Swap : public Neighbourhood<T> { public: virtual void alter(std::vector<T>& array, int i1, int i2); }; There's another subclass, just like this one, and alter is implemented in the cpp file. Ok, fine! Now I declare another method, in another class (including neighbourhood header file, of course), like this: void lSearch(/*parameters*/, Neighbourhood<LotSolutionInformation> nhood); It compiles fine and cleanly. When starting to link, I get the following error: 1>SolverFV.obj : error LNK2001: unresolved external symbol "public: virtual void __thiscall lsc::Neighbourhood<class LotSolutionInformation>::alter(class std::vector<class LotSolutionInformation,class std::allocator<class LotSolutionInformation> > &,int,int)" (?alter@?$Neighbourhood@VLotSolutionInformation@@@lsc@@UAEXAAV?$vector@VLotSolutionInformation@@V?$allocator@VLotSolutionInformation@@@std@@@std@@HH@Z)

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• Are .NET's regular expressions Turing complete?

  - by Robert

  Regular expressions are often pointed to as the classical example of a language that is not Turning complete. For example "regular expressions" is given in as the answer to this SO question looking for languages that are not Turing complete. In my, perhaps somewhat basic, understanding of the notion of Turning completeness, this means that regular expressions cannot be used check for patterns that are "balanced". Balanced meaning have an equal number of opening characters as closing characters. This is because to do this would require you to have some kind of state, to allow you to match the opening and closing characters. However the .NET implementation of regular expressions introduces the notion of a balanced group. This construct is designed to let you backtrack and see if a previous group was matched. This means that a .NET regular expressions: ^(?<p>a)*(?<-p>b)*(?(p)(?!))$ Could match a pattern that: ab aabb aaabbb aaaabbbb ... etc. ... Does this means .NET's regular expressions are Turing complete? Or are there other things that are missing that would be required for the language to be Turing complete?

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• Exclude subdirectory from rewrite rule in web.config

  - by Clog

  This question comes up often, but I can only find solutions for PHP, Apache, htaccess etc but not for web.config I would like my pages to return in HTTP not HTTPS, except for forms within certain subdirectories. I have created the following web.config file, but how do I exclude a subdirectory called forms. <configuration> <system.webServer> <rewrite> <rules> <rule name="Force all to HTTP" stopProcessing="true"> <match url="(.*)" /> <conditions> <add input="{HTTPS}" pattern="on" ignoreCase="true" /> </conditions> <action type="Redirect" redirectType="Found" url="http://www.mysite.com/{R:1}" /> </rule> </rules> </rewrite> </system.webServer> </configuration> Many thanks all you clever clogs.

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• figuring out which field to look for a value in with SQL and perl

  - by Micah

  I'm not too good with SQL and I know there's probably a much more efficient way to accomplish what I'm doing here, so any help would be much appreciated. Thanks in advance for your input! I'm writing a short program for the local school high school. At this school, juniors and seniors who have driver's licenses and cars can opt to drive to school rather than ride the bus. Each driver is assigned exactly one space, and their DLN is used as the primary key of the driver's table. Makes, models, and colors of cars are stored in a separate cars table, related to the drivers table by the License plate number field. My idea is to have a single search box on the main GUI of the program where the school secretary can type in who/what she's looking for and pull up a list of results. Thing is, she could be typing a license plate number, a car color, make, and model, someone driver's name, some student driver's DLN, or a space number. As the programmer, I don't know what exactly she's looking for, so a couple of options come to mind for me to build to be certain I check everywhere for a match: 1) preform a couple of SELECT * FROM [tablename] SQL statements, one per table and cram the results into arrays in my program, then search across the arrays one element at a time with regex, looking for a matched pattern similar to the search term, and if I find one, add the entire record that had a match in it to a results array to display on screen at the end of the search. 2) take whatever she's looking for into the program as a scaler and prepare multiple select statements around it, such as SELECT * FROM DRIVERS WHERE DLN = $Search_Variable SELECT * FROM DRIVERS WHERE First_Name = $Search_Variable SELECT * FROM CARS WHERE LICENSE = $Search_Variable and so on for each attribute of each table, sticking the results into a results array to show on screen when the search is done. Is there a cleaner way to go about this lookup without having to make her specify exactly what she's looking for? Possibly some kind of SQL statement I've never seen before?

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• Recoverable error while running XSL

  - by Kate

  XSL: <?xml version="1.0" encoding="utf-8"?> <xsl:stylesheet xmlns:xsl="http://www.w3.org/1999/XSL/Transform" xmlns:ve="http://schemas.openxmlformats.org/markup-compatibility/2006" xmlns:o="urn:schemas-microsoft-com:office:office" xmlns:r="http://schemas.openxmlformats.org/officeDocument/2006/relationships" xmlns:m="http://schemas.openxmlformats.org/officeDocument/2006/math" xmlns:v="urn:schemas-microsoft-com:vml" xmlns:wp="http://schemas.openxmlformats.org/drawingml/2006/wordprocessingDrawing" xmlns:w10="urn:schemas-microsoft-com:office:word" xmlns:w="http://schemas.openxmlformats.org/wordprocessingml/2006/main" xmlns:wne="http://schemas.microsoft.com/office/word/2006/wordml" exclude-result-prefixes="wp wne w10 w ve o r m v" version="2.0"> <xsl:output method="text"/> <xsl:param name="styleName"/> <xsl:template match="w:p"> <xsl:apply-templates/><xsl:text>&#10;</xsl:text> </xsl:template> <xsl:template match="w:r[not ((parent::w:hyperlink[@w:anchor[matches(.,concat('^(',$styleName,')')),'i']]))]"> <xsl:value-of select="replace(., '.', '&#xFF00;')"/> </xsl:template> </xsl:stylesheet> While processing the above XSL, I am getting the below error, Recoverable Error: Recoverable error on line 11 FORG0006: An error occurred matching pattern {w:r[not ((parent::w:hyperlink[@w:anchor[matches(.,concat('^(',$styleName,')')),'i']]))]}: Effective boolean value is not defined for a sequence of two or more items starting with a boolean Please Help. I am not able to figure out this.

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• Regular Expression with Names and Emails

  - by Nina

  I am having a problem with regular expressions at the moment. What I'm trying to do is that for each line through the iteration, it checks for this type of pattern: Lastname, Firstname If it finds the name, then it will take the first letter of the first name, and the first six letters of the lastname and form it as an email. I have the following: $checklast = "[A-z],"; $checkfirst = "[A-z]"; if (ereg($checklast, $parts[1])||ereg($checkfirst, $parts[2])){ $first = preg_replace($checkfirst, $checkfirst{1,1}, $parts[2]); print "<a href='mailto:[email protected];'> $parts[$i] </a>"; } This one obviously broke the code. But I was initially attempting to find only the first letter of the firstname and then after that the first six letters of the lastname followed by the @email.com This didn't work out too well. I'm not sure what to do at this point. Any help is much appreciated.

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• Questions about the Backpropogation Algorithm

  - by Colemangrill

  I have a few questions concerning backpropogation. I'm trying to learn the fundamentals behind neural network theory and wanted to start small, building a simple XOR classifier. I've read a lot of articles and skimmed multiple textbooks - but I can't seem to teach this thing the pattern for XOR. Firstly, I am unclear about the learning model for backpropogation. Here is some pseudo-code to represent how I am trying to train the network. [Lets assume my network is setup properly (ie: multiple inputs connect to a hidden layer connect to an output layer and all wired up properly)]. SET guess = getNetworkOutput() // Note this is using a sigmoid activation function. SET error = desiredOutput - guess SET delta = learningConstant * error * sigmoidDerivative(guess) For Each Node in inputNodes For Each Weight in inputNodes[n] inputNodes[n].weight[j] += delta; // At this point, I am assuming the first layer has been trained. // Then I recurse a similar function over the hidden layer and output layer. // The prime difference being that it further divi's up the adjustment delta. I realize this is probably not enough to go off of, and I will gladly expound on any part of my implementation. Using the above algorithm, my neural network does get trained, kind of. But not properly. The output is always XOR 1 1 [smallest number] XOR 0 0 [largest number] XOR 1 0 [medium number] XOR 0 1 [medium number] I can never train the [1,1] [0,0] to be the same value. If you have any suggestions, additional resources, articles, blogs, etc for me to look at I am very interested in learning more about this topic. Thank you for your assistance, I appreciate it greatly!

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• How strict should I be in the "do the simplest thing that could possible work" while doing TDD

  - by Support - multilanguage SO

  For TDD you have to Create a test that fail Do the simplest thing that could possible work to pass the test Add more variants of the test and repeat Refactor when a pattern emerge With this approach you're supposing to cover all the cases ( that comes to my mind at least) but I'm wonder if am I being too strict here and if it is possible to "think ahead" some scenarios instead of simple discover them. For instance, I'm processing a file and if it doesn't conform to a certain format I am to throw an InvalidFormatException So my first test was: @Test void testFormat(){ // empty doesn't do anything... processor.validate("empty.txt"); try { processor.validate("invalid.txt"); assert false: "Should have thrown InvalidFormatException"; } catch( InvalidFormatException ife ) { assert "Invalid format".equals( ife.getMessage() ); } } I run it and it fails because it doesn't throw an exception. So the next thing that comes to my mind is: "Do the simplest thing that could possible work", so I : public void validate( String fileName ) throws InvalidFormatException { if(fileName.equals("invalid.txt") { throw new InvalidFormatException("Invalid format"); } } Doh!! ( although the real code is a bit more complicated, I found my self doing something like this several times ) I know that I have to eventually add another file name and other test that would make this approach impractical and that would force me to refactor to something that makes sense ( which if I understood correctly is the point of TDD, to discover the patterns the usage unveils ) but: Q: am I taking too literal the "Do the simplest thing..." stuff?

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• Why does this Haskell code produce the "infinite type" error?

  - by Charlie Flowers

  I am new to Haskell and facing a "cannot construct infinite type" error that I cannot make sense of. In fact, beyond that, I have not been able to find a good explanation of what this error even means, so if you could go beyond my basic question and explain the "infinite type" error, I'd really appreciate it. Here's the code: intersperse :: a -> [[a]] -> [a] -- intersperse '*' ["foo","bar","baz","quux"] -- should produce the following: -- "foo*bar*baz*quux" -- intersperse -99 [ [1,2,3],[4,5,6],[7,8,9]] -- should produce the following: -- [1,2,3,-99,4,5,6,-99,7,8,9] intersperse _ [] = [] intersperse _ [x] = x intersperse s (x:y:xs) = x:s:y:intersperse s xs And here's the error trying to load it into the interpreter: Prelude :load ./chapter.3.ending.real.world.haskell.exercises.hs [1 of 1] Compiling Main ( chapter.3.ending.real.world.haskell.exercises.hs, interpreted ) chapter.3.ending.real.world.haskell.exercises.hs:147:0: Occurs check: cannot construct the infinite type: a = [a] When generalising the type(s) for `intersperse' Failed, modules loaded: none. Thanks. EDIT: Thanks to the responses, I have corrected the code and I also have a general guideline for dealing with the "infinite type" error in Haskell: Corrected code intersperse _ [] = [] intersperse _ [x] = x intersperse s (x:xs) = x ++ s:intersperse s xs What the problem was: My type signature states that the second parameter to intersperse is a list of lists. Therefore, when I pattern matched against "s (x:y:xs)", x and y became lists. And yet I was treating x and y as elements, not lists. Guideline for dealing with the "infinite type" error: Most of the time, when you get this error, you have forgotten the types of the various variables you're dealing with, and you have attempted to use a variable as if it were some other type than what it is. Look carefully at what type everything is versus how you're using it, and this will usually uncover the problem.

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• Do I need to write a trigger for such a simple constraint?

  - by Paul Hanbury

  I really had a hard time knowing what words to put into the title of my question, as I am not especially sure if there is a database pattern related to my problem. I will try to simplify matters as much as possible to get directly to the heart of the issue. Suppose I have some tables. The first one is a list of widget types: create table widget_types ( widget_type_id number(7,0) primary key, description varchar2(50) ); The next one contains icons: create table icons ( icon_id number(7,0) primary key, picture blob ); Even though the users get to select their preferred widget, there is a predefined subset of widgets that they can choose from for each widget type. create table icon_associations ( widget_type_id number(7,0) references widget_types, icon_id number(7,0) references icons, primary key (widget_type_id, icon_id) ); create table icon_prefs ( user_id number(7,0) references users, widget_type_id number(7,0), icon_id number(7,0), primary key (user_id, widget_type_id), foreign key (widget_type_id, icon_id) references icon_associations ); Pretty simple so far. Let us now assume that if we are displaying an icon to a user who has not set up his preferences, we choose one of the appropriate images associated with the current widget. I'd like to specify the preferred icon to display in such a case, and here's where I run into my problem: alter table icon_associations add ( is_preferred char(1) check( is_preferred in ('y','n') ) ) ; I do not see how I can enforce that for each widget_type there is one, and only one, row having is_preferred set to 'y'. I know that in MySQL, I am able to write a subquery in my check constraint to quickly resolve this issue. This is not possible with Oracle. Is my mistake that this column has no business being in the icon_associations table? If not where should it go? Is this a case where, in Oracle, the constraint can only be handled with a trigger? I ask only because I'd like to go the constraint route if at all possible. Thanks so much for your help, Paul

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• How should I handle expected errors? eg. "username already exists"

  - by Pheter

  I am struggling to understand how I should design the error handling parts of my code. I recently asked a similar question about how I should go about returning server error codes to the user, eg. 404 errors. I learnt that I should handle the error from within the current part of the application; seem's simple enough. However, what should I do when I can't handle the error from the current link in the chain? For example, I may have a class that is used to manage authentication. One of it's methods could be createUser($username, $password). Within that function, I need to determine if the username already exists. If this is true, how should I alert the calling code about this? Returning null instead of a user object is one way. But how do I then know what caused the error? How should I handle errors in such a way that calling code can easily find out what caused the error? Is there a design pattern commonly used for this kind of situation?

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• Entity Framework POCO objects

  - by Dan

  I'm struggling with understanding Entity Framework and POCO objects. Here's what I'm trying to achieve. 1) Separate the DAL from the Business Layer by having my business layer use an interface to my DAL. Maybe use Unity to create my context. 2) Use Entity Framework inside my DAL. I have a Domain model with objects that reside in my business layer. I also have a database full of tables that doesn't really represent my domain model. I setup Entity Framework and generated POCO objects by using the ADO.NET POCO Generator extension. This gave me an object for each table in my database. Now I want to be able to say context.GetAll<User>(); and have it return a list of my User objects. The User object is in my business layer. Is that possible? Does that make sense or am I totally off and should start over? I'm guessing I need to use the repository pattern to achieve this, but I'm not sure. Can anyone help?

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• How to face observable object containing an observable field

  - by iseek

  Hello, I need a hint concerning MVC and Observer-Pattern. For example a model contains the classes "Address" and "Person". The Address class contains the fields street:String, zipcode:String, location:String. Whereas the Person class contains the fields name:String, firstName:String, address:Address. My approach so far looks something like this: Both, Address and Person are observable. If one of their setters is being called, I validate whether the current value and new value differ. Only in this case an update event is fired. The event contains the source, the name of the changed field, the old and the new value. The class for the view contains text fields to display and edit the information of a person: name, firstname, street, zipcode, location. It knows the Person model and is an subscribed observer for the person. So it gets the update events from the person object. My questions concerns the address field from type Address in the person class, since an address is observable on its own. If the view gets an update event from person when a new address has been set, I can update all of the address related fields in the view. But what if a field of the address changes? Should the view also register for update events from the address? Any hints about common design approaches would be appreciated. Greetings.

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