functional-programming

问题 I am coding with Racket for educational reasons. I was given a task in which I should create a function that, without filter, would receive a list as an input and return another list only with the even numbers of the first list. I came up with this recursive definition of an iterative process: (define (add-even lista) (define (iter lista accu) (cond ((null? lista) accu) ((even? (car lista)) (iter (cdr lista) (cons (car lista) accu))) (else (iter (cdr lista) accu)))) (iter lista empty)) It

问题 in most articles about Haskell you'll find a statement like "Data in Haskell is immutable". I don't quite understand why. For example: let a = 123 let a = 456 in the main method works. I just changed the data of a from 123 to 456 . What am I missing? It's probably a stupid mistake in my train of thought :/ Have a good day! 回答1: Actually, a hasn't changed. Try this in ghci to see: > a = 123 > action = print a > a = 456 > action 123 Compare with a language that has mutable variables, e.g.

问题 I have two files: myUnionFind.ml and myUnionFind_test.ml . Both files are in the same directory . myUnionFind.ml open Batteries module type MyUnionFindSig = sig type union_find val print_array : 'a array -> unit val create_union : int -> union_find val union_weighted : union_find -> int -> int -> unit val is_connected_weighted : union_find -> int -> int -> bool end;; module MyUnionFind : MyUnionFindSig = struct let print_array ary = print_endline (BatPervasives.dump ary);; type union_find =

问题 From a FP course: type Set = Int => Boolean // Predicate /** * Indicates whether a set contains a given element. */ def contains(s: Set, elem: Int): Boolean = s(elem) Why would that make sense? assert(contains(x => true, 100)) Basically what it does is provide the value 100 to the function x => true . I.e., we provide 100, it returns true . But how is this related to sets? Whatever we put, it returns true . Where is the sense of it? I understand that we can provide our own set implementation

问题 Background I have a function that makes a request to a server. If the request fails, I want to: 1. log the error 2. run a terminal command 2.1 log if the command failed or succeeded To achieve this I have the following code: const createRequest = ( { request, logger, terminal } ) => ( { endpoint, timeout } ) => request.get( endpoint, { timeout } ) .then( response => logger.info( { event: "Heartbeat request succeeded.", status: response.status } ) ) .catch( err => logger.error( { event:

问题 Colouring problem : Hello, I'm trying to implement a bool function that returns true when a color can be extended to a country and false otherwise but I'm having trouble working with sets as we cannot pattern match them... My code : type Country = string;; type Chart = Set<Country*Country>;; type Colour = Set<Country>;; type Colouring = Set<Colour>;; (* This is how you tell that two countries are neghbours. It requires a chart.*) let areNeighbours ct1 ct2 chart = Set.contains (ct1,ct2) chart

问题 I am trying to write a function like zip. I am not good at explaining what I mean, so i will just show 'code' of what i'm trying to do. a = [1,2,3,[4,5]] b = a[:] zip(a, b) == [(1,1), (2,2), (3,3), ([4,5],[4,5])] myzip(a, b) == [(1,1), (2,2), (3,3), [(4,4), (5,5)]] I am so stuck on this it's not even funny. I am trying to write it in a simple functional way with recursive lambdas, to make my code prettier. I want myzip like this because i want to use its output with another function I wrote

问题 Say I have double xSquared( const double ) { return x*x; } ... std::function<double (double)> func = &xSquared; ... which works fine for the (more complicated) purposes I use this structure, up till now. Now I have a function that accepts a std::function of the above form and I need to create a new std::function that extends the original: typedef std::function<double (double)> func1D; double someFunction( const func1D &func, const double a ) { func1D extendedFunc = func/(x-a); // I know this

问题 Say I have double xSquared( const double ) { return x*x; } ... std::function<double (double)> func = &xSquared; ... which works fine for the (more complicated) purposes I use this structure, up till now. Now I have a function that accepts a std::function of the above form and I need to create a new std::function that extends the original: typedef std::function<double (double)> func1D; double someFunction( const func1D &func, const double a ) { func1D extendedFunc = func/(x-a); // I know this

问题 I want to write a function that takes in a sequence <1,1,2,2,3> and returns the sequence with equal elements grouped like <<1,1>, <2,2>, <3>>. I'm using sequences, not lists, but some of the functions are similar. Some of the functions I am thinking of using are map, reduce, tabulate, filter, append etc.. Reduce takes in an associative function and returns the sequence that is "reduced" by that operator. So, reduce op+ 0 <1,2,3> = 6. My first thought was to use map to raise the sequence by