A lambda is a functor – just defined with a shorter syntax. The problem is that this syntax is limited. It doesn’t always allow you to solve a problem in the most efficient and flexible way – or at all. Until C++14, the operator() couldn’t even be a template. Furthermore, a lambda has exactly one … Read more
This question has been answered for some time now, but I was quite surprised that most of the answers say what OCaml features are missing in F# – this is definitely good to know if you want to port existing OCaml programs to F# (which is probably the motivation of most of the referenced articles). … Read more
You’re not modifying your original array. You’re modifying the objects in the array. If you want to avoid mutating the objects in your array, you can use Object.assign to create a new object with the original’s properties plus any changes you need: const freeProduct = function(products) { return products.map(x => { return Object.assign({}, x, { … Read more
This can be used to create “functors”, objects that act like functions: class Multiplier { public: Multiplier(int m): multiplier(m) {} int operator()(int x) { return multiplier * x; } private: int multiplier; }; Multiplier m(5); cout << m(4) << endl; The above prints 20. The Wikipedia article linked above gives more substantial examples.
Here’s a fully parametric example: data Weird a = Weird a (a -> a) instance Foldable Weird where foldMap f (Weird a b) = f $ b a Weird is not a Functor because a occurs in a negative position.
It is, but in general I would encourage just using a proper functor or a lambda: Using a lambda: std::sort(payloadIndices.begin(), payloadIndices.end(), [this](int a, int b){ return this->sortByWeights(a, b); }); Alternatively using std::mem_fn: auto sorter = std::bind(std::mem_fn(SimpleGreedyMappingTechnique::sortByWeights), this); std::sort(payloadIndices.begin(), payloadIndices.end(), sorter); Alternatively using a functor: namespace{ struct indicies_less_than { const SimpleGreedyMappingTechnique & mapping_tech; indicies_less_than(const SimpleGreedyMappingTechnique & … Read more
It’s a frequently asked question: How do I extract ‘the’ value from my monad, not only in Haskell, but in other languages as well. I have a theory about why this question keeps popping up, so I’ll try to answer according to that; I hope it helps. Containers of single values You can think of … Read more
The word “functor” comes from category theory, which is a very general, very abstract branch of mathematics. It has been borrowed by designers of functional languages in at least two different ways. In the ML family of languages, a functor is a module that takes one or more other modules as a parameter. It’s considered … Read more
A function object is just that. Something which is both an object and a function. Aside: calling a function object a “functor” is a serious abuse of the term: a different kind of “functors” are a central concept in mathematics, and one that has a direct role in computer science (see “Haskell Functors”). The term … Read more
You cannot directly pass a pointer to a C++ functor object as a function pointer to C code (or even to C++ code). Additionally, to portably pass a callback to C code it needs to be at least declared as an extern “C” non-member function. At least, because some APIs require specific function call conventions … Read more