If you type :i (->) in GHCi and hit Enter it returns the following: data (->) t1 t2 — Defined in ‘GHC.Prim’ infixr 0 `(->)` instance Monad ((->) r) – Defined in ‘GHC.Base’ instance Functor ((->) r) – Defined in ‘GHC.Base’ instance Applicative ((->) a) – Defined in ‘GHC.Base’ instance Monoid b => Monoid (a -> b) – Defined… Read More What is (->) in Haskell?
Predefined functors need to be in-place instantiated (with empty parentheses) for use in algorithms but not as type parameters for container adapters such as priority_queue. Why the difference? #include <queue> #include <vector> #include <numeric> int main(){ std::priority_queue<int, std::vector<int>, // parentheses are NOT neded here: std::greater<> std::greater<>> pq; pq.push(1); pq.push(2); pq.push(3); std::vector<int> v = {1, 2,… Read More C+ selective predefined functor initializaiton
I want to have a Map datastructure for "env" in my lisp interpreter. My code in env.mli is module Env = Map.Make(String) type t = (Ast.expression) Env.t val empty: t val add: string -> Ast.expression -> t -> t val lookup: string -> t -> Ast.expression But my code in env.ml is: module Env =… Read More The module Map.Make is a functor, it cannot have any components
I have a class which contains an integer among some other things: class Foo { public: Foo() = default; Foo(int x) : x(x) {} int operator+(const Foo& rhs) { return x + rhs.x; } private: int x; float whatever = 0.0f; }; I also have a vector of these objects: std::vector<Foo> foos{1, 2, 3, 4};… Read More std::accumulate won't' peform sum of elements with std::plus functor
I have a functor and the overloaded operator() of it returns a double. In an easy way I can write: int main(){ auto f=[](double t){return 1.0/(pow(7.0*t,2.0)+1.0);}; std::vector<double> nodes(n); bestpolchevnod(f,nodes); //calculate nodes = f_i(cheby_point(i)) ChebyPoly c_p = ChebyPoly(nodes);//call the constructor with the calculated nodes std::cout << c_p(0.6) << std::endl; //evaluate at c_p(0.6) as an approx of… Read More save result of for_each algorithm inside a vector
In order to learn more about Go, generics, and functional programming, I implemented a List type (excerpt of list.go): package funcprog type Function[T any] func(T) T type List[T any] []T func (l List[T]) Map(f Function[T]) List[T] { var ys List[T] for _, x := range l { y := f(x) ys = append(ys, y) }… Read More Go: own List type incompatible with own Functor type
why does the following code not cause an error? Instead, it exits immediately. someAction :: IO (Either String ()) someAction = return $ Left "Error" main :: IO () main = either error id <$> someAction >Solution : When an action of type IO () is executed, its return value (the unit) is not evaluated.… Read More Why doesn't fmap either error id over an Either work?
Why does this fmap (replicate 3) Just "JOHN" return this [Just "JOHN",Just "JOHN",Just "JOHN"] ? I know that fmap (replicate 3) Just "JOHN" is equivalent with fmap (replicate 3) Just $ "JOHN", but why does that even compile ? How can we apply fmap to Just which isn’t even of concrete type ? >Solution :… Read More About some basic functor in haskell