The Map function is quite simple — but then that’s the nature of the standard higher order functions; that’s what makes them the great building blocks they are in functional programming. Map takes a list of elements and a function to call with each element in turn. Then it constructs a new list from the results of the function calls and returns that new list.

Using iterators in C#, Map can be implemented lazily. Here is the implementation from FCSlib:

static IEnumerable<R> Map<T, R>(Converter<T, R> function, IEnumerable<T> list)


  foreach (T sourceVal in list)

    yield return function(sourceVal);


The Converter<T,R> delegate type is a function that receives a parameter of type T and returns an element of type R. The name Converter can be a bit misleading, since the function doesn’t necessarily convert anything. It might just as well extract something, which is a major use case of the Map function. For example, given a list of objects, Map can be used to extract a particular property from each of the objects:

var people = new List<Person> {

  new Person {Name = "Harry", Age = 32},

  new Person {Name = "Anna", Age = 45},

  new Person {Name = "Willy", Age = 43},

  new Person {Name = "Rose", Age = 37}


var names = Functional.Map(p => p.Name, people);

Of course, actual calculations can be performed just as easily:

var squares = Functional.Map(i => i * i, Enumerable.Range(1, 10));

Using Criteria: Filter

Filter is a function that applies criteria to ...

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