functional-programming

问题 Can I unsugar list comprehension in this expression: [(i,j) | i <- [1..4], j <- [i+1..4]] This is the output: [(1,2),(1,3),(1,4),(2,3),(2,4),(3,4)] How can I, with map, filter and so on, write that piece of code? edit Here an other: [(i,j,k) | i <- [1..6], j <- [i+1..6],k <- [j+1..6]] This is the output: [(1,2,3),(1,2,4),(1,2,5),(1,2,6),(1,3,4),(1,3,5),(1,3,6),(1,4,5),(1,4,6),(1,5,6),(2,3,4),(2,3,5),(2,3,6),(2,4,5),(2,4,6),(2,5,6),(3,4,5),(3,4,6),(3,5,6),(4,5,6)] 回答1: List comprehensions (in

问题 Can I unsugar list comprehension in this expression: [(i,j) | i <- [1..4], j <- [i+1..4]] This is the output: [(1,2),(1,3),(1,4),(2,3),(2,4),(3,4)] How can I, with map, filter and so on, write that piece of code? edit Here an other: [(i,j,k) | i <- [1..6], j <- [i+1..6],k <- [j+1..6]] This is the output: [(1,2,3),(1,2,4),(1,2,5),(1,2,6),(1,3,4),(1,3,5),(1,3,6),(1,4,5),(1,4,6),(1,5,6),(2,3,4),(2,3,5),(2,3,6),(2,4,5),(2,4,6),(2,5,6),(3,4,5),(3,4,6),(3,5,6),(4,5,6)] 回答1: List comprehensions (in

问题 In short, this works: [1, 2, 3].reduce(function (a, b) { return Math.max(a, b); }); => 3 But this doesn't: [1, 2, 3].reduce(Math.max); => NaN Pure puzzlement. This is in Firefox 3.5.9, which I presume is using the mozilla standard implementation of reduce, FWIW. 回答1: Math.max can be used as a higher-order function. The problem is .reduce will call the function with 4 arguments: Math.max(accumulator, value, index, the_array) here is the_array is an array, so Math.max returns NaN. I don't think

问题 Ok, only just in F# and this is how I understand it now : Some problems are recursive in nature (building or reading out a treestructure to name just one) and then you use recursion. In these cases you preferably use tail-recursion to give the stack a break Some languagues are pure functional, so you have to use recursion in stead of while-loops, even if the problem is not recursive in nature So my question : since F# also support the imperative paradigm, would you use tail recursion in F#

问题 Ok, only just in F# and this is how I understand it now : Some problems are recursive in nature (building or reading out a treestructure to name just one) and then you use recursion. In these cases you preferably use tail-recursion to give the stack a break Some languagues are pure functional, so you have to use recursion in stead of while-loops, even if the problem is not recursive in nature So my question : since F# also support the imperative paradigm, would you use tail recursion in F#

问题 Most of the problems I have to solve in my job as a developer have to do with data modeling. For example in a OOP Web Application world I often have to change the data properties that are in a object to meet new requirements. If I'm lucky I don't even need to programmatically add new "behavior" code (functions,methods). Instead I can declarative add validation and even UI options by annotating the property (Java). In Functional Programming it seems that adding new data properties requires

问题 Most of the problems I have to solve in my job as a developer have to do with data modeling. For example in a OOP Web Application world I often have to change the data properties that are in a object to meet new requirements. If I'm lucky I don't even need to programmatically add new "behavior" code (functions,methods). Instead I can declarative add validation and even UI options by annotating the property (Java). In Functional Programming it seems that adding new data properties requires

问题 I am currently reading Learn You a Haskell for Great Good! and am stumbling on the explanation for the evaluation of a certain code block. I've read the explanations several times and am starting to doubt if even the author understands what this piece of code is doing. ghci> (+) <$> (+3) <*> (*100) $ 5 508 An applicative functor applies a function in some context to a value in some context to get some result in some context. I have spent a few hours studying this code block and have come up

问题 I am trying to find a better way to implement this: d = {"a": {"b": {"c": 4}}} l = ["a", "b", "c"] for x in l: d = d[x] print (d) # 4 I am learning functional programming so I am just trying random example that come to my head :) 回答1: Use reduce(): reduce(dict.__getitem__, l, d) or better still, using operator.getitem(): from operator import getitem reduce(getitem, l, d) Demo: >>> d = {"a": {"b": {"c": 4}}} >>> l = ["a", "b", "c"] >>> from operator import getitem >>> reduce(getitem, l, d) 4

问题 This question already has answers here : Error: Immutable value passed on reduce function (3 answers) Closed 4 years ago . I'm using Xcode 6.4 I have an array of UIViews and I want to convert to a Dictionary with keys "v0", "v1"... . Like so: var dict = [String:UIView]() for (index, view) in enumerate(views) { dict["v\(index)"] = view } dict //=> ["v0": <view0>, "v1": <view1> ...] This works, but I'm trying to do this in a more functional style. I guess it bothers me that I have to create the