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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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  • Why are there no package management systems for C and C++?

    - by m0nhawk
    There are some programming languages for which exist their own package management systems: CTAN for TeX CPAN for Perl Pip & Eggs for Python Maven for Java cabal for Haskell Gems for Ruby Is there any other languages with such systems? What about C and C++? (that's the main question!) Why there are no such systems for them? And isn't creating packages for yum, apt-get or other general package management systems better?

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  • What does "general purpose system" mean for Java SE Embedded?

    - by Majid Azimi
    The Oracle website says this about Java SE Embedded license: development is free, but royalties are required upon deployment on anything other than general purpose systems What does "general purpose system" mean here? We have a sensor network around the country. On each box we have installed, there is a micro controller based board that gets data from the environment and send data on serial port to a ARM based embedded board. On this board system there is a Java process which reads and submits data to our central server using JMS. Is this categorized as general purpose system? Sorry I'm asking this here. We are in Iran, there is no Oracle office here to ask.

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  • Free OS with MS Windows Archetecture and capabilities

    - by Nayana Adassuriya
    Currently most of the PC users mostly depend on the windows OS and they would not go away from that beaus of the hand on usage knowledge about and also because of the look and feel habituation. But there are plenty of Linux base Desktop operation systems there such as UBUNTU, FEDORA. Users do not tend to go for those OSs (specially office environments) because most of the 3rd party software and tools (such as Photoshop, flash, Visual Studio) mostly can install only in windows operating system. So I'm thinking why we cant create a free OS same as Windows. That is capable to install software that created for windows. that can communicate with windows servers and exchange etc.. . Simply it should be a free OS with all the capabilities of Windows OS. How about your idea?

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  • Is it a waste of time to free resources before I exit a process?

    - by Martin
    Let's consider a fictional program that builds a linked list in the heap, and at the end of the program there is a loop that frees all the nodes, and then exits. For this case let's say the linked list is just 500K of memory, and no special space managing is required. Is that a wast of time, because the OS will do that anyway? Will there be a different behavior later? Is that different according to the OS version? I'm mainly interested in UNIX based systems, but any information will be appreciated. I had today my first lesson in OS course and I'm wondering about that now.

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  • How difficult is it to change from Embedded programming to a high level programming [on hold]

    - by anudeep shetty
    I have a background in Computer Science. I worked on Embedded programming on Linux file systems, after I finished my Bachelor's degree, for over a year. After that I pursued my masters where most of my course choices involved working on web, java and databases. Now I have an offer to work with a company that is offering a job to work on the OS level. The company is pretty good but I am feeling that my Masters has gone to waste. I wanted to know is it common that a Computer Science major works on low-level coding and is there a possibility that I can work in this company for some years and then move onto an opportunity where I can work on high-level coding? Also is working on low-level programming a safe choice in terms of job opportunities?

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  • What are best practices for testing programs with stochastic behavior?

    - by John Doucette
    Doing R&D work, I often find myself writing programs that have some large degree of randomness in their behavior. For example, when I work in Genetic Programming, I often write programs that generate and execute arbitrary random source code. A problem with testing such code is that bugs are often intermittent and can be very hard to reproduce. This goes beyond just setting a random seed to the same value and starting execution over. For instance, code might read a message from the kernal ring buffer, and then make conditional jumps on the message contents. Naturally, the ring buffer's state will have changed when one later attempts to reproduce the issue. Even though this behavior is a feature it can trigger other code in unexpected ways, and thus often reveals bugs that unit tests (or human testers) don't find. Are there established best practices for testing systems of this sort? If so, some references would be very helpful. If not, any other suggestions are welcome!

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  • How to organize my site's file system properly?

    - by Wolfpack'08
    Doing some reading on Stack Overflow, I've found a lot of information suggesting that proper organization of a file system is crucial to a well-written web app. One of the key pieces of evidence is high-frequency references to "separation of concerns" in questions related to keeping programs organized. Now, I've found some information on organizing file systems (Filesystem Hierarchy Standard) from 2004. It raises only two concerns: first, the standard's a bit dated, so I believe it may be possible to do better given the changes in technology over the past 8 years; second, and most important, my application is very small compared to an entire Linux distro. I think that the file system should be organized very differently because of that. Here's what I'm looking at, currently: /scripts, /databases, /www -> /dev, /production -> login, router, admin pages, /sites -> content types, static pages /modules, /includes, /css, /media -> /module-specific-media

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  • What are steps in making an operating system in C ? [duplicate]

    - by ps06756
    This question already has an answer here: Compiler/OS Design - Where to start [closed] 3 answers I am trying to make an my own OS. This is for educational purpose only, so that I get to understand the internals as well as get a good idea of low level programming. I have some prior application development experience in C#/python/C++/C. I am a noob in assembly language(very less experience and knowledge). I understand that in writing an operating system,we can't go without assembly language. Currently, I have just printed a string in assembly language in the boot sector using qemu and BIOS interrupts. What I want is that, can someone specifically point out the steps that I need to follow to make my operating systems run C programs. So that, I can start writing my OS in C. Any other piece of advice to help a newbie, regarding the same is also welcome. Although, I have looked into many os development related tutorials/websites, I can't seem to find this information anywhere.

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  • Windows 7 disk backup and clone for deployment to multiple systems

    - by gregmac
    I'm in the process of deploying some new PCs (there's only 8), all identical hardware. What I'd like to do is install Windows 7 (64bit), join to domain etc, install a bunch of other software, and then clone that drive to multiple other machines. I'd also like to be able to use it as a backup image, so the machine can be restored back to that image at some future date. I understand this involves at least sysprep, but I am confused after reading some tutorials that talk about using Windows Automated Installation Kit, or hacks with the registry and custom-build batch files. This process seems overly complex to me: I did something similar 10+ years ago, and and don't remember it being this bad. Surely things have improved in a decade? There's also some products that involve having network servers running deployment software, network boot, etc etc.. this is way more than I want to set up. My systems are all identical hardware. Is there a simplified way to clone PCs? Preferably (since I'm a lazy developer, and not an IT admin) I'd like to find some off-the-shelf product that I can run after I get the machine setup, that will spit out a bootable DVD I can run on all the other systems, which will boot up, ask for a computer name, join it to the domain, and that's it. Does such as product exist?

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  • Distributed File Systems.

    - by GruffTech
    So, I've been reading several articles around ServerFault as well as google. (For Example, this link) My Requirements are very similar to the link above, however i'd like to also have dynamic or at least resizeable file volumes, so if necessary i can add 4-5 servers to the pool, and then expand the volume. Any Distributed File systems that support that, to save me some time? Thanks! LustreFS will be my next test cluster to build. GlusterFS I've build a 3-machine test GlusterFS cluster, However i quickly became aware of several of its limitations that it doesn't seem to make clearly public. One, i can't seem to resize a volume. Once a volume is created, its done. Which seems retarded, why have a fully scalable file system if i can't scale a volume? So maybe i'm doing something wrong. I'm not sure. AmazonS3 while gives the cheapest startup adds too much cost when broken down to per client per month, so its out. Building my own system when prorated over several years with no bandwidth costs makes it significantly cheaper. MogileFS isn't an option as we'd like this server to be a SAN-Replacement, for storing tons of media from a multitude of systems, which for us means it needs to be POSIX compliant so it can be remotely mounted via NFS or CIFS.

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  • Recommendation on remote access setup for accessing customer systems

    - by gregmac
    I'm looking for a product recommendation (open or commercial) that will allow remote access to customer sites for tech support purposes. We need to be able to gain access to help troubleshoot problems on servers. Currently end up using anything from RDP on public IP, to various VPNs that clients happen to have, to webex-type sessions that require lots of interaction from both sides to get things working. This often means a problem that could take 10 minutes to solve takes an extra 30+ minutes messing around trying to get a connection up. There are multiple customer sites, which should NOT have access to each other. At each site, there is anywhere from 1 to 8 servers (Windows 2003 or 2008) that need to be accessed. Support connection to machines even if they're behind a firewall/router with no public IP Be able to selectively allow/deny access from customer site. Customer site should not be able to connect outbound to anywhere else (our systems, or other customer sites) Support multiple users from our end If not a VPN connection (where RDP could be used over top), should support: Remote desktop access, including copy/paste File transfers Preferably would have some way to list all remote systems, showing online/offline. Anyone have any suggestions?

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  • How much system and business analysis should a programmer be reasonably expected to do?

    - by Rahul
    In most places I have worked for, there were no formal System or Business Analysts and the programmers were expected to perform both the roles. One had to understand all the subsystems and their interdependencies inside out. Further, one was also supposed to have a thorough knowledge of the business logic of the applications and interact directly with the users to gather requirements, answer their queries etc. In my current job, for ex, I spend about 70% time doing system analysis and only 30% time programming. I consider myself a good programmer but struggle with developing a good understanding of the business rules of a complex application. Often, this creates a handicap because while I can write efficient algorithms and thread-safe code, I lose out to guys who may be average programmers but have a much better understanding of the business processes. So I want to know - How much business and systems knowledge should a programmer have ? - How does one go about getting this knowledge in an immensely complex software system (e.g. trading applications) with several interdependent business processes but poorly documented business rules.

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  • Examples of limitations in IT due to different bit length by design

    - by Alaudo
    I am teaching the course "Introduction in Programming" for the first-year students and would like to find interesting examples where the datatype size in bits, chosen by design, led to certain known restrictions or important values. Here are some examples: Due to the fact that the Bell teleprinter used 7-bit-code (later accepted as ASCII) until now have we often to encode attachments in electronic messages to contain only 7 bit data. Classical limitation of 32-bit address space leads to the 4Gb maximal RAM size available for 32-bit systems and 4Gb maximal file size in FAT32. Do you know some other interesting examples how the choice of the data type (and especially its binary length) influenced the modern IT world. Added after some discussion in comments: I am not going to teach how to overcome limitations. I just want them to know that 1 byte can hold the values from -127..0..+127 o 0..255, 2 bytes cover the range 0..65535 etc by proving examples they know from other sources, like the above-mentioned base64 encoding etc. We are just learning the basic datatypes and I am trying to find a good reference for "how large" these types are.

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  • Moving From IT to Embedded software Developing

    - by Ameer Adel
    i worked for two years at a channel station, managing various Types of tasks, varying from printers installation, software solution, down to managing and maintaining server automation, to be honest, i always been enthusiastic about programming, i studied at some affordable college and finished my IT path successfully, my graduation project was in C# ADO.NET couple of years ago. Obviously it was so much of a beginner spaghetti code than a well furnished code. I also had the chance; after leaving the IT career, to study about some ASP.NET MVC and web apps development. I have rookie level of coding skills due to the poor level of education i endured, and sufficient resources. Currently i m working as a trainee in a newly opened embedded software development company, that is being said, i am, as i sound, have a little idea about the algorithms included, as i was reading for the past couple of days, embedded system development requires more strict coding skills, including memory management, CPU optimization according to its architect, and couple of other tricks regarding the display, and power management if mobile.. etc. My question is, What type of Algorithms am i supposed to use in such cases, as i mentioned before, i am really enthusiastic about learning programming skills and algorithms related to embedded systems and programming languages, including C/C++, Java, C#, and some EC++ if still operational.

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  • What Computing/Programming Qualifications should I aspire for

    - by indevel
    I am a computing science post graduate in my first job from after my degree. During my 12 month review my boss posed the question "What can we do for you in terms of progressing your career?". This got me thinking, after university I hadn't really thought about what other qualifications were available. So this is my question, what courses/qualifications should I be looking to do. Which are highly regarded and which would be really useful to complete. I've searched Google it but all I see is a jumble of courses with no idea of the credibility of each. Any help is much appreciated. I'm traditionally a systems architect, but with this job I've turned to more embedded work so Id like to edge towards electronics, embedded programming, real time OS to help with my work also it would be more likely to be accepted if it was related to my job. Finally UK based courses/ qualifications are a must as travel is probably out of the question. Help me grow as a programmer.

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  • Cache coherence literature for big (>=16CPU) systems

    - by osgx
    Hello What books and articles can you recommend to learn basis of cache coherence problems in big SMP systems (which are NUMA and ccNUMA really) with =16 cpu sockets? Something like SGI Altix architecture analysis may be interesting. What protocols (MOESI, smth else) can scale up well?

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  • WordPress-based ticketing systems?

    - by CarlF
    I have been asked to set up a ticketing/help desk system for a small nonprofit. Our server runs Debian GNU/Linux. Because we already have WordPress installed and plan to exploit it heavily going forward, I'm wondering whether there are any WP-based ticketing systems. Obviously, it will simplify the admin's life to have less software installed on the server. Thanks.

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  • Better way to quad+ boot operating systems?

    - by Wijagels
    I currently have Windows 7, Ubuntu 12.04, Fedora 17, and open SUSE installed. I currently use BURG boot loader to load up all the systems. However, BURG does not work with windows(I still manage) and it is a little finicky. So, I want to make windows work and have all the other OSes I want all on one boot loader. I already tried easy BCD and for whatever reason Fedora took over and blocked out the other OSes.

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