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• Is there an imperative language with a Haskell-like type system?

  - by Graham Kaemmer

  I've tried to learn Haskell a few times over the last few years, and, maybe because I know mainly scripting languages, the functional-ness of it has always bothered me (monads seem like a huge mess for doing lots of I/O). However, I think it's type system is perfect. Reading through a guide to Haskell's types and typeclasses (like this), I don't really see a reason why they would require a functional language, and furthermore, they seem like they would be perfect for an industry-grade object-oriented language (like Java). This all begs the question: has anyone ever taken Haskell's typing system and made a imperative, OOP language with it? If so, I want to use it.

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• data validation on wpf passwordbox:type password, re-type password

  - by black sensei

  Hello Experts !! I've built a wpf windows application in with there is a registration form. Using IDataErrorInfo i could successfully bind the field to a class property and with the help of styles display meaningful information about the error to users.About the submit button i use the MultiDataTrigger with conditions (Based on a post here on stackoverflow).All went well. Now i need to do the same for the passwordbox and apparently it's not as straight forward.I found on wpftutorial an article and gave it a try but for some reason it wasn't working. i've tried another one from functionalfun. And in this Functionalfun case the properties(databind,databound) are not recognized as dependencyproperty even after i've changed their name as suggested somewhere in the comments plus i don't have an idea whether it will work for a windows application, since it's designed for web. to give you an idea here is some code on textboxes <Window.Resources> <data:General x:Key="recharge" /> <Style x:Key="validButton" TargetType="{x:Type Button}" BasedOn="{StaticResource {x:Type Button}}" > <Setter Property="IsEnabled" Value="False"/> <Style.Triggers> <MultiDataTrigger> <MultiDataTrigger.Conditions> <Condition Binding="{Binding ElementName=txtRecharge, Path=(Validation.HasError)}" Value="false" /> </MultiDataTrigger.Conditions> <Setter Property="IsEnabled" Value="True" /> </MultiDataTrigger> </Style.Triggers> </Style> <Style x:Key="txtboxerrors" TargetType="{x:Type TextBox}" BasedOn="{StaticResource {x:Type TextBox}}"> <Style.Triggers> <Trigger Property="Validation.HasError" Value="true"> <Setter Property="ToolTip" Value="{Binding RelativeSource={RelativeSource Self}, Path=(Validation.Errors)[0].ErrorContent}"/> <Setter Property="Validation.ErrorTemplate"> <Setter.Value> <ControlTemplate> <DockPanel LastChildFill="True"> <TextBlock DockPanel.Dock="Bottom" FontSize="8" FontWeight="ExtraBold" Foreground="red" Padding="5 0 0 0" Text="{Binding ElementName=showerror, Path=AdornedElement.(Validation.Errors)[0].ErrorContent}"></TextBlock> <Border BorderBrush="Red" BorderThickness="2"> <AdornedElementPlaceholder Name="showerror" /> </Border> </DockPanel> </ControlTemplate> </Setter.Value> </Setter> </Trigger> </Style.Triggers> </Style> </Window.Resources> <TextBox Margin="12,69,12,70" Name="txtRecharge" Style="{StaticResource txtboxerrors}"> <TextBox.Text> <Binding Path="Field" Source="{StaticResource recharge}" ValidatesOnDataErrors="True" UpdateSourceTrigger="PropertyChanged"> <Binding.ValidationRules> <ExceptionValidationRule /> </Binding.ValidationRules> </Binding> </TextBox.Text> </TextBox> <Button Height="23" Margin="98,0,0,12" Name="btnRecharge" VerticalAlignment="Bottom" Click="btnRecharge_Click" HorizontalAlignment="Left" Width="75" Style="{StaticResource validButton}">Recharge</Button> some C# : class General : IDataErrorInfo { private string _field; public string this[string columnName] { get { string result = null; if(columnName == "Field") { if(Util.NullOrEmtpyCheck(this._field)) { result = "Field cannot be Empty"; } } return result; } } public string Error { get { return null; } } public string Field { get { return _field; } set { _field = value; } } } So what are suggestion you guys have for me? I mean how would you go about this? how do you do this since the databinding first purpose here is not to load data onto the fields they are just (for now) for data validation. thanks for reading this.

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• Java conditional operator ?: result type

  - by Wangnick

  I'm a bit puzzled about the conditional operator. Consider the following two lines: Float f1 = false? 1.0f: null; Float f2 = false? 1.0f: false? 1.0f: null; Why does f1 become null and the second statement throws a NullPointerException? Langspec-3.0 para 15.25 sais: Otherwise, the second and third operands are of types S1 and S2 respectively. Let T1 be the type that results from applying boxing conversion to S1, and let T2 be the type that results from applying boxing conversion to S2. The type of the conditional expression is the result of applying capture conversion (§5.1.10) to lub(T1, T2) (§15.12.2.7). So for false?1.0f:null T1 is Float and T2 is the null type. But what is the result of lub(T1,T2)? This para 15.12.2.7 is just a bit too much ... BTW, I'm using 1.6.0_18 on Windows. PS: I know that Float f2 = false? (Float) 1.0f: false? (Float) 1.0f: null; doesn't throw NPE.

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• Binding type variables that only occur in assertions

  - by Giuseppe Maggiore

  Hi! I find it extremely difficult to describe my problem, so here goes nothing: I have a bunch of assertions on the type of a function. These assertions rely on a type variable that is not used for any parameter of the function, but is only used for internal bindings. Whenever I use this function it does not compile because, of course, the compiler has no information from which to guess what type to bind my type variable. Here is the code: {-# LANGUAGE MultiParamTypeClasses, FunctionalDependencies, FlexibleInstances, UndecidableInstances, FlexibleContexts, EmptyDataDecls, ScopedTypeVariables, TypeOperators, TypeSynonymInstances #-} class C a a' where convert :: a -> a' class F a b where apply :: a -> b class S s a where select :: s -> a data CInt = CInt Int instance S (Int,String) Int where select (i,_) = i instance F Int CInt where apply = CInt f :: forall s a b . (S s a, F a b) => s -> b f s = let v = select s :: a y = apply v :: b in y x :: Int x = f (10,"Pippo") And here is the generated error: FunctorsProblems.hs:21:4: No instances for (F a Int, S (t, [Char]) a) arising from a use of `f' at FunctorsProblems.hs:21:4-17 Possible fix: add an instance declaration for (F a Int, S (t, [Char]) a) In the expression: f (10, "Pippo") In the definition of `x': x = f (10, "Pippo") Failed, modules loaded: none. Prelude>

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• Type classe, generic memoization

  - by nicolas

  Something quite odd is happening with y types and I quite dont understand if this is justified or not. I would tend to think not. This code works fine : type DictionarySingleton private () = static let mutable instance = Dictionary<string*obj, obj>() static member Instance = instance let memoize (f:'a -> 'b) = fun (x:'a) -> let key = f.ToString(), (x :> obj) if (DictionarySingleton.Instance).ContainsKey(key) then let r = (DictionarySingleton.Instance).[key] r :?> 'b else let res = f x (DictionarySingleton.Instance).[key] <- (res :> obj) res And this ones complains type DictionarySingleton private () = static let mutable instance = Dictionary<string*obj, _>() static member Instance = instance let memoize (f:'a -> 'b) = fun (x:'a) -> let key = f.ToString(), (x :> obj) if (DictionarySingleton.Instance).ContainsKey(key) then let r = (DictionarySingleton.Instance).[key] r :?> 'b else let res = f x (DictionarySingleton.Instance).[key] <- (res :> obj) res The difference is only the underscore in the dictionary definition. The infered types are the same, but the dynamic cast from r to type 'b exhibits an error. 'this runtime coercition ... runtime type tests are not allowed on some types, etc..' Am I missing something or is it a rough edge ?

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• Best way to store data in database when you don't know the type

  - by stiank81

  I have a table in my database that represents datafields in a custom form. The DataField gives some representation of what kind of control it should be represented with, and what value type it should take. Simplified you can say that I have 2 entities in this table - Textbox taking any string and Textbox only taking numbers. Now I have the different values stored in a separate table, referencing the datafield definition. What is the best way to store the data value here, when the type differs? One possible solution is to have the FieldValue table hold one field per possible value type. Now this would certainly be redundant, but at least I would get the value stored in its correct form - simplifying queries later. FieldValue ---------- Id DataFieldId IntValue DoubleValue BoolValue DataValue .. Another possibility is just storing everything as String, and casting this in the queries. I am using .Net with NHibernate, and I see that at least here there is a Projections.Cast that can be used to cast e.g. string to int in the query. Either way in these two solutions I need to know which type to use when doing the query, but I will know that from the DataField, so that won't be a problem. Anyway; I don't think any of these solutions sounds good. Are they? Or is there a better way?

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• A more concise example that illustrates that type inference can be very costly?

  - by mrrusof

  It was brought to my attention that the cost of type inference in a functional language like OCaml can be very high. The claim is that there is a sequence of expressions such that for each expression the length of the corresponding type is exponential on the length of the expression. I devised the sequence below. My question is: do you know of a sequence with more concise expressions that achieves the same types? # fun a -> a;; - : 'a -> 'a = <fun> # fun b a -> b a;; - : ('a -> 'b) -> 'a -> 'b = <fun> # fun c b a -> c b (b a);; - : (('a -> 'b) -> 'b -> 'c) -> ('a -> 'b) -> 'a -> 'c = <fun> # fun d c b a -> d c b (c b (b a));; - : ((('a -> 'b) -> 'b -> 'c) -> ('a -> 'b) -> 'c -> 'd) -> (('a -> 'b) -> 'b -> 'c) -> ('a -> 'b) -> 'a -> 'd = <fun> # fun e d c b a -> e d c b (d c b (c b (b a)));; - : (((('a -> 'b) -> 'b -> 'c) -> ('a -> 'b) -> 'c -> 'd) -> (('a -> 'b) -> 'b -> 'c) -> ('a -> 'b) -> 'd -> 'e) -> ((('a -> 'b) -> 'b -> 'c) -> ('a -> 'b) -> 'c -> 'd) -> (('a -> 'b) -> 'b -> 'c) -> ('a -> 'b) -> 'a -> 'e = <fun> # fun f e d c b a -> f e d c b (e d c b (d c b (c b (b a))));; - : ((((('a -> 'b) -> 'b -> 'c) -> ('a -> 'b) -> 'c -> 'd) -> (('a -> 'b) -> 'b -> 'c) -> ('a -> 'b) -> 'd -> 'e) -> ((('a -> 'b) -> 'b -> 'c) -> ('a -> 'b) -> 'c -> 'd) -> (('a -> 'b) -> 'b -> 'c) -> ('a -> 'b) -> 'e -> 'f) -> (((('a -> 'b) -> 'b -> 'c) -> ('a -> 'b) -> 'c -> 'd) -> (('a -> 'b) -> 'b -> 'c) -> ('a -> 'b) -> 'd -> 'e) -> ((('a -> 'b) -> 'b -> 'c) -> ('a -> 'b) -> 'c -> 'd) -> (('a -> 'b) -> 'b -> 'c) -> ('a -> 'b) -> 'a -> 'f = <fun>

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• fatal error LNK1112: module machine type 'X86' conflicts with target machine type 'AMD64'

  - by KK

  Hi, I am using VS 2003 .Net on 32 bit XP OS. I have also installed "Microsoft Platform SDK" on my machine. Can I build vc++ application (binaries) targeted for 64 bit OS? I am using following project options : Name="VCLinkerTool" AdditionalOptions="/machine:AMD64 bufferoverflowU.lib" OutputFile="\bin\Release\MM64.dll" LinkIncremental="1" SuppressStartupBanner="TRUE" AdditionalLibraryDirectories="&quot;C:\Program Files\Microsoft Platform SDK\Lib\AMD64&quot;" GenerateDebugInformation="TRUE" ProgramDatabaseFile="\bin\Release\MM64.pdb" GenerateMapFile="TRUE" MapFileName="\bin\Release\MM64.map" MapExports="TRUE" MapLines="TRUE" OptimizeReferences="2" EnableCOMDATFolding="2" ImportLibrary=".\Release/MM64.lib" TargetMachine="0"/> I am getting following error: fatal error LNK1112: module machine type 'X86' conflicts with target machine type 'AMD64' Do I need to build project on 64 bit OS or I need to change project settings to resolve this error. Please help me to resolve this issue.

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• cannot convert from 'cli::array<Type> ^' to 'cli::array<Type> ^[]'

  - by user1576628

  I'm pretty new to C++/CLI and I am trying to convert a System::String to a System::Char array. Here's what I have so far: private: System::Void modeToolStripMenuItem_Click(System::Object^ sender, System::EventArgs^ e) { Mode frmMode; if(frmMode.ShowDialog() == System::Windows::Forms::DialogResult::OK){ array <Char>^ load [] = gcnew array<Char>(txtbxName->Text->ToCharArray()); } } txtbxName is a textbox inside a the form. Supposedly, this should work, but I get the compiler error: error C2440: cannot convert from 'cli::array<Type> ^' to 'cli::array<Type> ^[]' for the fourth line of the snippet.

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• Unable to cast object of type 'System.Object[]' to type 'System.String[]'

  - by salvationishere

  I am developing a C# VS 2008 / SQL Server website application. I am a newbie to ASP.NET. I am getting the above error, however, on the last line of the following code. Can you give me advice on how to fix this? This compiles correctly, but I encounter this error after running it. DataTable dt; Hashtable ht; string[] SingleRow; ... SqlConnection conn2 = new SqlConnection(connString); SqlCommand cmd = conn2.CreateCommand(); cmd.CommandText = "dbo.AppendDataCT"; cmd.Connection = conn2; SingleRow = (string[])dt.Rows[1].ItemArray; My error: System.InvalidCastException was caught Message="Unable to cast object of type 'System.Object[]' to type 'System.String[]'." Source="App_Code.g68pyuml" StackTrace: at ADONET_namespace.ADONET_methods.AppendDataCT(DataTable dt, Hashtable ht) in c:\Documents and Settings\Admin\My Documents\Visual Studio 2008\WebSites\Jerry\App_Code\ADONET methods.cs:line 88 InnerException:

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• Repeater itemdatabound event value type and reference type

  - by Sune

  Im trying to bind a list with datetime objects to my repeater. if (e.Item.ItemType == ListItemType.AlternatingItem || e.Item.ItemType == ListItemType.Item) { DateTime actualDate = e.Item.DataItem as DateTime; } When I want access the itemdatabound event on the repeater Then I get an errormessage which says that DateTime is a valuetype and not a reference type. My solution is that a wrap the datetime in a custom object (reference type) and pass that to the repeater datasource instead of the datetime. But Im wondering if there are other solutions where the repeater takes valuetypes (DateTime objects)........

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• Haskell: type inference and function composition

  - by Pillsy

  This question was inspired by this answer to another question, indicating that you can remove every occurrence of an element from a list using a function defined as: removeall = filter . (/=) Working it out with pencil and paper from the types of filter, (/=) and (.), the function has a type of removeall :: (Eq a) => a -> [a] -> [a] which is exactly what you'd expect based on its contract. However, with GHCi 6.6, I get gchi> :t removeall removeall :: Integer -> [Integer] -> [Integer] unless I specify the type explicitly (in which case it works fine). Why is Haskell inferring such a specific type for the function?

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• Get subclass type from projection with NHibernate

  - by TigerShark

  Hi I am trying to do a projection on a type called Log. Log references a superclass called Company. Each company type is mapped with table per subclass. Is it possible for me to get the type of Company when I do a projection on Log? I currently have an Enum property (which is not mapped) on each subclass, so I can perform switches on Company types, but since it is not mapped to anything, I can't do a projection on it. I have tried Projections.Property("log.Company.class") but that does not work :( PS: I couldn't find a lot of appropriate tags for this question. If anyone have an idea for more specific tags, please tell me.

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• What type of data is this JavaScript code!?

  - by SolidSnakeGTI

  Hello, Well, I'm completely new to JavaScript. Can you please tell me what type of data is this JavaScript code: var options = { sourceLanguage: 'en', destinationLanguage: ['hi', 'bn', 'fa', 'gu', 'kn', 'ml', 'mr', 'ne', 'pa', 'ta','te','ur'], shortcutKey: 'ctrl+g', transliterationEnabled: true }; I've reviewed JavaScript arrays, but it doesn't seem to be a traditional array. Still don't know if it's some kind of arrays or another data type!! Additionally, is there any way to set individual elements to that data type such as setting array elements individually. Thanks in advance

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• C++ min heap with user-defined type.

  - by bsg

  Hi, I am trying to implement a min heap in c++ for a struct type that I created. I created a vector of the type, but it crashed when I used make_heap on it, which is understandable because it doesn't know how to compare the items in the heap. How do I create a min-heap (that is, the top element is always the smallest one in the heap) for a struct type? The struct is below: struct DOC{ int docid; double rank; }; I want to compare the DOC structures using the rank member. How would I do this? I tried using a priority queue with a comparator class, but that also crashed, and it also seems silly to use a data structure which uses a heap as its underlying basis when what I really need is a heap anyway. Thank you very much, bsg

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• Resolving type parameter values passed to ancester type using reflection

  - by Tom Tucker

  I've asked a similar question before, but this one is much more challenging. How do I find a value of a specific type parameter that is passed to an ancestor class or an interface implemented by one of its ancestor classes using reflection? I basically need to write a method that looks like this. // Return the value of the type parameter at the index passed to the parameterizedClass from the clazz. Object getParameterValue(Class<?> clazz, Class<?> parameterizedClass, int index) For the example below, getParameterValue(MyClass.class, Map.class, 1) would return String.class public class Foo<K, V> implements Map<K, V>{ } public class Bar<V> extends Foo<Integer, V> { } public class MyClass extends Bar<String> { } Thanks!

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• Template type deduction with a non-copyable class

  - by Evan Teran

  Suppose I have an autolocker class which looks something like this: template <T> class autolocker { public: autolocker(T *l) : lock(l) { lock->lock(); } ~autolocker() { lock->unlock(); } private: autolocker(const autolocker&); autolocker& operator=(const autolocker&); private: T *lock; } Obviously the goal is to be able to use this autolocker with anything that has a lock/unlock method without resorting to virtual functions. Currently, it's simple enough to use like this: autolocker<some_lock_t> lock(&my_lock); // my_lock is of type "some_lock_t" but it is illegal to do: autolocker lock(&my_lock); // this would be ideal Is there anyway to get template type deduction to play nice with this (keep in my autolocker is non-copyable). Or is it just easiest to just specify the type?

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• implementing type inference

  - by deepblue

  well I see some interesting discussions here about static vs. dynamic typing I generally prefer static typing, due to compile type checking, better documented code,etc. However I do agree that they do clutter up the code if done the way Java does it, for example. so Im about to start building a language of my own and type inference is one of the things that I want to implement, in a functional style language... I do understand that it is a big subject, and Im not trying to create something that has not been done before, just basic inferencing... any pointers on what to read up that will help me with this? preferably something more pragmatic/practical as oppose to more theoretical category theory/type theory texts. If there's a implementation discussion text out here, with data structures/algorithms, that would just be lovely much appreciated

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• Scala: "Parameter type in structural refinement may not refer to an abstract type defined outside th

  - by raichoo

  Hi, I'm having a problem with scala generics. While the first function I defined here seems to be perfectly ok, the compiler complains about the second definition with: error: Parameter type in structural refinement may not refer to an abstract type defined outside that refinement def >>[B](a: C[B])(implicit m: Monad[C]): C[B] = { ^ What am I doing wrong here? trait Lifter[C[_]] { implicit def liftToMonad[A](c: C[A]) = new { def >>=[B](f: A => C[B])(implicit m: Monad[C]): C[B] = { m >>= (c, f) } def >>[B](a: C[B])(implicit m: Monad[C]): C[B] = { m >> a } } } IMPORTANT: This is NOT a question about Monads, it's a question about scala polymorphism in general. Regards, raichoo

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• C# Generic method type argument inference

  - by CaptainCasey

  Is there any way that I can generalise the type definitions here? Ideally, I'd like to be able to change the type of 'testInput' and have test correctly infer the type at compile time. public static void Run() { var testInput = 3; var test = ((Func<int, int>) Identity).Compose<int,int,int>(n => n)(testInput); Console.WriteLine(test); } public static Func<T, V> Compose<T, U, V>(this Func<U, V> f, Func<T, U> g) { return x => f(g(x)); } public static T Identity<T> (this T value) { return value; }

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• In C#: How to declare a generic Dictionary with a type as key and an enumeration of that type as val

  - by Marcel

  Hi all, I want to declare a dictionary that stores typed IEnumerable's of a specific type, with that exact type as key, like so: (Edited to follow johny g's comment) private IDictionary<Type, IEnumerable<T>> _dataOfType where T: BaseClass; //does not compile! The concrete classes I want to store, all derive from BaseClass, therefore the idea to use it as constraint. The compiler complains that it expects a semicolon after the member name. If it would work, I would expect this would make the later retrieval from the dictionary simple like: IEnumerable<ConcreteData> concreteData; _sitesOfType.TryGetValue(typeof(ConcreteType), out concreteData); How to define such a dictionary?

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• How do I determine if a value is an instance of a generic type, ignoring the type parameter, in vb.n

  - by Strilanc

  I have a class C(Of T). I want to determine if some given value has type C, regardless of what T is. For example, I might want to determine if a value is a strongly-typed list, regardless what type of items the list stores. I just need to know how to do it in VB.net. In C# the syntax is like this: var result = obj instanceof Gen2<?>;

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• Returned content type - Restlet

  - by DutrowLLC

  How do you set the content type in Restlet? In this case, I'd like to set the content type to ""text/xml". I have: public class SubResource extends ServerResource { @Get public Representation get(Representation representation){ setStatus(Status.SUCCESS_OK); StringRepresentation sr = new StringRepresentation(getSomeXml()); return sr; } } I'm unsure even if it is a value that is set in the Representation, or if it is set from the ServerResource class the same way that the return code is.

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• PHP Equivalent of Java Type-Casting Solution

  - by stjowa

  Since PHP has no custom-class type-casting, how would I go about doing the PHP equivalent of this Java code: CustomBaseObject cusBaseObject = cusBaseObjectDao.readCustomBaseObjectById(id); ((CustomChildObject) cusBaseObject).setChildAttribute1(value1); ((CustomChildObject) cusBaseObject).setChildAttribute2(value2); In my case, it would very nice if I could do this. However, trying this without type-casting support, it gives me an error that the methods do not exist for the object. Thanks, Steve

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• Making JSP page not set the response content-type

  - by doublep

  Is it possible to make JSP pages not set any content type on response? In my setup, JSP doesn't directly generate the response, but rather an intermediate presentation, which is then processed by additional Java code that creates HTML or JSON based on that. So, can I somehow make JSP not set content-type on the response and leave it to the intermediate code? If I just remove contentType="..." in a JSP, it still defaults to text/html.

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