functional-programming

问题 I'm writing a small interpreter for a C-like language in Scheme (R5RS) and trying to convert something like: for (i = 0; i < 100; i++) { if (isprime(i)) continue; else /* do something with i */ } to valid Scheme (the isprime function is just an example and not important). However, after trying for some time, I have not been able to find an efficient/simple way to add the equivalent of a continue statement to a do loop in Scheme. What would be even better would be a "for" macro which allows

问题 I've always wondered how the Haskell exception system fits in with the whole "Pure functional language" thing. For example see the below GHCi session. GHCi, version 8.0.1: http://www.haskell.org/ghc/ :? for help Prelude> head [] *** Exception: Prelude.head: empty list Prelude> :t head head :: [a] -> a Prelude> :t error error :: [Char] -> a Prelude> error "ranch" *** Exception: ranch CallStack (from HasCallStack): error, called at <interactive>:4:1 in interactive:Ghci1 Prelude> The type of

问题 In a functional language, functions are first class citizens and thus calling them is not the only thing I can do. I can also store them away. Now when I have a language, which is strict by default, then I am still not forced to evaluate a function call. I have the option to store the function and its parameters e.g. in a tuple for later evaluation. So instead of x = f a b c I do something like x = (f,a,b,c) And later, I can evaluate this thing with something like eval (f,a,b,c) = f a b c

问题 This is a rather general question. Functional-style programming promotes the idea that a program is about transforming data through functions, and that mutation should be avoided (except possibly within a function, seen as a basic unit of abstraction). But in this program: function foo (bar) { bar.k1 = "bananas"; return bar; } var o = { k1: "apples", k2: "oranges"}; var p = foo(o); the external variable o is mutated within foo because bar is a reference to o, and, in the end, o === p (they

问题 I want to zip even and odd elements in a list to make a list of pairs, like that: ["A", "B", "C", "D", "E", "F"] -> [("A", "B"), ("C", "D"), ("E", "F")] What is the most concise expression to do this in elegant in functional way? 回答1: In 2.8, you'd probably use methods: scala> val a = "ABCDEF".toList.map(_.toString) a: List[java.lang.String] = List(A, B, C, D, E, F) scala> a.grouped(2).partialMap{ case List(a,b) => (a,b) }.toList res0: List[(java.lang.String, java.lang.String)] = List((A,B),

问题 If I have a function that is part of a module, and I want a log entry while inside the function,I have to manually print the function namespace and name e.g. namespace MyModuleNamespace module MyModule = let AddTwoNums logger x y = logger.Info("MyModuleNamespace.AddTwoNums - Start") let res = x+y logger.Info("MyModuleNamespace.AddTwoNums - End") res Is there any way I can automatically work out what "MyModuleNamespace.AddTwoNums" is as it is very cumbersome/error prone especially when you

问题 I would like to apply a function to values of a dict inplace in the dict (like map in a functional programming setting). Let's say I have this dict : d = { 'a':2, 'b':3 } I want to apply the function divide by 2.0 to all values of the dict, leading to: d = { 'a':1., 'b':1.5 } What is the simplest way to do that? I use Python 3 . Edit: A one-liner would be nice. The divide by 2 is just an example, I need the function to be a parameter. 回答1: You may find multiply is still faster than dividing

问题 I was just searching for better way to handle this scenario using java 8 streams. Object A has list of object b. What I get is a list of object A (List). I need to stream through list of object A and get all the listB's in each of the object A as a one single list. class A { List<B> listB } I have tried the below way it throws compilation List<A> as = someObject.getAs(); List<B> listofBs = as.stream().map(in -> in.getListB()).collect(Collectors.toList()); 回答1: To get a single list of all B's,

问题 I want to create a list of variations of applying a function to every element of a list. Here is a quick example of what I mean. applyVar f [a, b, c] >> [[(f a), b, c], [a, (f b), c], [a, b, (f c)]] Essentially It applies a function to each element of a list individually and stores each possible application in an array. I'm not too sure how to approach a problem like this without using indexes as I have heard they are not very efficient. This is assuming that the function f returns the same

问题 trying to implement Kleisli category for a made-up Partial type in Scala (reading Bartosz Milewski's "category theory for programmers", that's exersize for chapter 4) object Kleisli { type Partial[A, B] = A => Option[B] implicit class KleisliOps[A, B](f1: Partial[A, B]) { def >=>[C](f2: Partial[B, C]): Partial[A, C] = (x: A) => for { y <- f1(x) z <- f2(y) } yield z def identity(f: Partial[A, B]): Partial[A, B] = x => f(x) } val safeRecip: Partial[Double, Double] = { case 0d => None case x =>