functor

I've been learning about functional programming and have come across Monads, Functors and Applicatives. From my understanding the following definitions apply: a) ( A=>B ) => C[A] => C[B] | Functor b) ( A=>C[B] ) => C[A] => C[B] | Monad c) ( C[A=>B] ) => C[A] => C[B] | Applicative (reference: https://thedet.wordpress.com/2012/04/28/functors-monads-applicatives-can-be-so-simple/ ) Furthermore, I understand a Monad is a special case of a Functor. As in, it applies a function that returns a wrapped value to a wrapped value and returns a wrapped value. When we use Promise.then(func) , we are

I've come across the term 'Functor' a few times while reading various articles on functional programming, but the authors typically assume the reader already understands the term. Looking around on the web has provided either excessively technical descriptions (see the Wikipedia article ) or incredibly vague descriptions (see the section on Functors at this ocaml-tutorial website ). Can someone kindly define the term, explain its use, and perhaps provide an example of how Functors are created and used? Edit : While I am interested in the theory behind the term, I am less interested in the

This is a follow up of this problem: Generic functor for functions with any argument list I have this functor class (full code see link above): template<typename... ARGS> class Foo { std::function<void(ARGS...)> m_f; public: Foo( std::function<void(ARGS...)> f ) : m_f(f) {} void operator()(ARGS... args) const { m_f(args...); } }; In operator() I can access the args... easily with a recursive "peeling" function as described here http://www2.research.att.com/~bs/C++0xFAQ.html#variadic-templates My problem is: I want to access the types of the arguments of f, i.e. ARGS..., in the constructor.

I'm trying to learn Haskell and I'm through all the basics. But now I'm stuck, trying to get my head around functors. I've read that "A functor transforms one category into another category". What does this mean? I know it's a lot to ask, but could anyone give me a plain english explanation of functors or maybe a simple use case ? A fuzzy explanation would be that a Functor is some sort of container and an associated function fmap that allows you to alter whatever is contained, given a function that transforms the contained. For instance, lists are this kind of container, such that fmap (+1)

Compare double average = CalculateAverage(values.begin(), values.end()); with double average = std::for_each(values.begin(), values.end(), CalculateAverage()); What are the benefits of using a functor over a function? Isn't the first a lot easier to read (even before the implementation is added)? Assume the functor is defined like this: class CalculateAverage { private: std::size_t num; double sum; public: CalculateAverage() : num (0) , sum (0) { } void operator () (double elem) { num++; sum += elem; } operator double() const { return sum / num; } }; Oliver Charlesworth At least four good