Let me answer this with a question: Which output do you expect for: main = print $ fmap (*2) (“funny”,2) You can have something as you want (using data Pair a = Pair a a or so), but as (,) may have different types in their first and second argument, you are out of luck.
This is right. There is also another quite important reason for this behavior: You can think of Either a b as a computation, that may succeed and return b or fail with an error message a. (This is also, how the monad instance works). So it’s only natural, that the functor instance won’t touch the … 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
You can write function_traits class as shown below, to discover the argument types, return type, and number of arguments: template<typename T> struct function_traits; template<typename R, typename …Args> struct function_traits<std::function<R(Args…)>> { static const size_t nargs = sizeof…(Args); typedef R result_type; template <size_t i> struct arg { typedef typename std::tuple_element<i, std::tuple<Args…>>::type type; }; }; Test code: struct … Read more
UDATE. See this new library providing functor and monad operators for plain callback-based functions that do not have the issues with theneables: https://github.com/dmitriz/cpsfy The JS Promise is neither a Functor nor an Applicative nor a Monad It is not a functor, because the composition preservation law (sending compositions of functions to compositions of their images) … Read more
From a programmer’s point of view the essence of functor-ness is being able to easily adapt things. What I mean by “adapt” here is that if I have an f a and I need an f b, I’d like an adaptor that will fit my f a in my f b-shaped hole. It seems intuitive … Read more
A functor may go from one category to a different one, an endofunctor is a functor for which start and target category are the same. Same as with endomorphisms versus morphisms. Now, why must monads be endofunctors? There is the famous quote that “Monads are just monoids in the category of endofunctors”. Fortunately, somebody else … Read more
I’ve accidently written a Haskell Functors Tutorial I’ll answer your question using examples, and I’ll put the types underneath in comments. Watch out for the pattern in the types. fmap is a generalisation of map Functors are for giving you the fmap function. fmap works like map, so let’s check out map first: map (subtract … Read more
I can’t explain why, but here’s the proof of the cycle: Assume k >= 2 and fmap^(4k) :: (a -> b) -> F1 F2 F3 a -> F1 F2 F3 b, where Fx stands for an unknown/arbitrary Functor. fmap^n stands for fmap applied to n-1 fmaps, not n-fold iteration. The induction’s start can be verified … 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