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  • Render Html from a lambda in MVC

    - by Thad
    I have the following code to generate a list and will allow developers to customize the output if needed. <% Html.List<MyList>(item => item.Property).Value(item => return "<div>" + item.Property + "<br/>" + item.AnotherProperty + "</div>").Render() %> This is not ideal, how can I allow the developers to add the html similar to other controls. <% Html.List<MyList>(item => item.Property).Value(item => %> <div><%=item.Property%><br/><%=item.AnotherProperty%></div><%).Render() %> This way is much cleaner and standard with the rest of mvc.

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  • Assign to a slice of a Python list from a lambda

    - by Bushman
    I know that there are certain "special" methods of various objects that represent operations that would normally be performed with operators (i.e. int.__add__ for +, object.__eq__ for ==, etc.), and that one of them is list.__setitem, which can assign a value to a list element. However, I need a function that can assign a list into a slice of another list. Basically, I'm looking for the expression equivalent of some_list[2:4] = [2, 3].

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  • lambda expression for a query on two tables that returns records from one table

    - by peetee
    I have two tables TableA (articles) int id int Type string name and TableB (compatibles) int linked_ID int tableA_ID TableA records: id=1, Type=0, name="ArticleA" id=2, Type=1, name="ArticleB" id=3, Type=2, name="ArticleC" id=4, Type=1, name="ArticleD" TableB records: linked_ID= 1, tableA_ID=2 linked_ID= 1, tableA_ID=3 linked_ID= 1, tableA_ID=4 TableB has a list of arcicels that are compatible to a certain article. I am quite new to queries (didn't need them in my projects yet). But as C# and WPF allow some pretty cool automation with Binding I would like to add a binding that returns the following: Give me all articles that are of Type 1 and compatible to my selected article (id=1). The simple part of it works well (articles has a list of all articles): private ObservableCollection<Article> _articles = new ObservableCollection<Article>(); [fill it with the available articles] and then: comboBoxArticles.ItemsSource = _articles.AsBindable().Where( c => c.Typ == 0 ); How can I extend the Where clause to query another table? Thanks a lot in advance.

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  • Need help with this basic Contains<>() extension method and Lambda expressions

    - by Polaris878
    Hi, Say I have the following class: class Foo { // ctor etc here public string Bar { get; } } Now, I have a LinkedList of Foos declared like so: LinkedList<Foo> How would I write a basic Contains<() for this? I want to be able to do this: Foo foo = new Foo(someString); LinkedList<Foo> list = new LinkedList<foo>(); // Populate list with Foos bool contains = list.Contains<Foo>(foo, (x => foo.Bar == x.Bar)); Am I trying to do this correctly? Thanks

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  • Converting ObsevableCollection foreach to lambda.

    - by Jitendra Jadav
    Hello Guys, I am working some ObservableCollection converting to lembda it will give me error this is my actual code . foreach (var item in Query) { userDetail.Add(new UserDatail(item.ID,item.Name, item.Address, item.City, item.Pin, item.Phone)); } and I am try to add as lembda Query.ToList().ForEach(x => userDetail.Add(x.ID,x.Name,x.Address,x.City,x.Pin,x.Phone)); This will give me error. Thanks..

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  • Convert C# Lambda to vb.net

    - by Joven
    Need Help in converting this to VB.NET public void GetCustomers(Action<IEnumerable<Customer>> onSuccess, Action<Exception> onFail) { Manager.Customers.ExecuteAsync(op => { if (op.CompletedSuccessfully) { if (onSuccess != null) onSuccess(op.Results); } else { if (onFail != null) { op.MarkErrorAsHandled(); onFail(op.Error); } } } ); }

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  • How to pass a Lambda Expression as method parameter with EF

    - by Registered User
    How do I pass an EF expression as a method argument? To illustrate my question I have created a pseudo code example: The first example is my method today. The example utilizes EF and a Fancy Retry Logic. What I need to do is to encapsulate the Fancy Retry Logic so that it becomes more generic and does not duplicate. In the second example is how I want it to be, with a helper method that accepts the EF expression as an argument. This would be a trivial thing to do with SQL, but I want to do it with EF so that I can benefit from the strongly typed objects. First Example: public static User GetUser(String userEmail) { using (MyEntities dataModel = new MyEntities ()) { var query = FancyRetryLogic(() => { (dataModel.Users.FirstOrDefault<User>(x => x.UserEmail == userEmail))); }); return query; } } Second Example: T RetryHelper<T>(Expression<Func<T, TValue>> expression) { using (MyEntities dataModel = new (MyEntities ()) { var query = FancyRetryLogic(() => { return dataModel.expression }); } } public User GetUser(String userEmail) { return RetryHelper<User>(<User>.FirstOrDefault<User>(x => x.UserEmail == userEmail)) }

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  • Restrictons of Python compared to Ruby: lambda's

    - by Shyam
    Hi, I was going over some pages from WikiVS, that I quote from: because lambdas in Python are restricted to expressions and cannot contain statements I would like to know what would be a good example (or more) where this restriction would be, preferably compared to the Ruby language. Thank you for your answers, comments and feedback!

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  • Nested for_each with lambda not possible?

    - by Ela782
    The following code does not compile in VS2012, it gives error C2064: term does not evaluate to a function taking 1 arguments on the line of the second for_each (line 4 below). vector<string> v1; for_each(begin(v1), end(v1), [](string s1) { vector<string> v2; for_each(begin(v2), end(v2), [](string s2) { cout << "..."; }); }); I found some related stuff like http://connect.microsoft.com/VisualStudio/feedback/details/560907/capturing-variables-in-nested-lambdas which shows a bug (they are doing something different) but on the other hand that shows that what I print above should be possible. What's wrong with the above code?

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  • Extract information from a Func<bool, T> or alike lambda

    - by Syska
    I''m trying to build a generic cache layer. ICacheRepository Say I have the following: public class Person { public int PersonId { get; set; } public string Firstname { get; set; } public string Lastname { get; set; } public DateTime Added { get; set; } } And I have something like this: list.Where(x => x.Firstname == "Syska"); Here I want to extract the above information, to see if the query supplied the "PersonId" which it did not, so I dont want to cache it. But lets say I run a query like this: list.Where(x => x.PersonId == 10); Since PersonId is my key ... I want to cache it. with the key like "Person_10" and I later can fetch it from the cache. I know its possible to extract the information with Expression<Func<>> but there seems to be a big overhead of doing this (when running compile and extract the Constant values etc. and a bunch of cache to be sure to parse right) Are there a framework for this? Or some smart/golden way of doing this ?

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  • Lambda expression will not compile

    - by John Soer
    I am very confused. I have this lamba expression: tvPatientPrecriptionsEntities.Sort((p1, p2) => p1.MedicationStartDate .Value .CompareTo(p2.MedicationStartDate.Value)); Visual Studio will not compile it and complains about syntax. I converted the lamba expression to an anonymous delegate as so: tvPatientPrecriptionsEntities.Sort( delegate(PatientPrecriptionsEntity p1, PatientPrecriptionsEntity p2) { return p1.MedicationStartDate .Value .CompareTo(p2.MedicationStartDate.Value); }); and it works fine. The project uses .NET 3.5 and I have a reference to System.Linq.

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  • How to pass data to a C++0x lambda function that will run in a different thread?

    - by Dimitri C.
    In our company we've written a library function to call a function asynchronously in a separate thread. It works using a combination of inheritance and template magic. The client code looks as follows: DemoThread thread; std::string stringToPassByValue = "The string to pass by value"; AsyncCall(thread, &DemoThread::SomeFunction, stringToPassByValue); Since the introduction of lambda functions I'd like to use it in combination with lambda functions. I'd like to write the following client code: DemoThread thread; std::string stringToPassByValue = "The string to pass by value"; AsyncCall(thread, [=]() { const std::string someCopy = stringToPassByValue; }); Now, with the Visual C++ 2010 this code doesn't work. What happens is that the stringToPassByValue is not copied. Instead the "capture by value" feature passes the data by reference. The result is that if the function is executed after stringToPassByValue has gone out of scope, the application crashes as its destructor is called already. So I wonder: is it possible to pass data to a lambda function as a copy? Note: One possible solution would be to modify our framework to pass the data in the lambda parameter declaration list, as follows: DemoThread thread; std::string stringToPassByValue = "The string to pass by value"; AsyncCall(thread, [=](const std::string stringPassedByValue) { const std::string someCopy = stringPassedByValue; } , stringToPassByValue); However, this solution is so verbose that our original function pointer solution is both shorter and easier to read. Update: The full implementation of AsyncCall is too big to post here. In short, what happens is that the AsyncCall template function instantiates a template class holding the lambda function. This class is derived from a base class that contains a virtual Execute() function, and upon an AsyncCall() call, the function call class is put on a call queue. A different thread then executes the queued calls by calling the virtual Execute() function, which is polymorphically dispatched to the template class which then executes the lambda function.

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  • C# LINQ Where Predicate Type Arguments

    - by blu
    I have an XElement with values for mock data. I have an expression to query the xml: Expression<Func<XElement, bool>> simpleXmlFunction = b => int.Parse(b.Element("FooId").Value) == 12; used in: var simpleXml = xml.Elements("Foo").Where(simpleXmlFunction).First(); The design time error is: The type arguments for method 'System.Linq.Enumerable.Where(System.Collections.Generic.IEnumerable, System.Func)' cannot be inferred from the usage. Try specifying the type arguments explicitly' The delegate supplied to Where should take in an XElement and return a bool, marking if the item matches the query, I am not sure how to add anything more to the delegate or the where clause to mark the type. Also, the parallel method for the real function against the Entity Framework does not have this issue. What is not correct with the LINQ-to-XML version?

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  • LinqToXML why does my object go out of scope? Also should I be doing a group by?

    - by Kettenbach
    Hello All, I have an IEnumerable<someClass>. I need to transform it into XML. There is a property called 'ZoneId'. I need to write some XML based on this property, then I need some decendent elements that provide data relevant to the ZoneId. I know I need some type of grouping. Here's what I have attempted thus far without much success. **inventory is an IEnumerable<someClass>. So I query inventory for unique zones. This works ok. var zones = inventory.Select(c => new { ZoneID = c.ZoneId , ZoneName = c.ZoneName , Direction = c.Direction }).Distinct(); No I want to create xml based on zones and place. ***place is a property of 'someClass'. var xml = new XElement("MSG_StationInventoryList" , new XElement("StationInventory" , zones.Select(station => new XElement("station-id", station.ZoneID) , new XElement("station-name", station.ZoneName)))); This does not compile as "station" is out of scope when I try to add the "station-name" element. However is I remove the paren after 'ZoneId', station is in scope and I retreive the station-name. Only problem is the element is then a decendant of 'station-id'. This is not the desired output. They should be siblings. What am I doing wrong? Lastly after the "station-name" element, I will need another complex type which is a collection. Call it "places'. It will have child elements called "place". its data will come from the IEnumerable and I will only want "places" that have the "ZoneId" for the current zone. Can anyone point me in the right direction? Is it a mistake select distinct zones from the original IEnumerable? This object has all the data I need within it. I just need to make it heirarchical. Thanks for any pointers all. Cheers, Chris in San Diego

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  • How can i filter a list of objects using lamda expression?

    - by Colour Blend
    I know i shouldn't have id's with the same value. This is just fictitious, so overlook that. I have: List<Car> carList = new List<Car>(); carList.Add(new Car() { id = 1, name = "Honda" }); carList.Add(new Car() { id = 2, name = "Toyota" }); carList.Add(new Car() { id = 1, name = "Nissan" }); I want to use Lamda Expression to retreive all cars that have an id of 1. Anticipated Result: -- Id: 1, Name: Honda -- Id: 1, Name: Nissan The problem is more filtering an object list based on a foriegn key. Please help me.

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  • c++ class member functions selected by traits

    - by Jive Dadson
    I am reluctant to say I can't figure this out, but I can't figure this out. I've googled and searched stackoverflow, and come up empty. The abstract, and possibly overly vague form of the question is, how can I use the traits-pattern to instantiate non-virtual member functions? The question came up while modernizing a set of multivariate function optimizers that I wrote more than 10 years ago. The optimizers all operate by selecting a straight-line path through the parameter space away from the current best point (the "update"), then finding a better point on that line (the "line search"), then testing for the "done" condition, and if not done, iterating. There are different methods for doing the update, the line-search, and conceivably for the done test, and other things. Mix and match. Different update formulae require different state-variable data. For example, the LMQN update requires a vector, and the BFGS update requires a matrix. If evaluating gradients is cheap, the line-search should do so. If not, it should use function evaluations only. Some methods require more accurate line-searches than others. Those are just some examples. The original version instatiates several of the combinations by means of virtual functions. Some traits are selected by setting mode bits. Yuck. It would be trivial to define the traits with #define's and the member functions with #ifdef's and macros. But that's so twenty years ago. It bugs me that I cannot figure out a whiz-bang modern way. If there were only one trait that varied, I could use the curiously recurring template pattern. But I see no way to extend that to arbitrary combinations of traits. I tried doing it using boost::enable_if, etc.. The specialized state info was easy. I managed to get the functions done, but only by resorting to non-friend external functions that have the this-pointer as a parameter. I never even figured out how to make the functions friends, much less member functions. Perhaps tag-dispatch is the key. I haven't gotten very deeply into that. Surely it's possible, right? If so, what is best practice?

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  • c++ class member functions instatiated by traits

    - by Jive Dadson
    I am reluctant to say I can't figure this out, but I can't figure this out. I've googled and searched stackoverflow, and come up empty. The abstract, and possibly overly vague form of the question is, how can I use the traits-pattern to instantiate non-virtual member functions? The question came up while modernizing a set of multivariate function optimizers that I wrote more than 10 years ago. The optimizers all operate by selecting a straight-line path through the parameter space away from the current best point (the "update"), then finding a better point on that line (the "line search"), then testing for the "done" condition, and if not done, iterating. There are different methods for doing the update, the line-search, and conceivably for the done test, and other things. Mix and match. Different update formulae require different state-variable data. For example, the LMQN update requires a vector, and the BFGS update requires a matrix. If evaluating gradients is cheap, the line-search should do so. If not, it should use function evaluations only. Some methods require more accurate line-searches than others. Those are just some examples. The original version instantiates several of the combinations by means of virtual functions. Some traits are selected by setting mode bits that are tested at runtime. Yuck. It would be trivial to define the traits with #define's and the member functions with #ifdef's and macros. But that's so twenty years ago. It bugs me that I cannot figure out a whiz-bang modern way. If there were only one trait that varied, I could use the curiously recurring template pattern. But I see no way to extend that to arbitrary combinations of traits. I tried doing it using boost::enable_if, etc.. The specialized state info was easy. I managed to get the functions done, but only by resorting to non-friend external functions that have the this-pointer as a parameter. I never even figured out how to make the functions friends, much less member functions. The compiler (vc++ 2008) always complained that things didn't match. I would yell, "SFINAE, you moron!" but the moron is probably me. Perhaps tag-dispatch is the key. I haven't gotten very deeply into that. Surely it's possible, right? If so, what is best practice?

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  • C++ class member functions instantiated by traits

    - by Jive Dadson
    I am reluctant to say I can't figure this out, but I can't figure this out. I've googled and searched Stack Overflow, and come up empty. The abstract, and possibly overly vague form of the question is, how can I use the traits-pattern to instantiate non-virtual member functions? The question came up while modernizing a set of multivariate function optimizers that I wrote more than 10 years ago. The optimizers all operate by selecting a straight-line path through the parameter space away from the current best point (the "update"), then finding a better point on that line (the "line search"), then testing for the "done" condition, and if not done, iterating. There are different methods for doing the update, the line-search, and conceivably for the done test, and other things. Mix and match. Different update formulae require different state-variable data. For example, the LMQN update requires a vector, and the BFGS update requires a matrix. If evaluating gradients is cheap, the line-search should do so. If not, it should use function evaluations only. Some methods require more accurate line-searches than others. Those are just some examples. The original version instantiates several of the combinations by means of virtual functions. Some traits are selected by setting mode bits that are tested at runtime. Yuck. It would be trivial to define the traits with #define's and the member functions with #ifdef's and macros. But that's so twenty years ago. It bugs me that I cannot figure out a whiz-bang modern way. If there were only one trait that varied, I could use the curiously recurring template pattern. But I see no way to extend that to arbitrary combinations of traits. I tried doing it using boost::enable_if, etc.. The specialized state information was easy. I managed to get the functions done, but only by resorting to non-friend external functions that have the this-pointer as a parameter. I never even figured out how to make the functions friends, much less member functions. The compiler (VC++ 2008) always complained that things didn't match. I would yell, "SFINAE, you moron!" but the moron is probably me. Perhaps tag-dispatch is the key. I haven't gotten very deeply into that. Surely it's possible, right? If so, what is best practice?

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  • Lamda functions in php

    - by Oden
    Hey, Im really interested in the way of using lamda functions. Does it make sense to use them in a high-level programming language? If yes, why? Is this really just a function embedded in a function, (Like this) or is there more behind?

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  • trouble accessing non-static functions from static functions in AS3

    - by Dogmatixed
    I have a class containing, among other things, a drop down menu. With the aim of saving space, and since the contents of the menu will never change, I've made a static DataProvider for the whole class that populates each instances menu. I was hoping to populate the list with actual functions like so: tmpArr.push({label:"Details...", funct:openDetailsMenu, args:""}); and then assign tmpArr to the DataProvider. Because the DataProvider is static the function that contains that code also needs to be static, but the functions in the array are non-static. At first it didn't seem like a problem, because when the user clicks on a menu item the drop down menu can call a non-static "executeFunction(funct, args)" on its parent. However, when I try to compile, the static function setting up the DataProvider it can't find the non-static functions being passed. If the compiler would just trust me the code would work fine! The simple solution is to just pass strings and use a switch statement to call functions based on that, but that's big, ugly, inelegant, and difficult to maintain, especially if something inherits from this class. The simpler solution is to just make the DataProvider non-static, but I'm wondering if anyone else has a good way of dealing with this? Making the static function able to see its non-static brethren? Thanks.

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  • Usefulness of Toggle functions

    - by roygbiv
    Is it better to write functions that explicitly do something (i.e. HideForm/ShowForm etc...) or is it better to write 'Toggle' type functions (i.e. ToggleVisibility)? I find Toggle type functions awkard because it's hard to track the state by reading the code. In what situations is a toggle type function useful?

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  • boost::function & boost::lambda again

    - by John Dibling
    Follow-up to post: http://stackoverflow.com/questions/2978096/using-width-precision-specifiers-with-boostformat I'm trying to use boost::function to create a function that uses lambdas to format a string with boost::format. Ultimately what I'm trying to achieve is using width & precision specifiers for strings with format. boost::format does not support the use of the * width & precision specifiers, as indicated in the docs: Width or precision set to asterisk (*) are used by printf to read this field from an argument. e.g. printf("%1$d:%2$.*3$d:%4$.*3$d\n", hour, min, precision, sec); This class does not support this mechanism for now. so such precision or width fields are quietly ignored by the parsing. so I'm trying to find other ways to accomplish the same goal. Here is what I have so far, which isn't working: #include <string> #include <boost\function.hpp> #include <boost\lambda\lambda.hpp> #include <iostream> #include <boost\format.hpp> #include <iomanip> #include <boost\bind.hpp> int main() { using namespace boost::lambda; using namespace std; boost::function<std::string(int, std::string)> f = (boost::format("%s") % boost::io::group(setw(_1*2), setprecision(_2*2), _3)).str(); std::string s = (boost::format("%s") % f(15, "Hello")).str(); return 0; } This generates many compiler errors: 1>------ Build started: Project: hacks, Configuration: Debug x64 ------ 1>Compiling... 1>main.cpp 1>.\main.cpp(15) : error C2872: '_1' : ambiguous symbol 1> could be 'D:\Program Files (x86)\boost\boost_1_42\boost/lambda/core.hpp(69) : boost::lambda::placeholder1_type &boost::lambda::`anonymous-namespace'::_1' 1> or 'D:\Program Files (x86)\boost\boost_1_42\boost/bind/placeholders.hpp(43) : boost::arg<I> `anonymous-namespace'::_1' 1> with 1> [ 1> I=1 1> ] 1>.\main.cpp(15) : error C2664: 'std::setw' : cannot convert parameter 1 from 'boost::lambda::placeholder1_type' to 'std::streamsize' 1> No user-defined-conversion operator available that can perform this conversion, or the operator cannot be called 1>.\main.cpp(15) : error C2872: '_2' : ambiguous symbol 1> could be 'D:\Program Files (x86)\boost\boost_1_42\boost/lambda/core.hpp(70) : boost::lambda::placeholder2_type &boost::lambda::`anonymous-namespace'::_2' 1> or 'D:\Program Files (x86)\boost\boost_1_42\boost/bind/placeholders.hpp(44) : boost::arg<I> `anonymous-namespace'::_2' 1> with 1> [ 1> I=2 1> ] 1>.\main.cpp(15) : error C2664: 'std::setprecision' : cannot convert parameter 1 from 'boost::lambda::placeholder2_type' to 'std::streamsize' 1> No user-defined-conversion operator available that can perform this conversion, or the operator cannot be called 1>.\main.cpp(15) : error C2872: '_3' : ambiguous symbol 1> could be 'D:\Program Files (x86)\boost\boost_1_42\boost/lambda/core.hpp(71) : boost::lambda::placeholder3_type &boost::lambda::`anonymous-namespace'::_3' 1> or 'D:\Program Files (x86)\boost\boost_1_42\boost/bind/placeholders.hpp(45) : boost::arg<I> `anonymous-namespace'::_3' 1> with 1> [ 1> I=3 1> ] 1>.\main.cpp(15) : error C2660: 'boost::io::group' : function does not take 3 arguments 1>.\main.cpp(15) : error C2228: left of '.str' must have class/struct/union 1>Build log was saved at "file://c:\Users\john\Documents\Visual Studio 2005\Projects\hacks\x64\Debug\BuildLog.htm" 1>hacks - 7 error(s), 0 warning(s) ========== Build: 0 succeeded, 1 failed, 0 up-to-date, 0 skipped ========== My fundamental understanding of boost's lambdas and functions is probably lacking. How can I get this to work?

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  • Are first-class functions a substitute for the Strategy pattern?

    - by Prog
    The Strategy design pattern is often regarded as a substitute for first-class functions in languages that lack them. So for example say you wanted to pass functionality into an object. In Java you'd have to pass in the object another object which encapsulates the desired behavior. In a language such as Ruby, you'd just pass the functionality itself in the form of an annonymous function. However I was thinking about it and decided that maybe Strategy offers more than a plain annonymous function does. This is because an object can hold state that exists independently of the period when it's method runs. However an annonymous function by itself can only hold state that ceases to exist the moment the function finishes execution. So my question is: when using a language that features first-class functions, would you ever use the Strategy pattern (i.e. encapsulate the functionality you want to pass around in an explicit object), or would you always use an annonymous function? When would you decide to use Strategy when you can use a first-class function?

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