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  • C# Generic Generics (A Serious Question)

    - by tahirhassan
    In C# I am trying to write code where I would be creating a Func delegate which is in itself generic. For example the following (non-Generic) delegate is returning an arbitrary string: Func<string> getString = () => "Hello!"; I on the other hand want to create a generic which acts similarly to generic methods. For example if I want a generic Func to return default(T) for a type T. I would imagine that I write code as follows: Func<T><T> getDefaultObject = <T>() => default(T); Then I would use it as getDefaultObject<string>() which would return null and if I were to write getDefaultObject<int>() would return 0. This question is not merely an academic excercise. I have found numerous places where I could have used this but I cannot get the syntax right. Is this possible? Are there any libraries which provide this sort of functionality?

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  • How do I defer execution of some Ruby code until later and run it on demand in this scenario?

    - by Kyle Kaitan
    I've got some code that looks like the following. First, there's a simple Parser class for parsing command-line arguments with options. class Parser def initialize(&b); ...; end # Create new parser. def parse(args = ARGV); ...; end # Consume command-line args. def opt(...); ...; end # Declare supported option. def die(...); ...; end # Validation handler. end Then I have my own Parsers module which holds some metadata about parsers that I want to track. module Parsers ParserMap = {} def self.make_parser(kind, desc, &b) b ||= lambda {} module_eval { ParserMap[kind] = {:desc => "", :validation => lambda {} } ParserMap[kind][:desc] = desc # Create new parser identified by `<Kind>Parser`. Making a Parser is very # expensive, so we defer its creation until it's actually needed later # by wrapping it in a lambda and calling it when we actually need it. const_set(name_for_parser(kind), lambda { Parser.new(&b) }) } end # ... end Now when you want to add a new parser, you can call make_parser like so: make_parser :db, "login to database" do # Options that this parser knows how to parse. opt :verbose, "be verbose with output messages" opt :uid, "user id" opt :pwd, "password" end Cool. But there's a problem. We want to optionally associate validation with each parser, so that we can write something like: validation = lambda { |parser, opts| parser.die unless opts[:uid] && opts[:pwd] # Must provide login. } The interface contract with Parser says that we can't do any validation until after Parser#parse has been called. So, we want to do the following: Associate an optional block with every Parser we make with make_parser. We also want to be able to run this block, ideally as a new method called Parser#validate. But any on-demand method is equally suitable. How do we do that?

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  • Obtain container type from (its) iterator type in C++ (STL)

    - by KRao
    It is easy given a container to get the associated iterators, example: std::vector<double>::iterator i; //An iterator to a std::vector<double> I was wondering if it is possible, given an iterator type, to deduce the type of the "corresponding container" (here I am assuming that for each container there is one and only one (non-const) iterator). More precisely, I would like a template metafunction that works with all STL containers (without having to specialize it manually for each single container) such that, for example: ContainerOf< std::vector<double>::iterator >::type evaluates to std::vector<double> Is it possible? If not, why? Thank you in advance for any help!

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  • "Inlining" (kind of) functions at runtime in C

    - by fortran
    Hi, I was thinking about a typical problem that is very JIT-able, but hard to approach with raw C. The scenario is setting up a series of function pointers that are going to be "composed" (as in maths function composition) once at runtime and then called lots and lots of times. Doing it the obvious way involves many virtual calls, that are expensive, and if there are enough nested functions to fill the CPU branch prediction table completely, then the performance with drop considerably. In a language like Lisp, I could probably process the code and substitute the "virtual" call by the actual contents of the functions and then call compile to have an optimized version, but that seems very hacky and error prone to do in C, and using C is a requirement for this problem ;-) So, do you know if there's a standard, portable and safe way to achieve this in C? Cheers

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  • Scope of Groovy's ExpandoMetaClass?

    - by TicketMonster
    Groovy exposes an ExpandoMetaClass that allows you to dynamically add instance and class methods/properties to a POJO. I would like to use it to add an instance method to one of my Java classes: public class Fizz { // ...etc. } Fizz fizz = new Fizz(); fizz.metaClass.doStuff = { String blah -> fizz.buzz(blah) } This would be the equivalent to refactoring the Fizz class to have: public class Fizz { // ctors, getters/setters, etc... public void doStuff(String blah) { buzz(blah); } } My question: Does this add doStuff(String blah) to only this particular instance of Fizz? Or do all instances of Fizz now have a doStuff(String blah) instance method? If the former, how do I get all instances of Fizz to have the doStuff instance method? I know that if I made the Groovy: fizz.metaClass.doStuff << { String blah -> fizz.buzz(blah) } Then that would add a static class method to Fizz, such as Fizz.doStuff(String blah), but that's not what I want. I just want all instances of Fizz to now have an instance method called doStuff. Ideas?

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  • C++: How to require that one template type is derived from the other

    - by Will
    In a comparison operator: template<class R1, class R2> bool operator==(Manager<R1> m1, Manager<R2> m2) { return m1.internal_field == m2.internal_field; } Is there any way I could enforce that R1 and R2 must have a supertype or subtype relation? That is, I'd like to allow either R1 to be derived from R2, or R2 to be derived from R1, but disallow the comparison if R1 and R2 are unrelated types.

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  • DLL Load Failed: %1 is not a valid win32 application

    - by glittershark
    So I have a situation where I need to make binary patches and then be able to apply them from within python. I found bsdiff which looks like a great algorithm and has a python extension module, but that extension module doesn't have a windows installer past Python 2.5 Alright, so having never even written C seriously, let alone attempted a python extension module, I set out to compile it myself. After a few hours of bashing my head against a DLL load error with MinGW32, I managed to get it compiled and built using the setup.py provided in the source of the project. However, upon importing the resulting bsdiff module, I get: ImportError: DLL load failed: %1 is not a valid Win32 application. Any advice?

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  • Debugging metaprograms [C++]

    - by atch
    Hi, Is there any way to check step by step what's going on in let's say template? I mean how it is instantiated step by step and so on? In book I've mentioned here , I found (2 minutes ago) quite interesting example of how binary could be implemented as a metafunction. template <unsigned long N> struct binary { static unsigned const value = binary<N/10>::value << 1 // prepend higher bits | N%10; // to lowest bit }; template <> // specialization struct binary<0> // terminates recursion { static unsigned const value = 0; }; and I think it could be quite useful to be able to see step by step what's been done during the instantiation of this template. Thanks for your replies.

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  • How can I iterate through all of the Models in my rails app?

    - by James
    I would like to be able to iterate over and inspect all the models in my rails app. In pseudo-code it would look something like: rails_env.models.each do |model| associations = model.reflect_on_all_associations(:has_many) ... do some stuff end My question is how do I inspect my rails app to get a collection of the models (rails_env.models) ?

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  • pygame parachute

    - by user1473612
    I am using GUI2Exe to compile my python/pygame, game to a .exe I have a problem with the font module. using python 2.7 and the py2exe option in GUI2Exe I have updated python, pygame and py2exe with the 2.7 versions. My program runs fine but after I compile it with py2exe I get this. Here is the error I get: Fatal Python error: (pygame parachute) Segmentation Fault This application has requested the Runtime to terminate it in an unusual way. Please contact the application's support team for more information. My game starts off as a console and that part runs. But as soon as the display starts I get the crash. Thanks

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  • Checking if a function has C-linkage at compile-time [unsolvable]

    - by scjohnno
    Is there any way to check if a given function is declared with C-linkage (that is, with extern "C") at compile-time? I am developing a plugin system. Each plugin can supply factory functions to the plugin-loading code. However, this has to be done via name (and subsequent use of GetProcAddress or dlsym). This requires that the functions be declared with C-linkage so as to prevent name-mangling. It would be nice to be able to throw a compiler error if the referred-to function is declared with C++-linkage (as opposed to finding out at runtime when a function with that name does not exist). Here's a simplified example of what I mean: extern "C" void my_func() { } void my_other_func() { } // Replace this struct with one that actually works template<typename T> struct is_c_linkage { static const bool value = true; }; template<typename T> void assertCLinkage(T *func) { static_assert(is_c_linkage<T>::value, "Supplied function does not have C-linkage"); } int main() { assertCLinkage(my_func); // Should compile assertCLinkage(my_other_func); // Should NOT compile } Is there a possible implementation of is_c_linkage that would throw a compiler error for the second function, but not the first? I'm not sure that it's possible (though it may exist as a compiler extension, which I'd still like to know of). Thanks.

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  • Enhance Predfined Methods in Scala

    - by fratnk
    Base question: Why can I write in Scala just: println(10) Why don't I need to write: Console println(10) Followup question: How can I introduce a new method "foo" which is everywhere visible and usable like "println"?

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  • C++ require that one template type is derived from the other

    - by Will
    In a comparison operator: template<class R1, class R2> bool operator==(Manager<R1> m1, Manager<R2> m2) { return p1.internal_field == p2.internal_field; } Is there any way I could enforce that R1 and R2 must have a supertype or subtype relation? That is, I'd like to allow either R1 to be derived from R2, or R2 to be derived from R1, but disallow the comparison if R1 and R2 are unrelated types.

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  • Using CookieJar in Python to log in to a website from "Google App Engine". What's wrong here?

    - by brilliant
    Hello everybody, I've been trying to find a python code that would log in to my mail box on yahoo.com from "Google App Engine" . Here (click here to see that page) I was given this code: import urllib, urllib2, cookielib url = "https://login.yahoo.com/config/login?" form_data = {'login' : 'my-login-here', 'passwd' : 'my-password-here'} jar = cookielib.CookieJar() opener = urllib2.build_opener(urllib2.HTTPCookieProcessor(jar)) form_data = urllib.urlencode(form_data) # data returned from this pages contains redirection resp = opener.open(url, form_data) # yahoo redirects to http://my.yahoo.com, so lets go there instead resp = opener.open('http://mail.yahoo.com') print resp.read() The author of this script looked into HTML script of yahoo log-in form and came up with this script. That log-in form contains two fields, one for users' Yahoo! ID and another one is for users' password. Here is how HTML code of that page for both of those fields looks like: User ID field: <input type="text" maxlength="96" class="yreg_ipt" size="17" value="" id="username" name="login"> Password field: <input type="password" maxlength="64" class="yreg_ipt" size="17" value="" id="passwd" name="passwd"> However, when I uploaded this code to Google App Engine I discovered that this log-in form keeps coming back to me, which, I assume, means that logging-in process didn't succeed. Why is it so?

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  • Using CookieJar in Python to log in to a website from "Google App Engine". What's wrong here?

    - by brilliant
    Hello everybody, I've been trying to find a python code that would log in to my mail box on yahoo.com from "Google App Engine" . Here (click here to see that page) I was given this code: import urllib, urllib2, cookielib url = "https://login.yahoo.com/config/login?" form_data = {'login' : 'my-login-here', 'passwd' : 'my-password-here'} jar = cookielib.CookieJar() opener = urllib2.build_opener(urllib2.HTTPCookieProcessor(jar)) form_data = urllib.urlencode(form_data) # data returned from this pages contains redirection resp = opener.open(url, form_data) # yahoo redirects to http://my.yahoo.com, so lets go there instead resp = opener.open('http://mail.yahoo.com') print resp.read() The author of this script looked into HTML script of yahoo log-in form and came up with this script. That log-in form contains two fields, one for users' Yahoo! ID and another one is for users' password. Here is how HTML code of that page for both of those fields looks like: User ID field: <input type="text" maxlength="96" class="yreg_ipt" size="17" value="" id="username" name="login"> Password field: <input type="password" maxlength="64" class="yreg_ipt" size="17" value="" id="passwd" name="passwd"> However, when I uploaded this code to Google App Engine I discovered that this log-in form keeps coming back to me, which, I assume, means that logging-in process didn't succeed. Why is it so?

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  • Why does Python's 'for ... in' work differently on a list of values vs. a list of dictionaries?

    - by Code Duck
    I'm wondering about some details of how for ... in works in Python. My understanding is for var in iterable on each iteration creates a variable, var, bound to the current value of iterable. So, if you do for c in cows; c = cows[whatever], but changing c within the loop does not affect the original value. However, it seems to work differently if you're assigning a value to a dictionary key. cows=[0,1,2,3,4,5] for c in cows: c+=2 #cows is now the same - [0,1,2,3,4,5] cows=[{'cow':0},{'cow':1},{'cow':2},{'cow':3},{'cow':4},{'cow':5}] for c in cows: c['cow']+=2 # cows is now [{'cow': 2}, {'cow': 3}, {'cow': 4}, {'cow': 5}, {'cow': 6}, {'cow': 7} #so, it's changed the original, unlike the previous example I see one can use enumerate to make the first example work, too, but that's a different story, I guess. cows=[0,1,2,3,4,5] for i,c in enumerate(cows): cows[i]+=1 # cows is now [1, 2, 3, 4, 5, 6] Why does it affect the original list values in the second example but not the first?

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  • Getting a list of patches in an HPSA patch policy

    - by asm
    I'm trying to get a list of patches contained in a Patch Policy in HPSA -- I can get what I need via the Twister web interface (under PatchPolicy.getPatches(), give it an ID, and it happily returns a list of patches contained.) -- I'm having a hard time getting this to work via the Pytwist interface, though... I haven't used the Pytwist interface for much besides some very basic Device manipulation, and Python is.. not my forte. I create the TwistServer object, then a PatchPolicy object from that (which I think is working..), but can't figure out how/where to call the getPatches() method from in Python-land. If there's a way to dig this out of the database itself, that would work, too, but I can't seem to find much in there along these lines besides the vendor-recommended patching stuff, and we use custom policies.

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  • How to install mod_wsgi 3.1 on Ubuntu 9.10

    - by pthulin
    I have a Python 3 web app so mod_wsgi < 3.1 doesn't cut it for me. However, on my Ubuntu 9.10 installation there doesn't seem to be a package for mod_wsgi 3.1. Is there an alternative repository that has a package for mod_wsgi 3.1? There's a new Ubuntu release not so long from now, will it contain mod_wsgi 3.1? Some other distro ready with mod_wsgi 3.1 to recommend? Maybe my best bet is to compile it myself? From a quick google it looks like I only need the python and apache dev packages installed. Thanks!

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  • What am I (a beginner) losing, by choosing Cherokee over Apache for serving dynamic content?

    - by Bad Learner
    I am a complete beginner and am planning to setup a photo sharing site. This is the setup I am planning, basically for a start - - Cherokee (instead of Apache) for serving dynamic content (python-based application), and Nginx for serving static files. Since, I am a beginner, what have I, to lose? So, can someone, from your experience, please tell me, what I'd lose by choosing Cherokee over Apache for serving dynamic content in PHP/Python/whatever? Anything other than the fact that there's lot of documentation, many people who can help when there's an issue etc as Apache is well established and the most popular web server? Again, my intention is not to spurt a flame war here. Just wanted to know if Cherokee would be better than Apache in terms of performance, reliability, and speed, when it matters (peak load times). Also, I heard it's a lot faster than Apache in serving dynamic content, is it true?

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  • Installation requirements of django installing in hostgator dedicated server

    - by jaypabs
    First, before I install OSQA on my dedicated server at hostgator, I want to know the requirements. I don't want to screw up my server so it's better to ask question first. I have read a lot of tutorial on the internet regarding Django but I want to clarify something before I proceed. On my dedicated server I don't use FCGI. Instead I use Mod SuPHP. A lot of tutorial is talking about installing python using FCGI. My question is if it is safe to install Python if I'm using SuPHP? Is it safe to use the tutorial on this link: http://wiki.osqa.net/display/docs/Installing+OSQA+on+CentOS6?focusedCommentId=4784144 Thanks in advance for your help.

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  • How to install matplotlib on OS X?

    - by Paperflyer
    I want to install matplotlib on OS X. If possible, using homebrew. I installed Python 2.7.1 using brew install python, I modified my path to use it I installed pip using brew install pip I installed numpy 1.5.1 using pip install numpy I installed scipy 0.8.0 using pip install scipy This is where it gets hairy. pip install matplotlib will fetch the wrong version of matplotlib, which is incompatible with the recent version of numpy. The solution is to fetch the correct version of matplotlib manually: pip install -f http://sourceforge.net/projects/matplotlib/files/matplotlib/matplotlib-1.0.1/matplotlib-1.0.1.tar.gz matplotlib But, that version fails to compile since it can't find the freetype headers: In file included from src/ft2font.cpp:1: src/ft2font.h:14:22: error: ft2build.h: No such file or directory These headers are actually installed in /usr/X11/include as part of the X11 developer tools. So, how can I make matplotlib use these headers?

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