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  • TypeScript or JavaScript for noob web developer [closed]

    - by Phil Murray
    Following the recent release by Microsoft of TypeScript I was wondering if this is something that should be considered for a experienced WinForm and XAML developer looking to get into more web development. From reviewing a number of sites and videos online it appears that the type system for TypeScript makes more sense to me as a thick client developer than the dynamic type system in Javascript. I understand that Typescript compiles down to JavaScript but it appears that the learning curve is shallower due to the current tooling provided by Microsoft. What are your thoughts?

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  • What are the 'must know' GDB commands?

    - by Chris Smith
    I'm starting to get the hang of GDB, but everything still feels much slower than when debugging in Eclipse or Visual Studio. Are there any GDB commands you find particularly useful/productive? My life became dramatically better when I discovered: list - Display source code near the current instruction But that is still pretty basic. (And unnecessary when running GDB from Emacs.) Is there any way to do things like setup a watch window? (Print and update the result of an expression every time execution stops.)

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  • Do I need a degree in Computer Science to get a Jr Programming job in the world? [closed]

    - by t84
    As above really, do I need to go to Uni to get a job as a Junior C# coder? I'm 26 and have been working in Games (Production) for 6 years and I am thinking of a change, I've had exposure to VB6, VBA, HTML, CSS, PHP, JavaScript over the past few years and did a web design NCFE at College, but other than that, nothing else! I'm teaching myself C# at the moment with books and I was wondering 'how much' I need to learn and also how I can improve my chances of getting a programming job! Am I a late started to learn coding? (I know many people who started at a very young age!) Thanks for the help :)

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  • How do you manage the testing of your Android software on physical devices?

    - by Philip Regan
    I'm in charge of managing mobile application development at my company, and I am currently building a mobile device "library" for testing. Essentially, we want to have a representative device in-house for each of the OSes we are developing for, currently iOS (iPhone-only), Blackberry, and Android. Simulators only go so far, but I'm placing into the process a step to test software on the devices themselves. The problem we're finding is with Android. I don't think any of us here ever really understood just how fragmented the whole platform is until we started looking at devices to acquire. We are going to wait until v2.3 of Android is released, but which products to choose? Do we go by the most popular by market share? Do we get a small range of products by specs from least to most powerful overall? We're trying to avoid having to manage a dozen different devices to test each app, if not because of cost if only for the repeated time sink. How do you manage the testing of your Android software on physical devices?

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  • Performing user authentication in a CodeIgniter controller constructor?

    - by msanford
    In "The Clean Code Talks -- Unit Testing" (http://youtu.be/wEhu57pih5w), Miško Hevery mentions that "as little work as possible should be done in constructors [to make classes more easily testable]'. It got me thinking about the way I have implemented my user authentication mechanism. Having delved into MVC development through CodeIgniter, I designed my first web application to perform user authentication for protected resources in controllers' constructors in cases where every public function in that controller requires the user to be authenticated. For controllers with public methods having mixed authentication requirements, I would naturally move the authentication from the constructor to each method requiring authentication (though I don't currently have a need for this). I made this choice primarily to keep the controller tight, and to ensure that all resources in the controller are always covered. As for code longevity and maintainability: given the application structure, I can't foresee a situation in which one of the affected controllers would need a public method that didn't require user authentication, but I can see this as a potential drawback in general with this implementation (i.e., requiring future refactoring). Is this a good idea?

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  • What's the best language combo for code generation?

    - by Peter Turner
    I read through Code Generation in Action but never bothered to make anything of it because Ruby just doesn't fit with my lifestyle at this juncture. The book came out more on the cusp of the C# revolution, and it said that C# "was a language designed to be generated", apparently using Ruby as the generator language. In your experience, what is the ideal combination of languages to generate the most useful code?

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  • Installing Perl modules and dependencies with non-root and without CPAN [migrated]

    - by Eegabooga
    I have been writing Perl scripts for my work and the machine that I have been given to work on makes installing Perl modules difficult: We cannot have gcc on my machine for security reasons, so I cannot use CPAN to install modules, for most modules. I do not have access to the root account. Usually, when I want to install a module, I put in a request and I have to wait a day or two before it gets installed. I know that nobody would have a problem with me installing them myself, so to save everyone's time and my sanity I would like to install them myself. It's just an issue of how to best do that. I have talked to various people and they said to use an RPM to install them (to get around not having gcc). However, when trying to install modules from RPMs, it does not handle the dependencies so I would manually need to handle the dependencies, which could take a while. How can I best install Perl modules with these limitations?

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  • What is Mozilla's new release management strategy ?

    - by RonK
    I saw today that FireFox released a new version (5). I tried reading about what was added and ran into this link: http://arstechnica.com/open-source/news/2011/06/firefox-5-released-arrives-only-three-months-after-firefox-4.ars It states that: Mozilla has launched Firefox 5, a new version of the popular open source Web browser. This is the first update that Mozilla has issued since adopting a new release management strategy that has drastically shortened the Firefox development cycle. I find this very intriguing - any idea what this new strategy is?

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  • English to French translation of computing terminology

    - by Rich
    I work in France as a Java programmer, mainly in French, but am a native English speaker. My level of French is pretty good (French wife!), but one thing I have problems with is working out whether to use English terminology or a French equivalent. Examples: lock (as in a synchronisation lock) - do I use the verb "locker" or do I use verrouiller? shard (databases) - "un shard" or "un tesson" (which means a shard of glass) ...and so-on... So, what do people recommend? Can anyone point out some good websites for translating this kind of terminology? The usual online translation tools are a bit too everyday English/French, not the slightly more specialised version that I find myself needing.

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  • Validating allowed characters or validating disallowed characters

    - by Tom
    I've always validated my user input based on a list of valid/allowed characters, rather than a list of invalid/disallowed characters (or simply no validation). It's just a habit I picked up, probably on this site and I've never really questioned it until now. It makes sense if you wish to, say, validate a phone number, or validate an area code, however recently I've realised I'm also validating input such as Bio Text fields, User Comments, etc. for which the input has no solid syntax. The main advantage has always seemed to be: Validating allowed chars reduces the risk of you missing a potentially malicious character, but increases the risk the of you not allowing a character which the user may want to use. The former is more important. But, providing I am correctly preventing SQL Injection (with prepared statements) and also escaping output, is there any need for this extra barrier of protection? It seems to me as if I am just allowing practically every character on the keyboard, and am forgetting to allow some common characters. Is there an accepted practice for this situation? Or am I missing something obvious? Thanks.

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  • What is the correlation between the quality of the software development process and the quality of the product?

    - by Ophir Yoktan
    I used to believe the practicing "good" software development methods tends to yield a better product in the long run. However, I've seen quite a few cases where "quick-and-dirty" \ "brute-force" \ "copy-paste" programming appeared to give decent results quicker, and cheaper. This appears especially in cases where time to market is more critical then maintenance overhead. Is there a correlation between the quality of the development process and techniques and the quality of the product?

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  • Are there programming languages that allow you to do set arithmetic on types?

    - by Will Brown
    Out of curiosity, are there languages that allow you to do set arithmetic on types to create new types? Something like: interface A { void a(); void b(); } interface B { void b(); void c(); } interface C = A & B; // has b() interface D = A | B; // has a(), b() and c() interface E = (A & B) ^ B; // has c() I know that in some languages these ideas can be expressed (i.e., Java has List<Comparable & Serializable> for the union of the interfaces) but I've never heard of a language that supports type arithmetic. Thanks!

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  • How to prepare for a programming competition? Graphs, Stacks, Trees, oh my! [closed]

    - by Simucal
    Last semester I attended ACM's (Association for Computing Machinery) bi-annual programming competition at a local University. My University sent 2 teams of 3 people and we competed amongst other schools in the mid-west. We got our butts kicked. You are given a packet with about 11 problems (1 problem per page) and you have 4 hours to solve as many as you can. They'll run your program you submit against a set of data and your output must match theirs exactly. In fact, the judging is automated for the most part. In any case.. I went there fairly confident in my programming skills and I left there feeling drained and weak. It was a terribly humbling experience. In 4 hours my team of 3 people completed only one of the problems. The top team completed 4 of them and took 1st place. The problems they asked were like no problems I have ever had to answer before. I later learned that in order to solve them some of them effectively you have to use graphs/graph algorithms, trees, stacks. Some of them were simply "greedy" algo's. My question is, how can I better prepare for this semesters programming competition so I don't leave there feeling like a complete moron? What tips do you have for me to be able to answer these problems that involve graphs, trees, various "well known" algorithms? How can I easily identify the algorithm we should implement for a given problem? I have yet to take Algorithm Design in school so I just feel a little out of my element. Here are some examples of the questions asked at the competitions: ACM Problem Sets Update: Just wanted to update this since the latest competition is over. My team placed 1st for our small region (about 6-7 universities with between 1-5 teams each school) and ~15th for the midwest! So, it is a marked improvement over last years performance for sure. We also had no graduate students on our team and after reviewing the rules we found out that many teams had several! So, that would be a pretty big advantage in my own opinion. Problems this semester ranged from about 1-2 "easy" problems (ie bit manipulation, string manipulation) to hard (graph problems involving fairly complex math and network flow problems). We were able to solve 4 problems in our 5 hours. Just wanted to thank everyone for the resources they provided here, we used them for our weekly team practices and it definitely helped! Some quick tips that I have that aren't suggested below: When you are seated at your computer before the competition starts, quickly type out various data structures that you might need that you won't have access to in your languages libraries. I typed out a Graph data-structure complete with floyd-warshall and dijkstra's algorithm before the competition began. We ended up using it in our 2nd problem that we solved and this is the main reason why we solved this problem before anyone else in the midwest. We had it ready to go from the beginning. Similarly, type out the code to read in a file since this will be required for every problem. Save this answer "template" someplace so you can quickly copy/paste it to your IDE at the beginning of each problem. There are no rules on programming anything before the competition starts so get any boilerplate code out the way. We found it useful to have one person who is on permanent whiteboard duty. This is usually the person who is best at math and at working out solutions to get a head start on future problems you will be doing. One person is on permanent programming duty. Your fastest/most skilled "programmer" (most familiar with the language). This will save debugging time also. The last person has several roles between assessing the packet of problems for the next "easiest" problem, helping the person on the whiteboard work out solutions and helping the person programming work out bugs/issues. This person needs to be flexible and be able to switch between roles easily.

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  • Decoupling software components via naming convention

    - by csteinmueller
    I'm currently evaluating alternatives to refactor a drivermanagement. In my multitier architecture I have Baseclass DAL.Device //my entity Interfaces BL.IDriver //handles the dataprocessing between application and device BL.IDriverCreator //creates an IDriver from a Device BL.IDriverFactory //handles the driver creation requests Every specialization of Device has a corresponding IDriver implementation and a corresponding IDriverCreator implementation. At the moment the mapping is fix via a type check within the business layer / DriverFactory. That means every new driver needs a) changing code within the DriverFactory and b) referencing the new IDriver implementation / assembly. On a customers point of view that means, every new driver, used or not, needs a complex revalidation of their hardware environment, because it's a critical process. My first inspiration was to use a caliburn micro like nameconvention see Caliburn.Micro: Xaml Made Easy BL.RestDriver BL.RestDriverCreator DAL.RestDevice After receiving the RestDevicewithin the IDriverFactory I can load all driver dlls via reflection and do a namesplitting/comparing (extracting the xx from xxDriverCreator and xxDevice) Another idea would be a custom attribute (which also leads to comparing strings). My question: is that a good approach above layer borders? If not, what would be a good approach?

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  • Design Pattern for Complex Data Modeling

    - by Aaron Hayman
    I'm developing a program that has a SQL database as a backing store. As a very broad description, the program itself allows a user to generate records in any number of user-defined tables and make connections between them. As for specs: Any record generated must be able to be connected to any other record in any other user table (excluding itself...the record, not the table). These "connections" are directional, and the list of connections a record has is user ordered. Moreover, a record must "know" of connections made from it to others as well as connections made to it from others. The connections are kind of the point of this program, so there is a strong possibility that the number of connections made is very high, especially if the user is using the software as intended. A record's field can also include aggregate information from it's connections (like obtaining average, sum, etc) that must be updated on change from another record it's connected to. To conserve memory, only relevant information must be loaded at any one time (can't load the entire database in memory at load and go from there). I cannot assume the backing store is local. Right now it is, but eventually this program will include syncing to a remote db. Neither the user tables, connections or records are known at design time as they are user generated. I've spent a lot of time trying to figure out how to design the backing store and the object model to best fit these specs. In my first design attempt on this, I had one object managing all a table's records and connections. I attempted this first because it kept the memory footprint smaller (records and connections were simple dicts), but maintaining aggregate and link information between tables became....onerous (ie...a huge spaghettified mess). Tracing dependencies using this method almost became impossible. Instead, I've settled on a distributed graph model where each record and connection is 'aware' of what's around it by managing it own data and connections to other records. Doing this increases my memory footprint but also let me create a faulting system so connections/records aren't loaded into memory until they're needed. It's also much easier to code: trace dependencies, eliminate cycling recursive updates, etc. My biggest problem is storing/loading the connections. I'm not happy with any of my current solutions/ideas so I wanted to ask and see if anybody else has any ideas of how this should be structured. Connections are fairly simple. They contain: fromRecordID, fromTableID, fromRecordOrder, toRecordID, toTableID, toRecordOrder. Here's what I've come up with so far: Store all the connections in one big table. If I do this, either I load all connections at once (one big db call) or make a call every time a user table is loaded. The big issue here: the size of the connections table has the potential to be huge, and I'm afraid it would slow things down. Store in separate tables all the outgoing connections for each user table. This is probably the worst idea I've had. Now my connections are 'spread out' over multiple tables (one for each user table), which means I have to make a separate DB called to each table (or make a huge join) just to find all the incoming connections for a particular user table. I've avoided making "one big ass table", but I'm not sure the cost is worth it. Store in separate tables all outgoing AND incoming connections for each user table (using a flag to distinguish between incoming vs outgoing). This is the idea I'm leaning towards, but it will essentially double the total DB storage for all the connections (as each connection will be stored in two tables). It also means I have to make sure connection information is kept in sync in both places. This is obviously not ideal but it does mean that when I load a user table, I only need to load one 'connection' table and have all the information I need. This also presents a separate problem, that of connection object creation. Since each user table has a list of all connections, there are two opportunities for a connection object to be made. However, connections objects (designed to facilitate communication between records) should only be created once. This means I'll have to devise a common caching/factory object to make sure only one connection object is made per connection. Does anybody have any ideas of a better way to do this? Once I've committed to a particular design pattern I'm pretty much stuck with it, so I want to make sure I've come up with the best one possible.

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  • Good PHP BOOKS for starters!

    - by Goma
    I started reading some PHP books. Most of them in their introduction say that this book , unlike other books, it follows a good habits and practices. Now, I do not know which book tells the truth, and which writer is the most experienced in PHP. These are the books that I had a quick look to their first chapter: PHP and MySQL Web Development (Developer's Library) by Luke Welling and Laura Thomson. Build Your Own Database Driven Web Site Using PHP & MySQL by Kevin Yank. PHP and MySQL for Dummies by Janet Valade. Now, it's your time to advise me and tell me about the excellent one that follows best practices, please give an advice from your experience. (It could be any other book!). Regards,

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  • Learning to program in C (coming from Python)

    - by Honza Pokorny
    If this is the wrong place to ask this question, please let me know. I'm a Python programmer by occupation. I would love to learn C. Indeed, I have tried many times, but I always get discouraged. In Python, you write a few lines and the program does wonders. In C, I can't seem to be able to do anything useful. It seems to be very complicated to even connect to the Internet. Do you have any suggestions on what I can do to learn C? Are there are any good websites? Any cool projects? Thanks

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  • Find vertices of a convex hull

    - by Jeff Bullard
    I am attempting to do this within CGAL. From a 3D point cloud, find the convex hull, then loop over the finite facets of the convex hull and print each facet's vertices. It seems like there should be a straightforward way to do this; I would have expected that 3D polyhedra would own a vector of facet objects, each of which in turn would own a vector of its edges, each of which in turn would own a vector of its vertices, and that their would be some access through this hierarchy using iterators. But so far I have been unable to find a simple way to navigate through this hierarchy (if it exists).

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  • Should I start learning WPF?

    - by questron
    Hi, I've been studying C# for about 2 months so far, I have a few years experience programming however in VBA and VB6, but nothing too in depth (I've written simple tools, but nothing too advanced). I am very interested in WPF and I was wondering whether it would be too early for me to start learning how to use it? For reference, I am about 400 pages into 'Head First C#' and I've written a program that can quickly tag image files for moving into a predefined folder or to delete (Allows the user to pull images off of a camera memory card and sort VERY quickly). That's the most advanced app I've written.

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  • Any suggested approaches to track bugs/defects?

    - by deostroll
    What is the best way to track defect sources in tfs? We have various teams for a project like the vulnerability team, the customer, pre-sales, etc. We give a build and these teams independently test it. They do not have access to our tfs system. So they usually send in their defects via email. It will usually be send in an excel format. Our testing team takes these up and logs them into tfs. Sometimes they modify the original defect description (in excel) and add the expected/actual results. Sometimes they miss to cite the source. I am talking about managing the various sources as such. Is there a way we can add these sources into tfs, and actually link this particular source with the defects, with individual comments associated with them (saying where in the source we can find the actual material for the defect). Edit: I don't know if there is a way to manage various sources. Consider this: the vulnerability assessment team has come out with defects/suggestions. They captured it into an excel and passed that on to the testing team (in my case). The testing team takes the responsibility of elaborating the defect and logging it in tfs. Now say that the excel has come with 20 defect items. This is my source. (It answers the question where did this defect come from). So ultimately when I am looking at a bug I know from where it came from - I'll ultimately be looking at the email sent from the VA team which has the excel or the excel file itself sent by the VA team. It may be one of the 20 items in that excel. How should the tester link to this source just once? On the contrary, it does not make sense for the tester to attach the same excel 20 times (i.e. attach the same excel for the 20 defects while logging it into tfs) right? I hope you get my point.

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  • Is there an appropriate coding style for implementing an algorithm during an interview?

    - by GlenPeterson
    I failed an interview question in C years ago about converting hex to decimal by not exploiting the ASCII table if (inputDigitByte > 9) hex = inputDigitByte - 'a'. The rise of Unicode has made this question pretty silly, but the point was that the interviewer valued raw execution speed above readability and error handling. They tell you to review algorithms textbooks to prepare for these interviews, yet these same textbooks tend to favor the implementation with the fewest lines of code, even if it has to rely on magic numbers (like "infinity") and a slower, more memory-intensive implementation (like a linked list instead of an array) to do that. I don't know what is right. Coding an algorithm within the space of an interview has at least 3 constraints: time to code, elegance/readability, and efficiency of execution. What trade-offs are appropriate for interview code? How much do you follow the textbook definition of an algorithm? Is it better to eliminate recursion, unroll loops, and use arrays for efficiency? Or is it better to use recursion and special values like "infinity" or Integer.MAX_VALUE to reduce the number of lines of code needed to write the algorithm? Interface: Make a very self-contained, bullet-proof interface, or sloppy and fast? On the one extreme, the array to be sorted might be a public static variable. On the other extreme, it might need to be passed to each method, allowing methods to be called individually from different threads for different purposes. Is it appropriate to use a linked-list data structure for items that are traversed in one direction vs. using arrays and doubling the size when the array is full? Implementing a singly-linked list during the interview is often much faster to code and easier remember for recursive algorithms like MergeSort. Thread safety - just document that it's unsafe, or say so verbally? How much should the interviewee be looking for opportunities for parallel processing? Is bit shifting appropriate? x / 2 or x >> 1 Polymorphism, type safety, and generics? Comments? Variable and method names: qs(a, p, q, r) vs: quickSort(theArray, minIdx, partIdx, maxIdx) How much should you use existing APIs? Obviously you can't use a java.util.HashMap to implement a hash-table, but what about using a java.util.List to accumulate your sorted results? Are there any guiding principals that would answer these and other questions, or is the guiding principal to ask the interviewer? Or maybe this should be the basis of a discussion while writing the code? If an interviewer can't or won't answer one of these questions, are there any tips for coaxing the information out of them?

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  • Javascript SDK on Facebook

    - by Eamonn Fox
    I am trying to use the Javascript SDK for Facebook but I keep getting the message : Given URL is not permitted by the application configuration.: One or more of the given URLs is not allowed by the App's settings. It must match the Website URL or Canvas URL, or the domain must be a subdomain of one of the App's domains but I have copied and pasted my canvas URL from the settings section. Anyone any ideas whats up?

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  • Is a university education really worth it for a good programmer?

    - by Jon Purdy
    The title says it all, but here's the personal side of it: I've been doing design and programming for about as long as I can remember. If there's a programming problem, I can figure it out. (Though admittedly StackOverflow has allowed me to skip the figuring out and get straight to the doing in many instances.) I've made games, esoteric programming languages, and widgets and gizmos galore. I'm currently working on a general-purpose programming language. There's nothing I do better than programming. However, I'm just as passionate about design. Thus when I felt leaving high school that my design skills were lacking, I decided to attend university for New Media Design and Imaging, a digital design-related major. For a year, I diligently studied art and programmed in my free time. As the next year progressed, however, I was obligated to take fewer art and design classes and more technical classes. The trouble was of course that these classes were geared toward non-technical students, and were far beneath my skill level at the time. No amount of petitioning could overcome the institution's reluctance to allow me to test out of such classes, and the major offered no promise for any greater challenge in the future, so I took the extreme route: I switched into the technical equivalent of the major, New Media Interactive Development. A lot of my credits moved over into the new major, but many didn't. It would have been infeasible to switch to a more rigorous technical major such as Computer Science, and having tutored Computer Science students at every level here, I doubt I would be exposed to anything that I haven't already or won't eventually find out on my own, since I'm so involved in the field. I'm now on track to graduate perhaps a year later than I had planned, which puts a significant financial strain on my family and my future self. My schedule continues to be bogged down with classes that are wholly unnecessary for me to take. I'm being re-introduced to subjects that I've covered a thousand times over, simply because I've always been interested in it all. And though I succeed in avoiding the cynical and immature tactic of failing to complete work out of some undeserved sense of superiority, I'm becoming increasingly disillusioned by the lack of intellectual stimulation. Further, my school requires students to complete a number of quarters of co-op work experience proportional to their major. My original major required two quarters, but my current requires three, delaying my graduation even more. To top it all off, college is putting a severe strain on my relationship with my very close partner of a few years, so I've searched diligently for co-op jobs in my area, alas to no avail. I'm now in my third year, and approaching that point past which I can no longer handle this. Either I keep my head down, get a degree no matter what it takes, and try to get a job with a company that will pay me enough to do what I love that I can eventually pay off my loans; or I cut my losses now, move wherever there is work, and in six months start paying off what debt I've accumulated thus far. So the real question is: is a university education really more than just a formality? It's a big decision, and one I can't make lightly. I think this is the appropriate venue for this kind of question, and I hope it sticks around for the sake of others who might someday find themselves in similar situations. My heartfelt thanks for reading, and in advance for your help.

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  • Changing frontend cache

    - by Utsav
    Our architecture consists of a front-end cache that most read only users obtain their data from directly. The front-end cache sits in front of a farm of webservers that serve pages written in PHP. We need to be able to detect certain conditions at the front-end cache level and pass those values through to the back-end via HTTP headers. For example we would like to manually tag the carrier network based on the IP address. So, for incoming traffic if the user is say coming from an IP address in the range of "41.202.192.0"/19 we would tag them as being a Orange Cameroon user by setting the appropriate HTTP request header, e.g., X-Carrier = "Orange Cameroon". Based on the setting of this header we would like to vary the cache and serve a different banner to the end user. How would you go about doing this? Keep in mind that we don't want to pollute the cache and we also don't want to create too many small cache segments. Assumptions: You can assume that the X-Carrier has already been detected in our cache. So, for the purposes of your test you can just set this value manually in your example script.

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  • Type checking and recursive types (Writing the Y combinator in Haskell/Ocaml)

    - by beta
    When explaining the Y combinator in the context of Haskell, it's usually noted that the straight-forward implementation won't type-check in Haskell because of its recursive type. For example, from Rosettacode [1]: The obvious definition of the Y combinator in Haskell canot be used because it contains an infinite recursive type (a = a -> b). Defining a data type (Mu) allows this recursion to be broken. newtype Mu a = Roll { unroll :: Mu a -> a } fix :: (a -> a) -> a fix = \f -> (\x -> f (unroll x x)) $ Roll (\x -> f (unroll x x)) And indeed, the “obvious” definition does not type check: ?> let fix f g = (\x -> \a -> f (x x) a) (\x -> \a -> f (x x) a) g <interactive>:10:33: Occurs check: cannot construct the infinite type: t2 = t2 -> t0 -> t1 Expected type: t2 -> t0 -> t1 Actual type: (t2 -> t0 -> t1) -> t0 -> t1 In the first argument of `x', namely `x' In the first argument of `f', namely `(x x)' In the expression: f (x x) a <interactive>:10:57: Occurs check: cannot construct the infinite type: t2 = t2 -> t0 -> t1 In the first argument of `x', namely `x' In the first argument of `f', namely `(x x)' In the expression: f (x x) a (0.01 secs, 1033328 bytes) The same limitation exists in Ocaml: utop # let fix f g = (fun x a -> f (x x) a) (fun x a -> f (x x) a) g;; Error: This expression has type 'a -> 'b but an expression was expected of type 'a The type variable 'a occurs inside 'a -> 'b However, in Ocaml, one can allow recursive types by passing in the -rectypes switch: -rectypes Allow arbitrary recursive types during type-checking. By default, only recursive types where the recursion goes through an object type are supported. By using -rectypes, everything works: utop # let fix f g = (fun x a -> f (x x) a) (fun x a -> f (x x) a) g;; val fix : (('a -> 'b) -> 'a -> 'b) -> 'a -> 'b = <fun> utop # let fact_improver partial n = if n = 0 then 1 else n*partial (n-1);; val fact_improver : (int -> int) -> int -> int = <fun> utop # (fix fact_improver) 5;; - : int = 120 Being curious about type systems and type inference, this raises some questions I'm still not able to answer. First, how does the type checker come up with the type t2 = t2 -> t0 -> t1? Having come up with that type, I guess the problem is that the type (t2) refers to itself on the right side? Second, and perhaps most interesting, what is the reason for the Haskell/Ocaml type systems to disallow this? I guess there is a good reason since Ocaml also will not allow it by default even if it can deal with recursive types if given the -rectypes switch. If these are really big topics, I'd appreciate pointers to relevant literature. [1] http://rosettacode.org/wiki/Y_combinator#Haskell

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