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  • Explain Type Classes in Haskell

    - by Tsubasa Gomamoto
    Hi, I am a C++ / Java programmer and the main paradigm I happen to use in everyday programming is OOP. In some thread I read a comment that Type classes are more intuitive in nature than OOP. Can someone explain the concept of type classes in simple words so that an OOP guy like me can understand it?

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  • Using <input type=file /> with J2EE/MySQL Backend

    - by Mark Hazlett
    Hey everyone, I'm wondering how I can hook up an input type=file to send a picture back to a backend servlet that will eventually be stored in a MySQL database as a BLOB? In other words, how can I upload a picture using the input and send that back to the servlet to insert into the database as a BLOB type? Thanks

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  • Haskell data serialization of some data implementing a common type class

    - by Evan
    Let's start with the following data A = A String deriving Show data B = B String deriving Show class X a where spooge :: a -> Q [ Some implementations of X for A and B ] Now let's say we have custom implementations of show and read, named show' and read' respectively which utilize Show as a serialization mechanism. I want show' and read' to have types show' :: X a => a -> String read' :: X a => String -> a So I can do things like f :: String -> [Q] f d = map (\x -> spooge $ read' x) d Where data could have been [show' (A "foo"), show' (B "bar")] In summary, I wanna serialize stuff of various types which share a common typeclass so I can call their separate implementations on the deserialized stuff automatically. Now, I realize you could write some template haskell which would generate a wrapper type, like data XWrap = AWrap A | BWrap B deriving (Show) and serialize the wrapped type which would guarantee that the type info would be stored with it, and that we'd be able to get ourselves back at least an XWrap... but is there a better way using haskell ninja-ery? EDIT Okay I need to be more application specific. This is an API. Users will define their As, and Bs and fs as they see fit. I don't ever want them hacking through the rest of the code updating their XWraps, or switches or anything. The most i'm willing to compromise is one list somewhere of all the A, B, etc. in some format. Why? Here's the application. A is "Download a file from an FTP server." B is "convert from flac to mp3". A contains username, password, port, etc. information. B contains file path information. A and B are Xs, and Xs shall be called "Tickets." Q is IO (). Spooge is runTicket. I want to read the tickets off into their relevant data types and then write generic code that will runTicket on the stuff read' from the stuff on disk. At some point I have to jam type information into the serialized data.

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  • How to use input type="tel" in a contact form

    - by user1678664
    PHP newbie here trying to use a simple contact form. The fields I have are: "Name", "Email", "Phone", and "Message". I am trying to figure out what to do with the "Phone" field – I want it to output to the same place that "Message" does, so that when the email is received it will output both the message and the phone number to the body of the email. How do I do this? if( isset( $_POST['submit'] ) ) : wp_mail( get_option( 'admin_email' ), 'Website Contact Form', ( isset($_POST['message'] ) ? $_POST['message'] : '(blank)' ), 'From: ' . ( isset( $_POST['from_name'] ) ? $_POST['from_name'] : 'Website Contact Form' ) . ' <' . ( isset( $_POST['from_email'] ) ? $_POST['from_email'] : get_option( 'admin_email' ) ). '>' . "\r\n" ); $output = '<p>Thank you! Your message has been sent.</p>'; else : $output = ' <form action="" method="post"><ul> <li> <label for="from_name">Name</label> <input type="text" name="from_name" id="from_name" /> </li> <li> <label for="from_email">Email</label> <input type="email" name="from_email" id="from_email" /> </li> <li> <label for="from_tel">Phone</label> <input type="tel" name="from_tel" id="from_tel" /> </li> <li> <textarea name="message" id="message"></textarea> </li> <li class="submit"> <input name="submit" type="submit" value="Say Hello!" /> </li> </ul></form>'; endif; return $output;

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  • How to define type member constant in F# ?

    - by zproxy
    In C# one can define a type member constant like this: class Foo { public const int Bar = 600; } The IL shall looks like this. .field public static literal int32 Bar = int32(600) How can I do the same within Visual F# / FSharp? I tried this to no avail: [<Sealed>] type Foo() = [<Literal>] let Bar = 600

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  • MySql TABLE data type

    - by Neil Aitken
    MS SQL Server has a TABLE data type which can be used in stored procedures, Does anybody know if MySQL has an equivalent data type? I've had a look over the docs but can't seem to find anything, so I presume it doesn't exist, but perhaps somebody has created a workaround

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  • Interop type cannot be embedded

    - by Jan
    I am creating an web application on the .net 4.0 framework (beta2 ) in C#. When I try to use a assembly called "ActiveHomeScriptLib", I get the following error: Interop type 'ActiveHomeScriptLib.ActiveHomeClass' cannot be embedded. Use the applicable interface instead. When I change the framework to version 3.5, I don't have any errors. What is an Interop Type and why does this only occur when I use the 4.0 framework.

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  • C++: get const or non-const reference type from trait

    - by maciekp
    I am writing a functor F which takes function of type void (*func)(T) and func's argument arg. Then functor F calls func with arg. I would like F not to copy arg, just to pass it as reference. But then I cannot simply write "void F(void (*func)(T), T&)" because T could be a reference. So I am trying to write a trait, which allows to get proper reference type of T: T -> T& T& -> T& const T -> const T& const T& -> const T& I come up with something like this: template<typename T> struct type_op { typedef T& valid_ref_type; }; template<typename T> struct type_op<T&> { typedef typename type_op<T>::valid_ref_type valid_ref_type; }; template<typename T> struct type_op<const T> { typedef const T& valid_ref_type; }; template<typename T> struct type_op<const T&> { typedef const T& valid_ref_type; }; Which doesn't work for example for void a(int x) { std::cout << x << std::endl; } F(&a, 7); Giving error: invalid initialization of non-const reference of type ‘int&’ from a temporary of type ‘int’ in passing argument 2 of ‘void f(void (*)(T), typename type_op::valid_ref_type) [with T = int]’ How to get this trait to work?

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  • java mapping for oracle number type

    - by aauser
    I have database with oracle number type. Could there any possible issues with using java Double when updating table with numeric column. Should i switch BigDecimal or Double is approriate type. As I fas as I know, BigDecimal is used when arbitrary precision required, when deal with prices for example. Consider simple API updatePrice(Double d) Should i persist double as is or BigDecimal.valueOf(d) is preferable ?

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  • C# Type Casting at Runtimefor Array.SetValue

    - by sprocketonline
    I'm trying to create an array using reflection, and insert values into it. I'm trying to do this for many different types so would like a createAndFillArray function capable of this : Type t1 = typeof(A); Type t2 = typeof(B); double exampleA = 22.5; int exampleB = 43; Array arrA = createAndFillArray(t1, exampleA); Array arrB = createAndFillArray(t2, exampleB); private Array createAndFillArray(Type t, object val){ Array arr = Array.CreateInstance( t, 1); //length 1 in this example only, real-world is of variable length. arr.SetValue( val, 0 ); //this causes the following error: "System.InvalidCastException : Object cannot be stored in an array of this type." return arr; } with the class A being as follows: public class A{ public A(){} private double val; public double Value{ get{ return val; } set{ this.val = value; } } public static implicit operator A(double d){ A a = new A(); a.Value = d; return a; } } and class B being very similar, but with int: public class B{ public B(){} private double val; public double Value{ get{ return val; } set{ this.val = value; } } public static implicit operator B(double d){ B b = new B(); b.Value = d; return b; } } I hoped that the implicit operator would have ensured that the double be converted to class A, or the int to class B, and the error avoided; but this is obviously not so. The above is used in a custom deserialization class, which takes data from a custom data format and fills in the corresponding .Net object properties. I'm doing this via reflection and at runtime, so I think both are unavoidable. I'm targeting the C# 2.0 framework. I've dozens, if not hundreds, of classes similar to A and B, so would prefer to find a solution which improved on the createAndFillArray method rather than a solution which altered these classes.

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  • How to convert a PGresult to custom data type with libpq (PostgreSQL)

    - by mocopera
    Hi everyone! I'm using the libpq library in C to accessing my PostgreSQL database. So, when I do res = PQexec(conn, "SELECT point FROM test_point3d"); I don't know how to convert the PGresult I got to my custom data type. I know I can use the PQgetValue function, but again I don't know how to convert the returning string to my custom data type. Any suggestion? Thanks in advice.

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  • Nullable type conversion in C#?

    - by dinesh
    Hi can we assign null value to struct type of variable? struct MyStruct { } MyStruct var = null; is this is possible in C# .net? if not ? then how C# is allowing Nullable < T struct type of variable can be assigned as null?

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  • F# compilation error: Unexpected type application

    - by Jim Burger
    In F#, given the following class: type Foo() = member this.Bar<'t> (arg0:string) = ignore() Why does the following compile: let f = new Foo() f.Bar<Int32> "string" While the following won't compile: let f = new Foo() "string" |> f.Bar<Int32> //The compiler returns the error: "Unexpected type application"

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  • Hook perm for more than one content type

    - by Andrew
    Drupal 6.x I have this module that manages four different content types. For that matter, how do I define permission for each content within the same module? Is that even possible? I can't figure out how to define permission for each content type cuz hook_perm has to be named with module name and it doesn't have any argument(like hook_access $node) to return permission base on content type. Any help would be highly appreciated.

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  • converting an interface to a type in MVC

    - by danielovich
    Hi. I have a small isuse regarding an interface. Consider this code: [HttpPost()] public void Update(IAuctionItem item) { RedirectToAction("List"); } Whenever I call this I get an exception saying I can't create an instance of type which i totally correct. But is there a way of telling what the interface should map to without actually using the concrete type ?

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  • Get drive type with SetupDiGetDeviceRegistryProperty

    - by new
    Dear all, I would like to know whether i can get the drive information using the SP_DEVICE_INTERFACE_DETAIL_DATA's DevicePath my device path looks like below "\?\usb#vid_04f2&pid_0111#5&39fe81e&0&2#{a5dcbf10-6530-11d2-901f-00c04fb951ed}" also please tell me in the winapi they say "To determine whether a drive is a USB-type drive, call SetupDiGetDeviceRegistryProperty and specify the SPDRP_REMOVAL_POLICY property." i too use SetupDiGetDeviceRegistryProperty like below while ( !SetupDiGetDeviceRegistryProperty( hDevInfo,&DeviceInfoData, SPDRP_REMOVAL_POLICY,&DataT,( PBYTE )buffer,buffersize,&buffersize )) but i dont know how can i get the drive type using the above.. Please help me up

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  • Reflection problem - Type Safety Warning

    - by jax
    Class<? extends Algorithm> alg = AlgorithmAllFrom9AndLastFrom10Impl.class Constructor<Algorithm> c = alg.getConstructors()[0]; For "alg.getConstructors()[0];" I am getting a warning in eclipse Type safety: The expression of type Constructor needs unchecked conversion to conform to Constructor How do I fix this?

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  • Get derived class type from a base's class static method

    - by Marco Bettiolo
    Hi, i would like to get the type of the derived class from a static method of its base class. How can this be accomplished? Thanks! class BaseClass { static void Ping () { Type t = this.GetType(); // should be DerivedClass, but it is not possible with a static method } } class DerivedClass : BaseClass {} // somewhere in the code DerivedClass.Ping();

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