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  • Passing in a lambda to a Where statement

    - by sonicblis
    I noticed today that if I do this: var items = context.items.Where(i => i.Property < 2); items = items.Where(i => i.Property > 4); Once I access the items var, it executes only the first line as the data call and then does the second call in memory. However, if I do this: var items = context.items.Where(i => i.Property < 2).Where(i => i.Property > 4); I get only one expression executed against the context that includes both where statements. I have a host of variables that I want to use to build the expression for the linq lambda, but their presence or absence changes the expression such that I'd have to have a rediculous number of conditionals to satisfy all cases. I thought I could just add the Where() statements as in my first example above, but that doesn't end up in a single expression that contains all of the criteria. Therefore, I'm trying to create just the lambda itself as such: //bogus syntax if (var1 == "something") var expression = Expression<Func<item, bool>>(i => i.Property == "Something); if (var2 == "somethingElse") expression = expression.Where(i => i.Property2 == "SomethingElse"); And then pass that in to the where of my context.Items to evaluate. A) is this right, and B) if so, how do you do it?

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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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  • If you had to reinvent a new syntax for regular expressions, what would it look like?

    - by Timwi
    Regular expressions as they are today are pretty much as concise and compact as they can be. Consequently, they are often criticised for being unreadable and hard to debug. If you had to reinvent a new syntax for regular expressions, what would it look like? Do you prefer the concise syntax they already have (or a different but similarly concise syntax)? If so, please justify why you think regular expressions deserve to be this concise, but your favourite programming language doesn’t (unless it’s Perl). Or do you think regular expressions should have a slightly more spaced-out syntax and look a bit more like operators and syntax elements normally do in programming languages? If so, provide examples of what you think the syntax should look like, and justify why it is better than the current syntax. Or do you think there shouldn’t even be a special syntax for regular expressions, and instead they should be constructed from syntax elements already present in the programming language? If so, give examples of a syntax that might be used to construct such regular expressions.

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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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  • Using Find/Replace with regular expressions inside a SSIS package

    - by jamiet
    Another one of those might-be-useful-again-one-day-so-I’ll-share-it-in-a-blog-post blog posts I am currently working on a SQL Server Integration Services (SSIS) 2012 implementation where each package contains a parameter called ETLIfcHist_ID: During normal execution this will get altered when the package is executed from the Execute Package Task however we want to make sure that at deployment-time they all have a default value of –1. Of course, they tend to get changed during development so I wanted a way of easily changing them all back to the default value. Opening up each package in turn and editing them was an option but given that we have over 40 packages and we might want to carry out this reset fairly frequently I needed a more automated method so I turned to Visual Studio’s Find/Replace… feature Of course, we don’t know what value will be in that parameter so I can’t simply search for a particular value; hence I opted to use a regular expression to identify the value to be change. In the rest of this blog post I’ll explain how to do that. For demonstration purposes I have taken the contents of a .dtsx file and stripped out everything except the element containing the parameters (<DTS:PackageParameters>), if you want to play along at home you can copy-paste the XML document below into a new XML file and open it up in Visual Studio: <?xml version="1.0"?> <DTS:Executable xmlns:DTS="www.microsoft.com/SqlServer/Dts">   <DTS:PackageParameters>     <DTS:PackageParameter       DTS:CreationName=""       DTS:DataType="3"       DTS:Description="InterfaceHistory_ID: used for Lineage"       DTS:DTSID="{635616DB-EEEE-45C8-89AA-713E25846C7E}"       DTS:ObjectName="ETLIfcHist_ID">       <DTS:Property         DTS:DataType="3"         DTS:Name="ParameterValue">VALUE_TO_BE_CHANGED</DTS:Property>     </DTS:PackageParameter>     <DTS:PackageParameter       DTS:CreationName=""       DTS:DataType="3"       DTS:Description="Some other description"       DTS:DTSID="{635616DB-EEEE-45C8-89AA-713E25845C7E}"       DTS:ObjectName="SomeOtherObjectName">       <DTS:Property         DTS:DataType="3"         DTS:Name="ParameterValue">SomeOtherValue</DTS:Property>     </DTS:PackageParameter>   </DTS:PackageParameters> </DTS:Executable> We are trying to identify the value of the parameter whose name is ETLIfcHist_ID – notice that in the XML document above that value is VALUE_TO_BE_CHANGED. The following regular expression will find the appropriate portion of the XML document: {\<DTS\:PackageParameter[\n ]*DTS\:CreationName="[A-Za-z0-9\:_\{\}- ]*"[\n ]*DTS\:DataType="[A-Za-z0-9\:_\{\}- ]*"[\n ]*DTS\:Description="[A-Za-z0-9\:_\{\}- ]*"[\n ]*DTS\:DTSID="[A-Za-z0-9\:_\{\}- ]*"[\n ]*DTS\:ObjectName="ETLIfcHist_ID"\>[\n ]*\<DTS\:Property[\n ]*DTS\:DataType="[A-Za-z0-9\:_\{\}- ]*"[\n ]*DTS\:Name="ParameterValue"\>}[A-Za-z0-9\:_\{\}- ]*{\<\/DTS\:Property\>} I have highlighted the name of the parameter that we’re looking for. I have also highlighted two portions identified by pairs of curly braces “{…}”; these are important because they pick out the two portions either side of the value I want to replace, in other words the portions highlighted here: <DTS:PackageParameters>     <DTS:PackageParameter       DTS:CreationName=""       DTS:DataType="3"       DTS:Description="InterfaceHistory_ID: used for Lineage"       DTS:DTSID="{635616DB-EEEE-45C8-89AA-713E25846C7E}"       DTS:ObjectName="ETLIfcHist_ID">       <DTS:Property         DTS:DataType="3"         DTS:Name="ParameterValue">VALUE_TO_BE_CHANGED</DTS:Property>     </DTS:PackageParameter> Those sections in the curly braces are termed tag expressions and can be identified in the replace expression using a backslash and a number identifying which tag expression you’re referring to according to its ordinal position. Hence, our replace expression is simply: \1-1\2 We’re saying the portion of our file identified by the regular expression should be replaced by the first curly brace section, then the literal –1, then the second curly brace section. Make sense? Give it a go yourself by plugging those two expressions into Visual Studio’s Find and Replace dialog. If you set it to look in “All Open Documents” then you can open up the code-behind of all your packages and change all of them at once. The Find and Replace dialog will look like this: That’s it! I realise that not everyone will be looking to change the value of a parameter but hopefully I have shown you a technique that you can modify to work for your own scenario. Given that this blog post is, y’know, on the web I have no doubt that someone is going to find a fault with my find regex expression and if that person is you….that’s OK. Let me know about it in the comments below and perhaps we can work together to come up with something better! Note that some parameters may have a different set of properties (for example some, but not all, of my parameters have a DTS:Required attribute) so your find regular expression may have to change accordingly. When researching this I found the following article to be invaluable: Visual Studio Find/Replace Regular Expression Usage @Jamiet

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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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  • Compute Scalars, Expressions and Execution Plan Performance

    - by Paul White
    The humble Compute Scalar is one of the least well-understood of the execution plan operators, and usually the last place people look for query performance problems. It often appears in execution plans with a very low (or even zero) cost, which goes some way to explaining why people ignore it. Some readers will already know that a Compute Scalar can contain a call to a user-defined function, and that any T-SQL function with a BEGIN…END block in its definition can have truly disastrous consequences...(read more)

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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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