higher-order-functions

问题 I am writing a small program that finds a winner of a marathon. Everything seems logical until I try to filter the vector for runners that are late for some amount of time. The vector remains same after the filter function, and if use iter_mut() it states type errors. fn main() { let mut input_line = String::new(); std::io::stdin().read_line(&mut input_line); let n = input_line.trim().parse::<u8>().unwrap(); let mut v = Vec::with_capacity(n as usize); for _ in 0..n { let mut input_line =

问题 I am writing a small program that finds a winner of a marathon. Everything seems logical until I try to filter the vector for runners that are late for some amount of time. The vector remains same after the filter function, and if use iter_mut() it states type errors. fn main() { let mut input_line = String::new(); std::io::stdin().read_line(&mut input_line); let n = input_line.trim().parse::<u8>().unwrap(); let mut v = Vec::with_capacity(n as usize); for _ in 0..n { let mut input_line =

问题 function each(collection, callback) { var arr = []; for(var i = 0; i < collection.length; i++) { var result = callback(collection[i]) if (typeof result !== 'undefined') { arr.push(callback(collection[i])); } } return arr } function isNumber(item) { if (typeof item === "number") { return item * 2; } } I am trying to understand higher order functions. The above works, but is apparently not best practice to return a value with isNumber . I have been told that a Boolean value should be returned.

问题 map2_List :: (a -> b -> c) -> [a] -> [b] -> [c] map2_List f [] _ = [] map2_List f (a:as) bs = map (f a) bs ++ map2_List f as bs This is an example from my lecture, which try to apply a binary function to all pairs of elements of two lists. The part (f a) makes me confused. Does it suppose to be a value but not a function? Then what does map value bs do? 回答1: "The part (f a) makes me confused." What is happening here is called currying and if you are coming to Haskell from imperative languages

问题 Taking an exemple derived from RWOCaml : utop # let divide ~first ~second = first / second;; val divide : first:int -> second:int -> int = <fun> utop # let apply_to_tuple_3 f (first,second) = f second first;; val apply_to_tuple_3 : ('a -> 'b -> 'c) -> 'b * 'a -> 'c = <fun> utop # apply_to_tuple_3 divide;; Error: This expression has type first:int -> second:int -> int but an expression was expected of type 'a -> 'b -> 'c Does it make sense to not match the types here ? apply_to_tuple_3 only

问题 Based on this post throttleFirst function: fun <T> throttleFirst( skipMs: Long = 700L, scope: CoroutineScope = viewModelScope, action: (T) -> Unit ): (T) -> Unit { var throttleJob: Job? = null return { param: T -> if (throttleJob?.isCompleted != false) { throttleJob = coroutineScope.launch { destinationFunction(param) delay(skipMs) } } } } I'm using it like this: View <Button android:onClick="@{viewModel.myClickListener}" .../> ViewModel: fun myClickListener() = View.OnClickListener { _ ->

问题 I am looking for an elegant way to generate booleans that will eventually be joined using && operator inside my callback function in filter method. I tried to loop through the filter conditions but I cannot find a way to join each iteration result into the following format: return Boolean && Boolean && Boolean && Boolean && Boolean Becasue += && Boolean doesn't work. Here is what I have and what is working: //data I am filtering this.preSearch = [ ["The Lord of the Rings", "J. R. R. Tolkien",

问题 function noisy(f) { return function (arg) { console.log("Calling with", arg); var val = f(arg); console.log("Called with", arg, "- got", val); return val; }; } noisy(Boolean)(0); // calling with 0 // Called with 0 - got false I understand that higher function can be functions that change other functions, like the example above. I understand why the second console.log gives Called with 0 - got false as it's output. I don't understand why the second parameter (0) is contained in a second pair

问题 I am trying to implement the K combinator from the SK combinator calculus in C++. The K combinator is a higher-order function that basically takes some value x , and returns something which in turn takes a value y and returns x from it. In other words, K(x)(y) == x or step-by-step: intermediate = K(x) intermediate(y) == x The ability to treat K(x) as a thing-in-itself, independent of y , is essential. Furthermore, it should not be necessary to specify the type of y when simply creating K(x)

问题 Why is the following valid? my @ys = map { $_ * $_ } @xs; And the following invalid? my @ys = map { $_ * $_ }, @xs; Is map a language construct and not really a function, or are there special rules for blocks? 回答1: map is list operator and a core function . It is simple Perl syntax that expects no comma after a block parameter to a subroutine. The special thing about map is that it can also take the form map EXPR, LIST . If this was used with a standard subroutine the EXPR would just be