lazy-evaluation

问题 How would one implement a list of prime numbers in Haskell so that they could be retrieved lazily? I am new to Haskell, and would like to learn about practical uses of the lazy evaluation functionality. 回答1: Here's a short Haskell function that enumerates primes from Literate Programs: primes :: [Integer] primes = sieve (2 : [3, 5..]) where sieve (p:xs) = p : sieve [x|x <- xs, x `mod` p > 0] Apparently, this is not the Sieve of Eratosthenes (thanks, Landei). I think it's still an instructive

问题 I often read that lazy is not the same as non-strict but I find it hard to understand the difference. They seem to be used interchangeably but I understand that they have different meanings. I would appreciate some help understanding the difference. I have a few questions which are scattered about this post. I will summarize those questions at the end of this post. I have a few example snippets, I did not test them, I only presented them as concepts. I have added quotes to save you from

问题 Background In order to speed up generating grouped summaries across multiple tables; as I'm doing most of that while in dplyr workflow, I've drafted a simple function that generates the desired metrics # Function to generate summary table generate_summary_tbl <- function(dataset, group_column, summary_column) { group_column <- enquo(group_column) summary_column <- enquo(summary_column) dataset %>% group_by(!!group_column) %>% summarise( mean = mean(!!summary_column), sum = sum(!!summary

问题 Excel's if function takes three arguments, a condition, an if-true value, and an if-false value. Does Excel work out the value of all three arguments, or does it only work out the value of the condition and the corresponding result? Clarification: I'm not wondering what the result of the if will be, I'm wondering whether or not it calculates the value of all arguments before calculating the result of the function. This is equivalent to asking whether or not the if function uses lazy or strict

问题 I'm trying to sort an array with a map() over an iterator. struct A { b: Vec<B>, } #[derive(PartialEq, Eq, PartialOrd, Ord)] struct B { c: Vec<i32>, } fn main() { let mut a = A { b: Vec::new() }; let b = B { c: vec![5, 2, 3] }; a.b.push(b); a.b.iter_mut().map(|b| b.c.sort()); } Gives the warning: warning: unused `std::iter::Map` that must be used --> src/main.rs:16:5 | 16 | a.b.iter_mut().map(|b| b.c.sort()); | ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ | = note: #[warn(unused_must_use)] on by

问题 How can lazy evaluation be achieved in Python? A couple of simple examples: >>> def foo(x): ... print(x) ... return x ... >>> random.choice((foo('spam'), foo('eggs'))) spam eggs 'eggs' Above, we didn't really need to evaluate all the items of this tuple, in order to choose one. And below, the default foo() didn't really need to be computed unless the lookup key was actually missing from the dict: >>> d = {1: "one"} >>> d.get(2, foo("default")) default 'default' >>> d.get(1, foo("default"))

问题 Scala does not permit to create laze vars, only lazy vals. It make sense. But I've bumped on use case, where I'd like to have similar capability. I need a lazy variable holder. It may be assigned a value that should be calculated by time-consuming algorithm. But it may be later reassigned to another value and I'd like not to call first value calculation at all. Example assuming there is some magic var definition lazy var value : Int = _ val calc1 : () => Int = ... // some calculation val

问题 Scala does not permit to create laze vars, only lazy vals. It make sense. But I've bumped on use case, where I'd like to have similar capability. I need a lazy variable holder. It may be assigned a value that should be calculated by time-consuming algorithm. But it may be later reassigned to another value and I'd like not to call first value calculation at all. Example assuming there is some magic var definition lazy var value : Int = _ val calc1 : () => Int = ... // some calculation val

问题 I understand that in Swift, static vars are implicitly lazy: https://stackoverflow.com/a/34667272/1672161 But I'm not clear on why this happens: protocol HatType {} class Hat: HatType { init() { print("real hat") } } class MockHat: HatType { init() { print("mock hat") } } struct HatInjector { static var hat: HatType = Hat() } HatInjector.hat = MockHat() // Output: // real hat // mock hat What I'm seeing is that the assignment to the static var, is also invoking the getter in a sense. This isn

问题 I'm trying to learn me a Haskell (for great good), and one of the many different things I'm doing is trying to tackle some Project Euler problems as I'm going along to test my mettle. In doing some of the Fibonacci based problems, I stumbled on and started playing around with the recursive infinite list version of the Fibonacci sequence: fibs = 1 : 2 : zipWith (+) fibs (tail fibs) For one of the PE problems, I needed to extract the subsequence of even Fibonacci numbers less than 4,000,000. I