functional-programming

问题 I'd like to write a Haskell program that uses GADTs interactively on a platform not supported by GHCi (namely, GNU/Linux on mipsel). The problem is, the construct that can be used to define a GADT in GHC, for example: data Term a where Lit :: Int -> Term Int Pair :: Term a -> Term b -> Term (a,b) ... doesn't seem working on Hugs. Can't GADTs really be defined in Hugs? My TA at a Haskell class said it was possible in Hugs, but he seemed unsure. If not, can GADT be encoded by using other syntax

问题 I am converting the "zxcvbn" password strength algorithm from JavaScript to Scala. I am looking for a pure functional algorithm for finding sequences of repeating characters in a string. I know that I can translate the imperative version from JavaScript, but I would like to keep this as side-effect free as possible, for all the reasons usually given for functional programming. The algorithm can be in Scala, Clojure, Haskell, F#, or even pseudocode. Thanks. 回答1: Using Haskell's standard higher

问题 I am using Sails' ORM (Waterline). I have written a geturl service that should return the url of several models/actions in my app. I am currently calling this service inside my templates. (As I am alone to develop this, don't hesitate to warn me if this design pattern is wrong) Now it occurs that Waterline's .find() method is asynchronous (as it should). I always use callbacks to do things when inserting or fetching things in database. Now I have seen everywhere that I cannot return any data

问题 I am struggling with understanding what I really need to do, and would like some outside input or a point to a good reference. I have been asked to use procedural representation to "implement sets of numbers." Each set will be a one argument function that takes a number and decides if the number is in the set. A few functions (that I have read can be defined in one line) that I have to create: A function that returns a function taking a number as an argument and checks if the number is in the

问题 I usually hear the term lifting, when people are talking about map , fold , or bind , but isn't basically every higher order function some kind of lifting? Why can't filter be a lift from a -> Bool to [a] -> [a] , heck even the bool function (which models an if statement) can be considered a lift from a -> a to Bool -> a . And if they are not, then why is ap from the Applicative type class considered a lift? If the important thing is going from ... a ... to ... f a ... , then ap wouldn't fit

问题 When working with arrays, intermediate representations are needed regularly - particularly in connection with functional programming, in which data is often treated as immutable: const square = x => x * x; const odd = x => (x & 1) === 1; let xs = [1,2,3,4,5,6,7,8,9]; // unnecessary intermediate array: xs.map(square).filter(odd); // [1,4,9,16,25,36,49,64,81] => [1,9,25,49,81] // even worse: xs.map(square).filter(odd).slice(0, 2); // [1,9] How can I avoid this behavior in Javascript/Ecmascript

问题 I'm clear about the generic functions and generic data-types. In the generic type: data SB = forall x. (show x) => SB x instance Show SB where show (SB x) = show x so for any given type x , if it has a signature of Show , and there sure be a show function corresponds to it. but when typing in ghci, e.g. :t 1 outputs 1 :: Num a => a if binds 1 to a name: let a=1 :t a now a has a true type. The question is: What is 1 's form in the run-time system (before it has a type, it has only Num ) ,

问题 I've been reading a nice answer to Difference between reduce and foldLeft/fold in functional programming (particularly Scala and Scala APIs)? provided by samthebest and I am not sure if I understand all the details: According to the answer ( reduce vs foldLeft ): A big big difference (...) is that reduce should be given a commutative monoid, (...) This distinction is very important for Big Data / MPP / distributed computing, and the entire reason why reduce even exists. and Reduce is defined

问题 Assuming the follow object how is it possible to use Ramda to perform a nested update in a criteria given an application, criteria ID and data? const application = { id: 'a1', features: [ { id: 'f1', criterias: [ { id: 'c1' } ] }, { id: 'f2', criterias: [ { id: 'c2' }, { id: 'c3' } ] } ] } The function would look something like this: const updateCriteria = (application, criteriaId, data) => // magic... updateCriteria(application, 'c2', { name: 'foo' }) // output: { // id: 'a1', // features: [

问题 I'm trying to return from a function a list of functions, each of which uses variables from the outside scope. This isn't working. Here's an example which demonstrates what's happening: a = [] for i in range(10): a.append(lambda x: x+i) a[1](1) # returns 10, where it seems it should return 2 Why is this happening, and how can I get around it in python 2.7 ? 回答1: The i refers to the same variable each time, so i is 9 in all of the lambdas because that's the value of i at the end of the loop.