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  • Lambda returning another lambda

    - by Yossarian
    Hello, is there any way how to return lambda from another lambda recursively? All I want to do is finite state machine, implemented as lambda, which returns lambda implementing another state (or null). nesting Func< won't work as I want. C#, .NET 3.5

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  • If Scheme is untyped, how can it have numbers and lists?

    - by Dokkat
    Scheme is said to be just an extension of the Untyped Lambda Calculus (correct me if I am wrong). If that is the case, how can it have Lists and Numbers? Those, to me, look like 2 base types. So I'd say Racket is actually an extension of the Simply Typed Lambda Calculus. No? Question: Is Scheme's type system actually based or more similar to Simply Typed or Untyped Lambda Calculus? In what ways does it differ from Untyped and or Simply Typed Lambda Calculus? (The same question is valid for "untyped" languages such as Python and JavaScript - all of which look like they have base types to me.)

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  • How to Process Lambda Expressions Passed as Argument Into Method - C# .NET 3.5

    - by Sunday Ironfoot
    My knowledge of Lambda expressions is a bit shaky, while I can write code that uses Lambda expressions (aka LINQ), I'm trying to write my own method that takes a few arguments that are of type Lambda Expression. Background: I'm trying to write a method that returns a Tree Collection of objects of type TreeItem from literally ANY other object type. I have the following so far: public class TreeItem { public string Id { get; set; } public string Text { get; set; } public TreeItem Parent { get; protected set; } public IList<TreeItem> Children { get { // Implementation that returns custom TreeItemCollection type } } public static IList<TreeItem> GetTreeFromObject<T>(IList<T> items, Expression<Func<T, string>> id, Expression<Func<T, string>> text, Expression<Func<T, IList<T>>> childProperty) where T : class { foreach (T item in items) { // Errrm!?? What do I do now? } return null; } } ...which can be called via... IList<TreeItem> treeItems = TreeItem.GetTreeFromObject<Category>( categories, c => c.Id, c => c.Name, c => c.ChildCategories); I could replace the Expressions with string values, and just use reflection, but I'm trying to avoid this as I want to make it strongly typed. My reasons for doing this is that I have a control that accepts a List of type TreeItem, whereas I have dozens of different types that are all in a tree like structure, and don't want to write seperate conversion methods for each type (trying to adhere to the DRY principle). Am I going about this the right way? Is there a better way of doing this perhaps?

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  • InvalidOperationException (Lambda parameter not in scope) when trying to Compile a Lambda Expression

    - by Moshe Levi
    Hello, I'm writing an Expression Parser to make my API more refactor friendly and less error prone. basicaly, I want the user to write code like that: repository.Get(entity => entity.Id == 10); instead of: repository.Get<Entity>("Id", 10); Extracting the member name from the left side of the binary expression was straight forward. The problems began when I tried to extract the value from the right side of the expression. The above snippet demonstrates the simplest possible case which involves a constant value but it can be much more complex involving closures and what not. After playing with that for some time I gave up on trying to cover all the possible cases myself and decided to use the framework to do all the heavy lifting for me by compiling and executing the right side of the expression. the relevant part of the code looks like that: public static KeyValuePair<string, object> Parse<T>(Expression<Func<T, bool>> expression) { var binaryExpression = (BinaryExpression)expression.Body; string memberName = ParseMemberName(binaryExpression.Left); object value = ParseValue(binaryExpression.Right); return new KeyValuePair<string, object>(memberName, value); } private static object ParseValue(Expression expression) { Expression conversionExpression = Expression.Convert(expression, typeof(object)); var lambdaExpression = Expression.Lambda<Func<object>>(conversionExpression); Func<object> accessor = lambdaExpression.Compile(); return accessor(); } Now, I get an InvalidOperationException (Lambda parameter not in scope) in the Compile line. when I googled for the solution I came up with similar questions that involved building an expression by hand and not supplying all the pieces, or trying to rely on parameters having the same name and not the same reference. I don't think that this is the case here because I'm reusing the given expression. I would appreciate if someone will give me some pointers on this. Thank you.

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  • Lambda Contains in SimpleRepository.Find

    - by Anton
    In SubSonic 3.04's SimpleRepository, I cannot seem to perform a Contains operation within a lambda expression. Here's a trivial example: SimpleRepository repo = new SimpleRepository("ConnectionString"); List<int> userIds = new List<int>(); userIds.Add(1); userIds.Add(3); List<User> users = repo.Find<User>(x => userIds.Contains(x.Id)).ToList(); I get the error message: variable 'x' of type 'User' referenced from scope '', but it is not defined Am I missing something here, or does SubSonic not support Contains in lambda expressions? If not, how would this be done?

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  • C# Joins/Where with Linq and Lambda

    - by David
    Hello, I'm having trouble with a query written in Linq and Lambda. So far, I'm getting allot of errors here's my code: int id = 1; var query = database.Posts.Join(database.Post_Metas, post => database.Posts.Where(x => x.ID == id), meta => database.Post_Metas.Where(x => x.Post_ID == id), (post, meta) => new { Post = post, Meta = meta }); I'm new to using Linq, so I'm not sure if this query is correct.

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  • LINQ/LAMBDA filter query by date [on hold]

    - by inquisitive_one
    I'm trying to use LINQ to SQL to retrieve earnings data for a particular date range. Currently the table is set up as follows: Comp Eps Year Quarter IBM .5 2012 2 IBM .65 2012 3 IBM .60 2012 4 IBM .5 2011 2 IBM .7 2013 1 IBM .8 2013 2 Except for Eps, all fields have a data type of string or char. Eps has a data type of double. Here's my code: var myData = myTable .Where(t => t.Comp.Equals("IBM") && Convert.Int32(string.Format("{0}{1}", t.Year, t.Quarter)) <= 20131); I get the following error when I tried that code: Method 'System.String Format(System.String, System.Object, System.Object)' has no supported translation to SQL How can I select all Eps that has a year & quarter less than "20132" using a lambda expression?

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  • .NET Lambda Pass Method Parameter

    - by RM
    Hi All, I hope i'm missing something obvious, but I'm having some troubles defining a method that takes a parameter of a method to fetch the method information for the passed method. I do NOT want actually execute the method. I want to be able to do: busObject.SetResolverMethod<ISomeInterface>(x=>x.GetNameById); Where GetNameById is a method defined on the interface ISomeInterface. In this case, an example of the method being passed in's signature would be: MyVarA GetNameById(int id){ .... } In the above example, the SetResolverMethod's body should be able to return / store the string "GetNameById". There is no standard signature the method being passed in will conform to (except that it will always return an object of some kind). Currently I'm setting the method as a string (i.e. "GetNameById"), but I want it to be compile time checked, hence this question.

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  • Lambda Expressions and Stored Procedures

    - by Jason Summers
    Hi Everyone. I'm trying to mimic the LINQ Where extension method for my ADO.NET DAL methods. Bascially, my aim is to have a single method that I can call. Such as: Product p = Dal.GetProduct(x => x.ProductId == 32); Product p2 = Dal.GetProduct(x => x.ProductName.Contains("Soap")); I then want to dissect those Predicates and send the filter options to parameters in an ADO.NET Stored Procedure call. Any comments greatly appreciated.

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  • Why doesn't Haskell have type-level lambda abstractions?

    - by Petr Pudlák
    Are there some theoretical reasons for that (like that the type checking or type inference would become undecidable), or practical reasons (too difficult to implement properly)? Currently, we can wrap things into newtype like newtype Pair a = Pair (a, a) and then have Pair :: * -> * but we cannot do something like ?(a:*). (a,a). (There are some languages that have them, for example, Scala does.)

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  • Query on Booleans in Lambda Calculus

    - by darkie15
    Hi All, I have following query on lambda calculus which am not able to understand: Here is the lambda calculus representation for the AND operator: lambda(m).lambda(n).lambda (a).lambda (b). m(n a b) b Can anyone help me in understanding this representation? Regards, darkie

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  • Query on Booleans in Lambda Calculus

    - by darkie15
    Hi All, I have following query on lambda calculus which am not able to understand: Here is the lambda calculus representation for the AND operator: lambda(m).lambda(n).lambda (a).lambda (b). m(n a b) b Can anyone help me in understanding this representation? Regards, darkie

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  • Ruby proc vs lambda in initialize()

    - by Jimmy Chu
    I found out this morning that proc.new works in a class initialize method, but not lambda. Concretely, I mean: class TestClass attr_reader :proc, :lambda def initialize @proc = Proc.new {puts "Hello from Proc"} @lambda = lambda {puts "Hello from lambda"} end end c = TestClass.new c.proc.call c.lambda.call In the above case, the result will be: Hello from Proc test.rb:14:in `<main>': undefined method `call' for nil:NilClass (NoMethodError) Why is that? Thanks!

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  • What is lifetime of lambda-derived implicit functors in C++ ?

    - by Fyodor Soikin
    The question is simple: what is lifetime of that functor object that is automatically generated for me by the C++ compiler when I write a lambda-expression? I did a quick search, but couldn't find a satisfactory answer. In particular, if I pass the lambda somewhere, and it gets remembered there, and then I go out of scope, what's going to happen once my lambda is called later and tries to access my stack-allocated, but no longer alive, captured variables? Or does the compiler prevent such situation in some way? Or what?

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  • boost lambda::bind return type selection

    - by psaghelyi
    I would like to call a member through lambda::bind. Unfortunately I have got to members with the same name but different return type. Is there a way to help the lambda::bind to deduce the right return type for a member function call? #include <vector> #include <iostream> #include <algorithm> #include <boost/bind.hpp> #include <boost/lambda/lambda.hpp> #include <boost/lambda/bind.hpp> using namespace std; using namespace boost; struct A { A (const string & name) : m_name(name) {} string & name () { return m_name; } const string & name () const { return m_name; } string m_name; }; vector<A> av; int main () { av.push_back (A ("some name")); // compiles fine find_if(av.begin(), av.end(), bind<const string &>(&A::name, _1) == "some name"); // error: call of overloaded 'bind(<unresolved overloaded function type>, const boost::lambda::lambda_functor<boost::lambda::placeholder<1> >&)' is ambiguous find_if(av.begin(), av.end(), lambda::bind(&A::name, lambda::_1) == "some name"); return 0; }

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  • Trying to use boost lambda, but my code won't compile

    - by hamishmcn
    Hi, I am trying to use boost lambda to avoid having to write trivial functors. For example, I want to use the lambda to access a member of a struct or call a method of a class, eg: #include <vector> #include <utility> #include <algorithm> #include <boost/lambda/lambda.hpp> using namespace std; using namespace boost::lambda; vector< pair<int,int> > vp; vp.push_back( make_pair<int,int>(1,1) ); vp.push_back( make_pair<int,int>(3,2) ); vp.push_back( make_pair<int,int>(2,3) ); sort(vp.begin(), vp.end(), _1.first > _2.first ); When I try and compile this I get the following errors: error C2039: 'first' : is not a member of 'boost::lambda::lambda_functor<T>' with [ T=boost::lambda::placeholder<1> ] error C2039: 'first' : is not a member of 'boost::lambda::lambda_functor<T>' with [ T=boost::lambda::placeholder<2> ] Since vp contains pair<int,int> I thought that _1.first should work. What I am doing wrong?

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  • What is the motivation behind c++0x lambda expressions?

    - by LoudNPossiblyRight
    I am trying to find out if there is an actual computational benefit to using lambda expressions in c++, namely "this code compiles/runs faster/slower because we use lambda expressions" OR is it just a neat development perk open for abuse by poor coders trying to look cool? Thanks. PS. I understand this question may seem subjective but i would much appreciate the opinion of the community on this matter.

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  • lambda expression based reflection vs normal reflection

    - by bitbonk
    What is the difference between normal reflection and the reflection that can be done with lambda expressions such as this (taken form build your own MVVM): public void NotifyOfPropertyChange<TProperty>(Expression<Func<TProperty>> property) { var lambda = (LambdaExpression)property; MemberExpression memberExpression; if (lambda.Body is UnaryExpression) { var unaryExpression = (UnaryExpression)lambda.Body; memberExpression = (MemberExpression)unaryExpression.Operand; } else memberExpression = (MemberExpression)lambda.Body; NotifyOfPropertyChange(memberExpression.Member.Name); } Is the lambda based reflection just using the normal reflection APIs internally? Or is this something significantly different. What is ther perfomance difference?

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  • What's the point of lambda in scheme?

    - by incrediman
    I am learning scheme. I know how to use both lambda and let expressions. However I'm struggling to figure out what the point is of using lambda. Can't you do everything with let that you can with lambda? It would be especially helpful to see an example of a situation where a lambda expression is a better choice than let. One other thing - are there also situations where let is more useful than lambda? If so such an example would be nice as well. Thanks!

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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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