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  • Ambiguous Evaluation of Lambda Expression on Array

    - by Joe
    I would like to use a lambda that adds one to x if x is equal to zero. I have tried the following expressions: t = map(lambda x: x+1 if x==0 else x, numpy.array) t = map(lambda x: x==0 and x+1 or x, numpy.array) t = numpy.apply_along_axis(lambda x: x+1 if x==0 else x, 0, numpy.array) Each of these expressions returns the following error: ValueError: The truth value of an array with more than one element is ambiguous. Use a.any() or a.all() My understanding of map() and numpy.apply_along_axis() was that it would take some function and apply it to each value of an array. From the error it seems that the the lambda is being evaluated as x=array, not some value in array. What am I doing wrong? I know that I could write a function to accomplish this but I want to become more familiar with the functional programming aspects of python.

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  • Extracting pair member in lambda expressions and template typedef idiom

    - by Nazgob
    Hi, I have some complex types here so I decided to use nifty trick to have typedef on templated types. Then I have a class some_container that has a container as a member. Container is a vector of pairs composed of element and vector. I want to write std::find_if algorithm with lambda expression to find element that have certain value. To get the value I have to call first on pair and then get value from element. Below my std::find_if there is normal loop that does the trick. My lambda fails to compile. How to access value inside element which is inside pair? I use g++ 4.4+ and VS 2010 and I want to stick to boost lambda for now. #include <vector> #include <algorithm> #include <boost\lambda\lambda.hpp> #include <boost\lambda\bind.hpp> template<typename T> class element { public: T value; }; template<typename T> class element_vector_pair // idiom to have templated typedef { public: typedef std::pair<element<T>, std::vector<T> > type; }; template<typename T> class vector_containter // idiom to have templated typedef { public: typedef std::vector<typename element_vector_pair<T>::type > type; }; template<typename T> bool operator==(const typename element_vector_pair<T>::type & lhs, const typename element_vector_pair<T>::type & rhs) { return lhs.first.value == rhs.first.value; } template<typename T> class some_container { public: element<T> get_element(const T& value) const { std::find_if(container.begin(), container.end(), bind(&typename vector_containter<T>::type::value_type::first::value, boost::lambda::_1) == value); /*for(size_t i = 0; i < container.size(); ++i) { if(container.at(i).first.value == value) { return container.at(i); } }*/ return element<T>(); //whatever } protected: typename vector_containter<T>::type container; }; int main() { some_container<int> s; s.get_element(5); return 0; }

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  • How to properly diagram lambda expressions or traversals through them in Architecture Explorer?

    - by MainMa
    I'm exploring a piece of code in Architecture Explorer in Visual Studio 2010 to study the relations between methods. I noticed a strange behavior. Take the following source code. It generates a hello message based on a template and a template engine, the template engine being a method (a sort of strategy pattern simplified at a maximum for demo purposes). public string GenerateHelloMessage(string personName) { return this.ApplyTemplate( this.DefaultTemplateEngine, this.GenerateLocalizedHelloTemplate(), personName); } private string GenerateLocalizedHelloTemplate() { return "Hello {0}!"; } public string ApplyTemplate( Func<string, string, string> templateEngine, string template, string personName) { return templateEngine(template, personName); } public string DefaultTemplateEngine(string template, string personName) { return string.Format(template, personName); } The graph generated from this code is this one: Change the first method from this: public string GenerateHelloMessage(string personName) { return this.ApplyTemplate( this.DefaultTemplateEngine, this.GenerateLocalizedHelloTemplate(), personName); } to this: public string GenerateHelloMessage(string personName) { return this.ApplyTemplate( (a, b) => this.DefaultTemplateEngine(a, b), this.GenerateLocalizedHelloTemplate(), personName); } and the graph becomes: While semantically identical, those two versions of code produce different dependency graphs, and Architecture Explorer shows no trace of the lambda expression (while Visual Studio's code coverage, for example, shows them, as well as Code analysis seems to be able to understand that the link exists). How would it be possible, without changing the source code, to: Either force Architecture Explorer to display everything, including lambda expressions, Or make it traverse lambda expressions while drawing a dependency through them (so in this case, drawing the dependency from GenerateHelloMessage to DefaultTemplateEngine in the second example)?

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  • What triggered the popularity of lambda functions in modern programming languages?

    - by Giorgio
    In the last few years anonymous functions (AKA lambda functions) have become a very popular language construct and almost every major / mainstream programming language has introduced them or is planned to introduce them in an upcoming revision of the standard. Yet, anonymous functions are a very old and very well-known concept in Mathematics and Computer Science (invented by the mathematician Alonzo Church around 1936, and used by the Lisp programming language since 1958, see e.g. here). So why didn't today's mainstream programming languages (many of which originated 15 to 20 years ago) support lambda functions from the very beginning and only introduced them later? And what triggered the massive adoption of anonymous functions in the last few years? Is there some specific event, new requirement or programming technique that started this phenomenon?

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  • custom C++ boost::lambda expression help

    - by aaa
    hello. A little bit of background: I have some strange multiple nested loops which I converted to flat work queue (basically collapse single index loops to single multi-index loop). right now each loop is hand coded. I am trying to generalized approach to work with any bounds using lambda expressions: For example: // RANGE(i,I,N) is basically a macro to generate `int i = I; i < N; ++i ` // for (RANGE(lb, N)) { // for (RANGE(jb, N)) { // for (RANGE(kb, max(lb, jb), N)) { // for (RANGE(ib, jb, kb+1)) { // is equivalent to something like (overload , to produce range) flat<1, 3, 2, 4>((_2, _3+1), (max(_4,_3), N), N, N) the internals of flat are something like: template<size_t I1, size_t I2, ..., class L1_, class L2, ..._> boost::array<int,4> flat(L1_ L1, L2_ L2, ...){ //boost::array<int,4> current; class variable bool advance; L2_ l2 = L2.bind(current); // bind current value to lambda { L1_ l1 = L1.bind(current); //bind current value to innermost lambda l1.next(); advance = !(l1 < l1.upper()); // some internal logic if (advance) { l2.next(); current[0] = l1.lower(); } } //..., } my question is, can you give me some ideas how to write lambda (derived from boost) which can be bound to index array reference to return upper, lower bounds according to lambda expression? thank you much

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  • scheme recursive lambda

    - by Mike
    Is there a way to have a recursive lambda expression in scheme without relying an external identifier? I know you can have (define fact (lambda (n) (if (= n 0) 1 (fact (- n 1)))) but it would be nice if fact wasn't hard coded in the lambda expression, it seems improper.

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  • boost::lambda bind expressions can't get bind to string's empty() to work

    - by navigator
    Hi, I am trying to get the below code snippet to compile. But it fails with: error C2665: 'boost::lambda::function_adaptor::apply' : none of the 8 overloads could convert all the argument types. Sepcifying the return type when calling bind does not help. Any idea what I am doing wrong? Thanks. #include <boost/lambda/lambda.hpp> #include <boost/lambda/bind.hpp> #include <string> #include <map> int main() { namespace bl = boost::lambda; typedef std::map<int, std::string> types; types keys_and_values; keys_and_values[ 0 ] = "zero"; keys_and_values[ 1 ] = "one"; keys_and_values[ 2 ] = "Two"; std::for_each( keys_and_values.begin(), keys_and_values.end(), std::cout << bl::constant("Value empty?: ") << std::boolalpha << bl::bind(&std::string::empty, bl::bind(&types::value_type::second, _1)) << "\n"); return 0; }

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  • varargs in lambda functions in Python

    - by brain_damage
    Is it possible a lambda function to have variable number of arguments? For example, I want to write a metaclass, which creates a method for every method of some other class and this newly created method returns the opposite value of the original method and has the same number of arguments. And I want to do this with lambda function. How to pass the arguments? Is it possible? class Negate(type): def __new__(mcs, name, bases, _dict): extended_dict = _dict.copy() for (k, v) in _dict.items(): if hasattr(v, '__call__'): extended_dict["not_" + k] = lambda s, *args, **kw: not v(s, *args, **kw) return type.__new__(mcs, name, bases, extended_dict) class P(metaclass=Negate): def __init__(self, a): self.a = a def yes(self): return True def maybe(self, you_can_chose): return you_can_chose But the result is totally wrong: >>>p = P(0) >>>p.yes() True >>>p.not_yes() # should be False Traceback (most recent call last): File "<pyshell#150>", line 1, in <module> p.not_yes() File "C:\Users\Nona\Desktop\p10.py", line 51, in <lambda> extended_dict["not_" + k] = lambda s, *args, **kw: not v(s, *args, **kw) TypeError: __init__() takes exactly 2 positional arguments (1 given) >>>p.maybe(True) True >>>p.not_maybe(True) #should be False True

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  • What triggered the popularity of lambda functions in modern mainstream programming languages?

    - by Giorgio
    In the last few years anonymous functions (AKA lambda functions) have become a very popular language construct and almost every major / mainstream programming language has introduced them or is planned to introduce them in an upcoming revision of the standard. Yet, anonymous functions are a very old and very well-known concept in Mathematics and Computer Science (invented by the mathematician Alonzo Church around 1936, and used by the Lisp programming language since 1958, see e.g. here). So why didn't today's mainstream programming languages (many of which originated 15 to 20 years ago) support lambda functions from the very beginning and only introduced them later? And what triggered the massive adoption of anonymous functions in the last few years? Is there some specific event, new requirement or programming technique that started this phenomenon? IMPORTANT NOTE The focus of this question is the introduction of anonymous functions in modern, main-stream (and therefore, maybe with a few exceptions, non functional) languages. Also, note that anonymous functions (blocks) are present in Smalltalk, which is not a functional language, and that normal named functions have been present even in procedural languages like C and Pascal for a long time. Please do not overgeneralize your answers by speaking about "the adoption of the functional paradigm and its benefits", because this is not the topic of the question.

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  • How to implement lambda as a function called "lambda" in Clojure?

    - by dirtyvagabond
    I'd like to be able to define lambdas using common Lisp syntax, in Clojure. For example: (lambda (myarg) (some-functions-that-refer-to myarg)) This needs to result in the same as: #(some-functions-that-refer-to %) In my case, I know I'll always have exactly one arg, so perhaps that simplifies things. (But it can be called anything -- "myarg" or whatever.) I suspect a workable solution is to "(defmacro lambda ...". If so, I'm not sure of the best way to proceed. How to cleanly translate the arg name to %? And how to end up with the correct function? Or, is there a simpler solution than writing my own macro that actually re-implements Clojure's... lambda?

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  • Anonymous Methods / Lambda's (Coding Standards)

    - by Mystagogue
    In Jeffrey Richter's "CLR via C#" (the .net 2.0 edtion page, 353) he says that as a self-discipline, he never makes anonymous functions longer than 3 lines of code in length. He cites mostly readability / understandability as his reasons. This suites me fine, because I already had a self-discipline of using no more than 5 lines for an anonymous method. But how does that "coding standard" advice stack against lambda's? At face value, I'd treat them the same - keeping a lambda equally as short. But how do others feel about this? In particular, when lambda's are being used where (arguably) they shine brightest - when used in LINQ statements - is there genuine cause to abandon that self-discipline / coding standard?

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  • Python lambda returning None instead of empty string

    - by yoshi
    I have the following lambda function: f = lambda x: x == None and '' or x It should return an empty string if it receives None as the argument, or the argument if it's not None. For example: >>> f(4) 4 >>> f(None) >>> If I call f(None) instead of getting an empty string I get None. I printed the type of what the function returned and I got NoneType. I was expecting string. type('') returns string, so I'd like to know why the lambda doesn't return an empty string when I pass None as an argument. I'm fairly new to lambdas so I might have misunderstood some things about how they work.

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  • C# Lambda Problem

    - by Chris Klepeis
    Probably something simple, but as I'm new to lambda expressions, the problem evades me: m => m.contactID == contactID && m.primaryAddress == true && (m.addressTypeID == 2 || m.addressTypeID == 3) I tried to use that lambda expression but I receive an invalid operator. Is there a way to simplify this so that it would work? Edit: The equivolent sql query would be: SELECT * FROM Contact WHERE contactID = 3 AND primaryAddress = 1 AND (addressTypeID = 2 OR addressTypeID = 3) I have a repository function defined like so: public E Single(Expression<Func<E, bool>> where) { return objectSet.Single<E>(where); } I'm passing the lambda expression above into this function. Invalid operator error.

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  • Examining a lambda expression at runtime in C#

    - by Ben Aston
    I have a method Get on a type MyType1 accepting a Func<MyType2, bool> as a parameter. An example of its use: mytype1Instance.Get(x => x.Guid == guid)); I would like create a stub implementation of the method Get that examines the incoming lambda expression and determines what the value of guid is. Clearly the lambda could be "anything", but I'm happy for the stub to make an assumption about the lambda, that it is trying to match on the Guid property. How can I do this? I suspect it involves the use of the built-in Expression type?

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  • List filtering: list comprehension vs. lambda + filter

    - by Agos
    I happened to find myself having a basic filtering need: I have a list and I have to filter it by an attribute of the items. My code looked like this: list = [i for i in list if i.attribute == value] But then i thought, wouldn't it be better to write it like this? filter(lambda x: x.attribute == value, list) It's more readable, and if needed for performance the lambda could be taken out to gain something. Question is: are there any caveats in using the second way? Any performance difference? Am I missing the Pythonic Way™ entirely and should do it in yet another way (such as using itemgetter instead of the lambda)? Thanks in advance

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  • Lambda Expression to be used in Select() query

    - by jameschinnock
    Hi, I am trying to build a lambda expression, containing two assignments (as shown further down), that I can then pass to a Queryable.Select() method. I am trying to pass a string variable into a method and then use that variable to build up the lambda expression so that I can use it in a LINQ Select query. My reasoning behind it is that I have a SQL Server datasource with many column names, I am creating a charting application that will allow the user to select, say by typing in the column name, the actual column of data they want to view in the y-axis of my chart, with the x-axis always being the DateTime. Therefore, they can essentially choose what data they chart against the DateTime value (it’s a data warehouse type app). I have, for example, a class to store the retrieved data in, and hence use as the chart source of: public class AnalysisChartSource { public DateTime Invoicedate { get; set; } public Decimal yValue { get; set; } } I have (purely experimentaly) built an expression tree for the Where clause using the String value and that works fine: public void GetData(String yAxis) { using (DataClasses1DataContext db = new DataClasses1DataContext()) { var data = this.FunctionOne().AsQueryable<AnalysisChartSource>(); //just to get some temp data in.... ParameterExpression pe = Expression.Parameter(typeof(AnalysisChartSource), "p"); Expression left = Expression.MakeMemberAccess(pe, typeof(AnalysisChartSource).GetProperty(yAxis)); Expression right = Expression.Constant((Decimal)16); Expression e2 = Expression.LessThan(left, right); Expression expNew = Expression.New(typeof(AnalysisChartSource)); LambdaExpression le = Expression.Lambda(left, pe); MethodCallExpression whereCall = Expression.Call( typeof(Queryable), "Where", new Type[] { data.ElementType }, data.Expression, Expression.Lambda<Func<AnalysisChartSource, bool>>(e2, new ParameterExpression[] { pe })); } } However……I have tried a similar approach for the Select statement, but just can’t get it to work as I need the Select() to populate both X and Y values of the AnalysisChartSource class, like this: .Select(c => new AnalysisChartSource { Invoicedate = c.Invoicedate, yValue = c.yValue}).AsEnumerable(); How on earth can I build such an expression tree….or….possibly more to the point…..is there an easier way that I have missed entirely?

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  • Lambda Expressions

    - by Asad Khan
    Can somebody explain me lambda expressions & what they can be used for. I have googled for it & have a rough idea. most of the examples give c# code. How about lambda expressions in plain old C...?

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  • Can you reverse order a string in one line with LINQ or a LAMBDA expression

    - by Student for Life
    Not that I would want to use this practically (for many reasons) but out of strict curiousity I would like to know if there is a way to reverse order a string using LINQ and/or LAMBDA expressions in one line of code, without utilising any framework "Reverse" methods. e.g. string value = "reverse me"; string reversedValue = (....); and reversedValue will result in "em esrever" EDIT Clearly an impractical problem/solution I know this, so don't worry it's strictly a curiosity question around the LINQ/LAMBDA construct.

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  • Specializing a template on a lambda in C++0x

    - by Tony A.
    I've written a traits class that lets me extract information about the arguments and type of a function or function object in C++0x (tested with gcc 4.5.0). The general case handles function objects: template <typename F> struct function_traits { template <typename R, typename... A> struct _internal { }; template <typename R, typename... A> struct _internal<R (F::*)(A...)> { // ... }; typedef typename _internal<decltype(&F::operator())>::<<nested types go here>>; }; Then I have a specialization for plain functions at global scope: template <typename R, typename... A> struct function_traits<R (*)(A...)> { // ... }; This works fine, I can pass a function into the template or a function object and it works properly: template <typename F> void foo(F f) { typename function_traits<F>::whatever ...; } int f(int x) { ... } foo(f); What if, instead of passing a function or function object into foo, I want to pass a lambda expression? foo([](int x) { ... }); The problem here is that neither specialization of function_traits<> applies. The C++0x draft says that the type of the expression is a "unique, unnamed, non-union class type". Demangling the result of calling typeid(...).name() on the expression gives me what appears to be gcc's internal naming convention for the lambda, main::{lambda(int)#1}, not something that syntactically represents a C++ typename. In short, is there anything I can put into the template here: template <typename R, typename... A> struct function_traits<????> { ... } that will allow this traits class to accept a lambda expression?

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  • Lambda&rsquo;s for .NET made easy&hellip;

    - by mbcrump
    The purpose of my blog is to explain things for a beginner to intermediate c# programmer. I’ve seen several blog post that use lambda expressions always assuming the audience is familiar with them. The purpose of this post is to make them simple and easily understood. Let’s begin with a definition. A lambda expression is an anonymous function that can contain expressions and statements, and can be used to create delegates or expression tree types. So anonymous function… delegates or expression tree types? I don’t get it??? Confused yet?   Lets break this into a few definitions and jump right into the code. anonymous function – is an "inline" statement or expression that can be used wherever a delegate type is expected. delegate - is a type that references a method. Once a delegate is assigned a method, it behaves exactly like that method. The delegate method can be used like any other method, with parameters and a return value. Expression trees - represent code in a tree-like data structure, where each node is an expression, for example, a method call or a binary operation such as x < y.   Don’t worry if this still sounds confusing, lets jump right into the code with a simple 3 line program. We are going to use a Function Delegate (all you need to remember is that this delegate returns a value.) Lambda expressions are used most commonly with the Func and Action delegates, so you will see an example of both of these. Lambda Expression 3 lines. using System; using System.Collections.Generic; using System.Linq; using System.Text;   namespace ConsoleApplication7 {     class Program     {          static void Main(string[] args)         {             Func<int, int> myfunc = x => x *x;             Console.WriteLine(myfunc(6).ToString());             Console.ReadLine();         }       } } Is equivalent to Old way of doing it. using System; using System.Collections.Generic; using System.Linq; using System.Text;   namespace ConsoleApplication7 {     class Program     {          static void Main(string[] args)         {               Console.WriteLine(myFunc(6).ToString());             Console.ReadLine();         }            static int myFunc(int x)          {              return x * x;            }       } } In the example, there is a single parameter, x, and the expression is x*x. I’m going to stop here to make sure you are still with me. A lambda expression is an unnamed method written in place of a delegate instance. In other words, the compiler converts the lambda expression to either a : A delegate instance An expression tree All lambda have the following form: (parameters) => expression or statement block Now look back to the ones we have created. It should start to sink in. Don’t get stuck on the => form, use it as an identifier of a lambda. A Lamba expression can also be written in the following form: Lambda Expression. using System; using System.Collections.Generic; using System.Linq; using System.Text;   namespace ConsoleApplication7 {     class Program     {          static void Main(string[] args)         {             Func<int, int> myFunc = x =>             {                 return x * x;             };               Console.WriteLine(myFunc(6).ToString());             Console.ReadLine();         }       } } This form may be easier to read but consumes more space. Lets try an Action delegate – this delegate does not return a value. Action Delegate example. using System; using System.Collections.Generic; using System.Linq; using System.Text;   namespace ConsoleApplication7 {     class Program     {          static void Main(string[] args)         {             Action<string> myAction = (string x) => { Console.WriteLine(x); };             myAction("michael has made this so easy");                                   Console.ReadLine();         }       } } Lambdas can also capture outer variables (such as the example below) A lambda expression can reference the local variables and parameters of the method in which it’s defined. Outer variables referenced by a lambda expression are called captured variables. Capturing Outer Variables using System; using System.Collections.Generic; using System.Linq; using System.Text;   namespace ConsoleApplication7 {     class Program     {          static void Main(string[] args)         {             string mike = "Michael";             Action<string> myAction = (string x) => {                 Console.WriteLine("{0}{1}", mike, x);          };             myAction(" has made this so easy");                                   Console.ReadLine();         }       } } Lamba’s can also with a strongly typed list to loop through a collection.   Used w a strongly typed list. using System; using System.Collections.Generic; using System.Linq; using System.Text;   namespace ConsoleApplication7 {     class Program     {          static void Main(string[] args)         {             List<string> list = new List<string>() { "1", "2", "3", "4" };             list.ForEach(s => Console.WriteLine(s));             Console.ReadLine();         }       } } Outputs: 1 2 3 4 I think this will get you started with Lambda’s, as always consult the MSDN documentation for more information. Still confused? Hopefully you are not.

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  • QotD: Maurizio Cimadamore on Project Lambda Binary Snapshots

    - by $utils.escapeXML($entry.author)
    I'm glad to announce that the first binary snapshots of the lambda repository are available at the following URL:http://jdk8.java.net/lambda/As you can imagine, as the implementation of the compiler/libraries is still under heavy development, there are still many rough corners that need to be polished. I'd like to thank you all for all the patience and the valuable feedback provided so far - please keep it coming!Maurizio Cimadamore announcing the Project Lambda binary snapshots on the lambda-dev OpenJDK mailing list.

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  • lambda traits inconsistency across C++0x compilers

    - by Sumant
    I observed some inconsistency between two compilers (g++ 4.5, VS2010 RC) in the way they match lambdas with partial specializations of class templates. I was trying to implement something like boost::function_types for lambdas to extract type traits. Check this for more details. In g++ 4.5, the type of the operator() of a lambda appears to be like that of a free standing function (R (*)(...)) whereas in VS2010 RC, it appears to be like that of a member function (R (C::*)(...)). So the question is are compiler writers free to interpret any way they want? If not, which compiler is correct? See the details below. template <typename T> struct function_traits : function_traits<decltype(&T::operator())> { // This generic template is instantiated on both the compilers as expected. }; template <typename R, typename C> struct function_traits<R (C::*)() const> { // inherits from this one on VS2010 RC typedef R result_type; }; template <typename R> struct function_traits<R (*)()> { // // inherits from this one g++ 4.5 typedef R result_type; }; int main(void) { auto lambda = []{}; function_traits<decltype(lambda)>::result_type *r; // void * } This program compiles on both g++ 4.5 and VS2010 but the function_traits that are instantiated are different as noted in the code.

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  • How to convert a lambda to an std::function using templates

    - by retep998
    Basically, what I want to be able to do is take a lambda with any number of any type of parameters and convert it to an std::function. I've tried the following and neither method works. std::function([](){});//Complains that std::function is missing template parameters template <typename T> void foo(function<T> f){} foo([](){});//Complains that it cannot find a matching candidate The following code does work however, but it is not what I want because it requires explicitly stating the template parameters which does not work for generic code. std::function<void()>([](){}); I've been mucking around with functions and templates all evening and I just can't figure this out, so any help would be much appreciated. As mentioned in a comment, the reason I'm trying to do this is because I'm trying to implement currying in C++ using variadic templates. Unfortunately, this fails horribly when using lambdas. For example, I can pass a standard function using a function pointer. template <typename R, typename...A> void foo(R (*f)(A...)) {} void bar() {} int main() { foo(bar); } However, I can't figure out how to pass a lambda to such a variadic function. Why I'm interested in converting a generic lambda into an std::function is because I can do the following, but it ends up requiring that I explicitly state the template parameters to std::function which is what I am trying to avoid. template <typename R, typename...A> void foo(std::function<R(A...)>) {} int main() { foo(std::function<void()>([](){})); }

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