functional-programming

问题 I'm studying Elixir and when I use only or except operators when importing functions from a module I need to specify an arity number. Why? e.g. import :math, only: [sqrt: 1] or import :math, except: [sin: 1, cos: 1] 回答1: Across the Erlang ecosystem functions are identified by name + arity. In most other languages you can overload functions by name. In other words, in the Erlang world foo/1 (that is, foo(one_arg)) is a completely different function than foo/2 (as in, foo(one_arg, two_arg)),

问题 I have a problem to prevent unauthorized users from accessing authorized-only routes/components - such as logged in users dashboard I have the following code: import React from 'react' //other imports import {withRouter} from 'react-router' class User extends React.Component { constructor(props) { super(props) console.log('props', props) let user = JSON.parse(localStorage.getItem('userDetails')) if(!user || !user.user || props.match.params.steamId !== user.user.steamId) { props.history.push('

问题 I want to divide two natural number. I have made function like this _/_ : N -> N -> frac m / one = m / one (suc m) / n = ?? I dont know what to write here. Please help. 回答1: As @gallais says you can use well-founded recursion explicitly, but I don't like this approach, because it's totally unreadable. This datatype record Is {α} {A : Set α} (x : A) : Set α where ¡ = x open Is ! : ∀ {α} {A : Set α} -> (x : A) -> Is x ! _ = _ allows to lift values to the type level, for example you can define a

问题 What is the difference and the term name between these underscore usage from these codes: (see the handler(resource) part) 1. def readFile[T](f: File)(handler: FileInputStream => Byte => T): T = { val resource = new java.io.FileInputStream(f) try { val hh = handler(resource)_ hh(2) } finally { resource.close() } } val bs = new Array[Byte](4) readFile(new File("scala.txt")) { input => b: Byte => println("Read: " + (input.read(bs) + b)) } I got compile error: Error:(55, 29) _ must follow method

问题 I have a hibernate-mapped Java object, JKL , which is full of a bunch of normal hibernate-mappable fields (like strings and integers). I'm added a new embedded field to it (which lives in the same table -- not a mapping), asdf , which is a fj.data.Option<ASDF> . I've made it an option to make it clear that this field may not actually contain anything (as opposed to having to handle null every time I access it). How do I set up the mapping in my JKL.hbm.xml file? I'd like hibernate to

问题 It is not hard to shuffle an array in O(n), with in place swapping, How to do it for list in OCaml, with O(n)? Requirement: No array or in place usage Consider this as an interview question 回答1: Lists are immutable, and there's often a log n price to pay for working with immutable data. If you're willing to pay this cost, there's an obvious n log n approach: tag each list element with a random value, sort based on random value, remove random values. This is the way I shuffle lists in my

问题 I'd like to make copies of my 2D array, which feels like the nice, functional, nondestructive way of handling arrays. What is the lispy way of doing this? 回答1: UPDATE : Nowadays, alexandria has a copy-array very similar to the implementation given below. Use that. OBSOLETE ANSWER : I used the following, which I believed was better than the alexandria version at the time: (defun copy-array (array &key (element-type (array-element-type array)) (fill-pointer (and (array-has-fill-pointer-p array)

问题 When thinking in a functional mindset, given that functions are supposed to be pure, one can conclude any function with no arguments is basically just a value. However, reallity gets in the way, and with different inputs, I might not need a certain function, and if that function is computationally expensive, I'd like to not evaluate it if it's not needed. I found a workaround, using let func _ = ... and calling it with func 1 or whatever, but that feels very non-idiomatic and confusing to the

问题 Say we have two sequences and we and we want to combine them using some method val a = Vector(1,2,3) val b = Vector(4,5,6) for example addition could be val c = a zip b map { i => i._1 + i._2 } or val c = a zip b map { case (i, j) => i + j } The repetition in the second part makes me think this should be possible in a single operation. I can't see any built-in method for this. I suppose what I really want is a zip method that skips the creation and extraction of tuples. Is there a prettier /

问题 I understand that you could be more efficient with memory in the implementation of map than in how you might do it over a loop. However, I see that using a map function over calling a function iterating over a loop has a speed boost as well. Is this coming from some optimization in where to store the memory? One example of what I mean by this is placement of memory done in such a way that it is contiguous. Also I can see that if the operations were being in run in parallel then there would