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  • Typeclass instances for unnamed types in Scala

    - by ncreep
    How would one encode the following constraint in Scala (pseudocode)? def foo(x: T forSome { type T has a Numeric[T] instance in scope }) = { val n= implicitly[...] // obtain the Numeric instance for x n.negate(x) // and use it with x } In words: I need a type class instance for my input argument, but I don't care about the argument's type, I just need to obtain the instance and use it on my argument. It doesn't have to be an existential type, but I need to avoid type parameters in the def's signature. Thanks.

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  • Tools to thumbnail multiple extension types

    - by Joomala
    My requirements are to be able to thumbnail the major image and file extensions: doc(x), txt, xls(x), pdf, rte, and as many others as possible. We have been hooking into Office extensions to do this in the past, but they are not really supported on Vista and Windows 7 operating systems. Are there any third party thumbnailing applications that you have had success implementing? Preferably ones that are easy to invoke from .Net

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  • problems with Haskell's Number Types

    - by mindeavor.
    I have the following haskell code: fac n = product [1..n] taylor3s w0 f f' f'' t h = w1 : taylor3s w1 f f' f'' (t+h) h where hp i = h^i / fac i w1 = w0 + (hp 1) * f t w0 + (hp 2) * f' t w0 + (hp 3) * f'' t w0 taylor_results = take 4 $ taylor3s 1 f f' f'' 1 0.25 where f t x = t^4 - 4*x/t f' t x = 4*t^3 - 4*(f t x)/t + 4*x/t^2 f'' t x = 12*t^2 - 4*(f' t x)/t + 8*(f t x)/t^2 - 8*x/t^3 taylor_results is supposed to be a use case of taylor3s. However, there is something wrong with the number type inferencing. When I try to compile, this is the error I get: practice.hs:93:26: Ambiguous type variable `a' in the constraints: `Integral a' arising from a use of `taylor3s' at practice.hs:93:26-51 `Fractional a' arising from a use of `f' at practice.hs:93:37 Possible cause: the monomorphism restriction applied to the following: taylor_results :: [a] (bound at practice.hs:93:0) Probable fix: give these definition(s) an explicit type signature or use -XNoMonomorphismRestriction Can someone help me with understanding what the problem is?

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  • Python: Using `copyreg` to define reducers for types that already have reducers

    - by cool-RR
    (Keep in mind I'm working in Python 3, so a solution needs to work in Python 3.) I would like to use the copyreg module to teach Python how to pickle functions. When I tried to do it, the _Pickler object would still try to pickle functions using the save_global function. (Which doesn't work for unbound methods, and that's the motivation for doing this.) It seems like _Pickler first tries to look in its own dispatch for the type of the object that you want to pickle before looking in copyreg.dispatch_table. I'm not sure if this is intentional. Is there any way for me to tell Python to pickle functions with the reducer that I provide?

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  • Reflection for Class of generic parameter in Java?

    - by hatboysam
    Imagine the following scenario: class MyClass extends OtherClass<String>{ String myName; //Whatever } class OtherClass<T> { T myfield; } And I am analyzing MyClass using reflection specifically (MyClass.class).getDeclaredFields(), in this case I will get the following fields (and Types, using getType() of the Field): myName --> String myField --> T I want to get the actual Type for T, which is known at runtime due to the explicit "String" in the extends notation, how do I go about getting the non-genetic type of myField?

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  • Dilemma with two types and operator +

    - by user35443
    I have small problem with operators. I have this code: public class A { public string Name { get; set; } public A() { } public A(string Name) { this.Name = Name; } public static implicit operator B(A a) { return new B(a.Name); } public static A operator+(A a, A b) { return new A(a.Name + " " + b.Name); } } public class B { public string Name { get; set; } public B() { } public B(string Name) { this.Name = Name; } public static implicit operator A(B b) { return new A(b.Name); } public static B operator +(B b, B a) { return new B(b.Name + " " + a.Name); } } Now I want to know, which's conversion operator will be called and which's addition operator will be called in this operation: new A("a") + new B("b"); Will it be operator of A, or of B? (Or both?) Thanks....

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  • MVC4 not binding a list of basic types

    - by admanb
    I cannot, for the life of me, get this data to bind. Here's my JavaScript: var params = { 'InvItemIDs': ["188475", "188490"]}; $.post("api/Orders/OrderFromInventory?" + $.param(params)) and the Controller action: public HttpResponseMessage OrderFromInventory(IList<int> InvItemIDs) { return new HttpResponseMessage(); } I've built the query string so that it's sending: ?InvItemIDs=188475&InvItemIDs=188490 as well as ?InvItemIDs[]=188475&InvItemIDs[]=188490 and even ?InvItemIDs[0]=188475&InvItemIDs[1]=188490 and none of them are binding. InvItemIDs is always null. What am I doing wrong? EDIT: So it turns out all this is a bug (or something) in the new Web API controller code in MVC4. As soon as I moved the exact same code over to a standard controller it started working. I'm still interested if anyone has any insight as to why the Web API would break this binding.

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  • How to write function where argument is type but not typed value?

    - by ssp
    I want to convert a string representations of few dozen enum types to enum values. It's easy to convert string to concrete type: Enum.Parse(typeof<FontStyle>,"Bold") |> unbox<FontStyle> but for now i want to write function where type and string are parameters. The best one i can write is: > let s2e (_: 'a) s = Enum.Parse(typeof<'a>,s) |> unbox<'a>;; val s2e : 'a -> string -> 'a > s2e FontStyle.Regular "Bold";; val it : FontStyle = Bold Is there any option to write something like this but with type itself as first argument?

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  • Incompatible format types

    - by nebffa
    I'm playing around with strncpy in C and am having some trouble. The code is as follows: #include <stdio.h> #include <string.h> int main(int argc, char *argv[]) { char src[] = "Benjamin Franklin"; char dest[5]; strncpy(src, dest, sizeof(dest) / sizeof(char)); dest[5] = '\0'; printf("%s\n", dest); return 0; } which compiles with no errors using: gcc -Wall -g -Werror test.c -o test and prints out gibberish like p4?? I cannot really understand what I'm doing wrong especially since I have played around with it a lot and been looking online for answers. Perhaps since I am using arrays I am passing the address to printf without realising it?

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  • Forcing entry of certain character types in UITextField

    - by cannyboy
    If I have a UITextField which the user is inputing a registration number, which has the format: 11-11-1111 (that is 2 digits, a dash, 2 digits, a dash, four digits) How do I force the user to enter this kind of data only... so they can't enter anything except 0-9 in the first character, and only '-' for the third character etc.

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  • Mapping integers to types using C++ template fails in a specific case

    - by Shailesh Kumar
    I am attempting to compile the following template based code in VC++ 2005. #include <iostream> using namespace std; /* * T is a template which maps an integer to a specific type. * The mapping happens through partial template specialization. * In the following T<1> is mapped to char, T<2> is mapped to long * and T<3> is mapped to float using partial template specializations */ template <int x> struct T { public: }; template<> struct T<1> { public: typedef char xType; }; template<> struct T<2> { public: typedef long xType; }; template<> struct T<3> { public: typedef float xType; }; // We can easily access the specific xType for a specific T<N> typedef T<3>::xType x3Type; /*! * In the following we are attempting to use T<N> inside another * template class T2<R> */ template<int r> struct T2 { //We can map T<r> to some other type T3 typedef T<r> T3; // The following line fails typedef T3::xType xType; }; int main() { T<1>::xType a1; cout << typeid(a1).name() << endl; T<2>::xType a2; cout << typeid(a2).name() << endl; T<3>::xType a3; cout << typeid(a3).name() << endl; return 0; } There is a particular line in the code which doesn't compile: typedef T3::xType xType; If I remove this line, compilation goes fine and the result is: char long float If I retain this line, compilation errors are observed. main.cpp(53) : warning C4346: 'T<x>::xType' : dependent name is not a type prefix with 'typename' to indicate a type main.cpp(54) : see reference to class template instantiation 'T2<r>' being compiled main.cpp(53) : error C2146: syntax error : missing ';' before identifier 'xType' main.cpp(53) : error C4430: missing type specifier - int assumed. Note: C++ does not support default-int I am not able to figure out how to make sure that T::xType can be treated as a type inside the T2 template. Any help is highly appreciated.

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  • Go - Data types for validation

    - by nevalu
    How to create a new data type for Go which to can check/validate its schema when is created a new variable (of that type)? By example, to validate if a string has 20 characters, I tried: // Format: 2006-01-12T06:06:06Z func date(str string) { if len(str) != 20 { fmt.Println("error") } } var Date = date() type Account struct { domain string username string created Date } but it fails because Date is not a type.

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  • C# underlying types of enums

    - by Marlon
    What is the point of having enum SomeEnum : byte // <---- { SomeValue = 0x01, ... } when you have to make a cast just to assign it to the same type of variable as the enums underlying type? byte b = (byte)SomeEnum.SomeValue;

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  • File System Types in .Net

    - by Avi
    I don't get the abstractions and the terminology :-( For example, DirectoryInfo.FullName is defined as the full path of the directory or file, but it's a string! So is DirectoryInfo.Name, FileInfo.FullName, Path.GetDirectoyName and so on. This means that in .Net there is no "depth" (or "meat" - my English isn't so good) for the file system objects. There's no protection from a type system. I can't, for example, define two Path objects and ask if one of them is "above" the other - I have to manipulate the strings. I can't differentiate between a Path that identifies a directory and a path that identifies a file. I can't do anything!-( Just manipulate strings. Is this correct (or am I simply missing something). If correct, are there any alternatives?

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  • Intersection between sets containing different types of variables

    - by Gacek
    Let's assume we have two collections: List<double> values List<SomePoint> points where SomePoint is a type containing three coordinates of the point: SomePoint { double X; double Y; double Z; } Now, I would like to perform the intersection between these two collections to find out for which points in points the z coordinate is eqal to one of the elements of values I created something like that: HashSet<double> hash = new HashSet<double>(points.Select(p=>p.Z)); hash.IntersectWith(values); var result = new List<SomePoints>(); foreach(var h in hash) result.Add(points.Find(p => p.Z == h)); But it won't return these points for which there is the same Z value, but different X and Y. Is there any better way to do it?

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  • Compiler error when using abstract types

    - by Dylan
    I'm trying to implement a "protocol helper" trait that is responsible for matching up Prompts and Responses. The eventual goal is to have an object that defines the various Prompt and Response classes as subclasses of a sealed trait, then have a class that mixes in the ProtocolSupport trait for that Protocol object. The problem is that my current approach won't compile, even though I'm fairly sure it should. Here's a distilled version of what I've got: trait Protocol { type Response type Prompt <: BasePrompt trait BasePrompt { type Data def validate(response: Response): Validated[Data] } } trait ProtocolSupport[P <: Protocol] { def foo(prompt: P#Prompt, response: P#Response) = { // compiler error prompt.validate(response) } } The compiler doesn't like the response as an argument to prompt.validate: [error] found : response.type (with underlying type P#Response) [error] required: _4.Response where val _4: P [error] prompt.validate(response) [error] ^ This isn't very helpful.. it seems to say that it wants a P.Response but that's exactly what I'm giving it, so what's the problem?

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  • Testing variable types in Python

    - by Jasper
    Hello, I'm creating an initialising function for the class 'Room', and found that the program wouldn't accept the tests I was doing on the input variables. Why is this? def __init__(self, code, name, type, size, description, objects, exits): self.code = code self.name = name self.type = type self.size = size self.description = description self.objects = objects self.exits = exits #Check for input errors: if type(self.code) != type(str()): print 'Error found in module rooms.py!' print 'Error number: 110' elif type(self.name) != type(str()): print 'Error found in module rooms.py!' print 'Error number: 111' elif type(self.type) != type(str()): print 'Error found in module rooms.py!' print 'Error number: 112' elif type(self.size) != type(int()): print 'Error found in module rooms.py!' print 'Error number: 113' elif type(self.description) != type(str()): print 'Error found in module rooms.py!' print 'Error number: 114' elif type(self.objects) != type(list()): print 'Error found in module rooms.py!' print 'Error number: 115' elif type(self.exits) != type(tuple()): print 'Error found in module rooms.py!' print 'Error number: 116' When I run this I get this error: Traceback (most recent call last): File "/Users/Jasper/Development/Programming/MyProjects/Game Making Challenge/Europa I/rooms.py", line 148, in <module> myRoom = Room(101, 'myRoom', 'Basic Room', 5, '<insert description>', myObjects, myExits) File "/Users/Jasper/Development/Programming/MyProjects/Game Making Challenge/Europa I/rooms.py", line 29, in __init__ if type(self.code) != type(str()): TypeError: 'str' object is not callable

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  • Types in a struct in C

    - by drigoSkalWalker
    In this article : http://publib.boulder.ibm.com/infocenter/macxhelp/v6v81/index.jsp?topic=/com.ibm.vacpp6m.doc/language/ref/clrc03defst.htm What's means the sentence "In C, a structure member may be of any type except "function returning T" (for some type T)" Thanks for all the answers!

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  • C# newbie problem with variable types

    - by ile
    int newWidth = 100; int newHeight = 100; double ratio = 0; if (img1.Width > img1.Height) { ratio = img1.Width / img1.Height; newHeight = (int)(newHeight / ratio); } else { ratio = img1.Height / img1.Width; newWidth = (int)(newWidth / ratio); } Image bmp1 = img1.GetThumbnailImage(newWidth, newHeight, null, IntPtr.Zero); bmp1.Save(Server.MapPath("~/Uploads/Photos/Thumbnails/") + photo.PhotoID + ".jpg"); I always get Image with both height and width having same values (100) I am obiously doing something wrong with type conversion?

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  • Data types for validation

    - by nevalu
    How to create a new data type which to can check/validate its schema when is created a new variable (of that type)? By example, to validate if a string has 20 characters, I tried: {{{ // Format: 2006-01-12T06:06:06Z func date(str string) { if len(str) != 20 { fmt.Println("error") } } var Date = date() type Account struct { domain string username string created Date } }}} but it faills because Date is not a type.

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