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  • What can Haskell's type system do that Java's can't and vice versa?

    - by Matt Fenwick
    I was talking to a friend about the differences between the type systems of Haskell and Java. He asked me what Haskell's could do that Java's couldn't, and I realized that I didn't know. After thinking for a while, I came up with a very short list of minor differences. Not being heavy into type theory, I'm left wondering whether they're formally equivalent. To try and keep this from becoming a subjective question, I'm asking: what are the major, non-syntactical differences between their type systems? I realize some things are easier/harder in one than in the other, and I'm not interested in talking about those. And to make it more specific, let's ignore Haskell type extensions since there's so many out there that do all kinds of crazy/cool stuff.

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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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  • How to solve 404 for static files with Django and Nginx?

    - by Lucio
    I setup a Trusty VM with Django + Nginx (other stuffs too). The server is working, I get the "Welcome to Django" page. But when I enter to servername/admin it loads the HTML page but fails to load the static content. And the admin page have this links to static content: <link rel="stylesheet" type="text/css" href="/static/admin/css/base.css" /> <link rel="stylesheet" type="text/css" href="/static/admin/css/login.css" /> Both of the CSS files give me 404, as the Nginx log shows: 192.168.56.1 - - [05/Jun/2014:12:04:09 -0300] "GET /admin HTTP/1.1" 301 5 "-" "Mozilla/5.0 (X11; Ubuntu; Linux x86_64; rv:29.0) Gecko/20100101 Firefox/29.0" 192.168.56.1 - - [05/Jun/2014:12:04:09 -0300] "GET /admin/ HTTP/1.1" 200 833 "-" "Mozilla/5.0 (X11; Ubuntu; Linux x86_64; rv:29.0) Gecko/20100101 Firefox/29.0" 192.168.56.1 - - [05/Jun/2014:12:04:10 -0300] "GET /static/admin/css/base.css HTTP/1.1" 404 142 "http://ubuntu-server/admin/" "Mozilla/5.0 (X11; Ubuntu; Linux x86_64; rv:29.0) Gecko/20100101 Firefox/29.0" 192.168.56.1 - - [05/Jun/2014:12:04:10 -0300] "GET /static/admin/css/login.css HTTP/1.1" 404 142 "http://ubuntu-server/admin/" "Mozilla/5.0 (X11; Ubuntu; Linux x86_64; rv:29.0) Gecko/20100101 Firefox/29.0" I think that the error is on my nginx.conf file, but do not know how to solve it.

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  • Dynamic vs Statically typed languages for websites

    - by Bradford
    Wanted to hear what others thought about this statement: I’ll contrast that with building a website. When rendering web pages, often you have very many components interacting on a web page. You have buttons over here and little widgets over there and there are dozens of them on a webpage, as well as possibly dozens or hundreds of web pages on your website that are all dynamic. With a system with a really large surface area like that, using a statically typed language is actually quite inflexible. I would find it painful probably to program in Scala and render a web page with it, when I want to interactively push around buttons and what-not. If the whole system has to be coherent, like the whole system has to type check just to be able to move a button around, I think that can be really inflexible. Source: http://www.infoq.com/interviews/kallen-scala-twitter

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  • Types of semantic bugs, logic errors [closed]

    - by C-Otto
    I am a PhD student and currently focus on automatically finding instances of new types of bugs in (Java) programs that cannot be found by existing tools like FindBugs. The existing tool currently is used to prove/disprove termination of (Java) programs. I have some ideas (see below), but I could need more input from you (experienced programmers, potential users of my tool). What kind of bugs do you wish to find? What types of bugs exist and might be suitable for my analysis? One strength of the approach I use is detailled information about the heap. So in contrast to FindBugs, I can work with knowledge of the form "variable x and variable y are disjoint on the heap" or "variable z is not cyclic". It is also possible to see if a method might have side effects (and if so, which variables may/may not be affected by it). Example 1: Vacuous call: Graph graphOne = createGraph(); Graph graphTwo = createGraph(); Node source = graphTwo.getRootNode(); for (Node n : graphOne.getNodes()) { if (areConnected(source, n)) { graphTwo.addNode(n); } } Imagine createGraph() creates a fresh graph, so that graphOne and graphTwo are disjoint on the heap. Then, because source is taken from graphTwo instead of graphOne, the call to areConnected always returns false. In this situation I could find out that the call areConnected is useless (because it does not have any side effect and the return value always is false) which helps finding the real bug (taking source from the wrong graph). For this the information that x and y are disjoint (because graphOne and graphTwo are disjoint) is crucial. This bug is related to calling x.equals(y) where x and y are objects of different classes. In this scenario, most implementations of equals() always return false, which most likely is not the intended result. FindBugs already finds this bug (hardcoded to equals(), semantics of implementation is not checked). Example 2: Useless code: someCode(); while (something()) { yetMoreSomething(); } moreCode(); In the case that the loop (so the code in something() and yetMoreSomething()) does not modify anything visible outside the loop, it does not make sense to run this code - the program has the same behaviour as someCode(); moreCode() (i.e., without the loop). To find this out, one needs detailled information about the side effects of the (possibly useless) code. If I can prove that the code does not have any side effect that can be observed afterwards (in the example: in moreCode() or later), then the code indeed is useless. Of course, here Input/Output of any form must be seen as a side effect, so that a System.out.println(...) is not considered useless. Example 3: Ignored return value: Instead of x = foo(); and making use of x, the method is called without storing the result: foo();. If the method does not have any side effect, its invocation is useless and can be dropped. Most likely, the bug here is that the returned value should have been used. Here, too, detailled information about side effects are needed. Can you think of similar types of bugs that might be detected (only) with detailled information about the heap, side effects, semantics of called methods, ...? Did you encounter bugs related to the ones shown below in "real life"? By the way, the tool is AProVE and Java related publications can be found on my homepage. Thanks a lot, Carsten

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  • Seeking a free Lint for C which programmers will *want* to use

    - by Mawg
    When I try to persuade others to Lint their code I always get excuses - too difficult to set up, too difficult to understand, false positives, etc (most of which translates to too lazy, too stupid or too afraid of new things). Is there any way that I can make Linting easier? We code in C using Netbeans. Can I incorporate Splint into Netbeans? I did find a Splint GUI which was quite good, but there was no way to lint a directory tree. Any ideas? Thanks in advance

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  • IPv4 private address assignment

    - by helloworld922
    I'm working on a private network which uses static IPv4 addresses as well as DHCP addressing for the physical LAN network. At a previous company I worked at they would assign static addresses in the 10.*.*.* space and all DHCP/LAN addresses were assigned in the 192.168.*.* space. Both of these address spaces are defined in the IPv4 private address space and there were never any internal conflicts. From personal experience at home, school, at work, and pretty much any other machine I've dealt with extensively (Windows and a few Linux distros), the DHCP server would always by default choose an address from the 192.168.*.* address space. Now my question is can I rely on this behavior? Do DHCP servers always by default assign from the 192.168.*.* pool (or any pool other than the 10.*.*.* pool), leaving the 10.*.*.* pool free for private static addressing? If not, under what conditions might a DHCP server choose an address in the 10.*.*.* address space?

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  • Can I use a 302 redirect to serve up static content from a url with escaped_fragment?

    - by Starfs
    We would like to serve up seo-friendly ajax-driven content. We are following this documentation. Has anyone ever tried to write a 302 redirect into the htaccess file, that takes the '?_escaped_fragment=' string and send that to a static page? For example /snapshot/yourfilename/ How will Google react to this? I've gone through the documentation and it's not very clear. The below quote is from Google's documentation this is what I find. I'm not sure if they are saying that you can redirect the _escaped_fragment_ url to a different static page, or if this is to redirect the hashtag URL to static content? Thoughts? From Google's site: Question: Can I use redirects to point the crawler at my static content? Redirects are okay to use, as long as they eventually get you to a page that's equivalent to what the user would see on the #! version of the page. This may be more convenient for some webmasters than serving up the content directly. If you choose this approach, please keep the following in mind: Compared to serving the content directly, using redirects will result in extra traffic because the crawler has to follow redirects to get the content. This will result in a somewhat higher number of fetches/second in crawl activity. Note that if you use a permanent (301) redirect, the url shown in our search results will typically be the target of the redirect, whereas if a temporary (302) redirect is used, we'll typically show the #! url in search results. Depending on how your site is set up, showing #! may produce a better user experience, because the user will be taken straight into the AJAX experience from the Google search results page. Clicking on a static page will take them to the static content, and they may experience avoidable extra page load time if the site later wants to switch them to the AJAX experience.

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  • Nginx Cache-Control

    - by optixx
    Iam serving my static content with ngnix. location /static { alias /opt/static/blog/; access_log off; etags on; etag_hash on; etag_hash_method md5; expires 1d; add_header Pragma "public"; add_header Cache-Control "public, must-revalidate, proxy-revalidate"; } The resulting header looks like this: Cache-Control:public, must-revalidate, proxy-revalidate Cache-Control:max-age=86400 Connection:close Content-Encoding:gzip Content-Type:application/x-javascript; charset=utf-8 Date:Tue, 11 Sep 2012 08:39:05 GMT Etag:e2266fb151337fc1996218fafcf3bcee Expires:Wed, 12 Sep 2012 08:39:05 GMT Last-Modified:Tue, 11 Sep 2012 06:22:41 GMT Pragma:public Server:nginx/1.2.2 Transfer-Encoding:chunked Vary:Accept-Encoding Why is nginx sending 2 Cache-Control entries, could this be a problem for the clients?

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  • Are the only types of data "sources" static and dynamic?

    - by blunders
    Thinking that there might be others, but not sure -- but before getting into that, let me explain what I mean by static and dynamic data sources. Static (or datastore) - Meaning that the data's state is non-changing, and if was changed, that would be a new state, and the old data would be considered stateless; meaning it no longer is known to exist, or not exist. Another way of possibly looking at a static data source might be that if read and written back without modification, the checksum for before and after should be exactly the same regardless of the duration of time between the reading and rewriting of the data. Examples: Photos, Files, Database Record, Dynamic (or datastream) - Meaning that the data's state is known to be in flux, and never expected to be the same per input. Example: Live video/audio feed, Stock Market feed, First let me say, the above is a very loose mapping of the concepts, and I'd welcome any feedback. Next, onto the core of the question, that being are these the only two types of data sources. My guess, is that yes, they are -- but that there are hybrid versions of the two. That being, streaming data that has a fixed state. For example, the data being streamed has a checksum given and each unique checksum is known to be a single instance of static data. On the flip side, static data could be chained via say a version control system; when played back, each version might be viewed as a segment of a stream; thing is, the very fact that it can be played back makes the data source static. Another type might be that the data source is being organically discovered, and it's simply unknown what the state is. Questions, feedback, requests -- just comment, thanks!!

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  • Dynamically vs Statically typed languages studies

    - by Winston Ewert
    Do there exist studies done on the effectiveness of statically vs dynamically typed languages? In particular: Measurements of programmer productivity Defect Rate Also including the effects of whether or not unit testing is employed. I've seen lots of discussion of the merits of either side but I'm wondering whether anyone has done a study on it. Edit Sadly, only one of the papers shown is actually a study and it does nothing but conclude that the language matters. This leads me to ponder: what if I proposed doing such a study with volunteers from this site?

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  • Can I use a 302 redirect to serve up static content from an URL with escaped_fragment?

    - by Starfs
    We would like to serve up SEO-friendly Ajax-driven content. We are following this documentation. Has anyone ever tried to write a 302 redirect into the .htaccess file, that takes the ?_escaped_fragment= string and send that to a static page?, for example /snapshot/yourfilename/. How will Google react to this? I've gone through the documentation and it's not very clear. The below quote is from Google's documentation this is what I find. I'm not sure if they are saying that you can redirect the _escaped_fragment_ URL to a different static page, or if this is to redirect the hashtag URL to static content? Thoughts? From Google's site: Question: Can I use redirects to point the crawler at my static content? Redirects are okay to use, as long as they eventually get you to a page that's equivalent to what the user would see on the #! version of the page. This may be more convenient for some webmasters than serving up the content directly. If you choose this approach, please keep the following in mind: Compared to serving the content directly, using redirects will result in extra traffic because the crawler has to follow redirects to get the content. This will result in a somewhat higher number of fetches/second in crawl activity. Note that if you use a permanent (301) redirect, the url shown in our search results will typically be the target of the redirect, whereas if a temporary (302) redirect is used, we'll typically show the #! url in search results. Depending on how your site is set up, showing #! may produce a better user experience, because the user will be taken straight into the AJAX experience from the Google search results page. Clicking on a static page will take them to the static content, and they may experience avoidable extra page load time if the site later wants to switch them to the AJAX experience.

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  • helper functions as static functions or procedural functions?

    - by fayer
    i wonder if one should create a helper function in a class as a static function or just have it declared as a procedural function? i tend to think that a static helper function is the right way to go cause then i can see what kind of helper function it is eg. Database::connect(), File::create(). what is best practice?

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  • cannot declare instance members in a static class in C#

    - by acadia
    Hello, I have a Public Static Class and I am trying to access appsettings from my app.config file in C# and I get the above error public static class employee { NameValueCollection appSetting = ConfigurationManager.AppSettings; } How do I get this to work? PS: I pasted just a few lines of code. thanks

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  • Calling a static method on a generic type parameter

    - by Remi Despres-Smyth
    I was hoping to do something like this, but it appears to be illegal in C#: public Collection MethodThatFetchesSomething<T>() where T : SomeBaseClass { return T.StaticMethodOnSomeBaseClassThatReturnsCollection(); } I get a compile-time error: "'T' is a 'type parameter', which is not valid in the given context." Given a generic type parameter, how can I call a static method on the generic class? The static method has to be available, given the constraint.

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  • Static analysis framework for eclipse?

    - by autobiographer
    i just wanted to use eclipse tptp, a framework for static code analysis but the support for code analysis ended with tptp 4.5.0. 1. it seems that this version can not be integrated into the current eclipse galileo. am i right? 2. which language independant framework for eclipse would you use as an alternative for tptp static analysis which works with eclipse galileo?

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  • Function-Local Static Const variable Initialization semantics.

    - by Hassan Syed
    The questions are in bold, for those that cannot be bothered reading a question in depth. This is a followup to this question. It is to do with the initialization semantics of static variables in functions. Static variables should be initialized once, and their internal state might be altered later - as I (currently) do in the linked question. However, the code in question does not require the feature to change the state of the variable later. Let me clarrify my position, since I don't require the string object's internal state to change. The code is for a trait class for meta programming, and as such would would benifit from a const char * const ptr -- thus Ideally a local cost static const variable is needed. My educated guess is that in this case the string in question will be optimally placed in memory by the link-loader, and that the code is more secure and maps to the intended semantics. This leads to the semantics of such a variable "The C++ Programming language Third Edition -- Stroustrup" does not have anything (that I could find) to say about this matter. All that is said is that the variable is initialized once when the flow of control of the thread first reaches the code. This leads me to ponder if the following code would be sensible, and if not what are the intended semantics ?. #include <iostream> const char * const GetString(const char * x_in) { static const char * const x = x_in; return x; } int main() { const char * const temp = GetString("yahoo"); std::cout << temp << std::endl; const char * const temp2 = GetString("yahoo2"); std::cout << temp2 << std::endl; } The following compiles on GCC and prints "yahoo" twice. Which is what I want -- However it might not be standards compliant (which is why I post this question). It might be more elegant to have two functions, "SetString" and "String" where the latter forwards to the first. If it is standards compliant does someone know of a templates implementation in boost (or elsewhere) ?

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  • SSH into Ubuntu Linux on a box without a static IP address

    - by Steven Xu
    Basically, how do I do it? I'd like to connect to my home computer from work, but my internet is routed through my apartment building's network, so I don't have the static IP address I'm accustomed to having. How do I go about accessing my home computer through SSH (I'll be using Putty at work if it matters) if my home computer doesn't have a static IP address?

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  • VB6's private static in C# ?

    - by blez
    In VB6 there are local static variables that keep their values after the exit of procedure. It's like using public vars but on local block. For example: sub count() static x as integer x = x + 1 end sub After 10 calls, x will be 10. I tried to search the same thing in .NET (and even Java) but there was none. Why? Does it break the OOP model in some way, and is there a way to emulate that.

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  • Static lock in Python?

    - by roddik
    Hello. I've got the following code: import time import threading class BaseWrapper: #static class lock = threading.Lock() @staticmethod def synchronized_def(): BaseWrapper.lock.acquire() time.sleep(5) BaseWrapper.lock.release() def test(): print time.ctime() if __name__ is '__main__': for i in xrange(10): threading.Thread(target = test).start() I want to have a method synchronized using static lock. However the above code prints the same time ten times, so it isn't really locking. How can I fix it? TIA

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  • SSL with external static content server

    - by SirMoreno
    I have a .Net web application that for performance issues gets all the static data (CSS, Images, JS) from an external server that is on different location and different hosting company. I want to enable SSL on my site without the users getting a message: "Page contains both secure and insecure elements" Does this means I’ll have to get two SSL Certificates one for each server? If I want the users to continue getting the static content from the external server what other options do I have? Thanks.

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  • C# - Sharing static data between multiple processes

    - by Murtaza Mandvi
    I have a WCF service (instantiated within a Console application on NetTCP), this service has static data (large volume) which gets instantiated on the load. I have multiple instances of this Console application running at once, and all of them are doing the same static data initialization , is there a way that I can have a single data source and share the data among processes so that each process does not have to consume large amount of memory?

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  • static function in an abstract class

    - by Alexander
    How to implement a static function in an abstract class?? Where do I implement this? class Item{ public: // // Enable (or disable) debug descriptions. When enabled, the String produced // by descrip() includes the minimum width and maximum weight of the Item. // Initially, debugging is disabled. static enableDebug(bool); };

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