function call
Per MDN, A function call is an expression that passes control and
arguments (if any) to a function and has the form: expression
(expression-listopt)
callback function
A callback function is a lucky function that gets passed into the enclosing higher-order function: The callback function gets executed (called) inside the higher order function, but not necessarily immediately.
https://thenewstack.io/mastering-javascript-callbacks-bind-apply-call/
function call is term related to program syntax. it related to generally programs and programming languages. There are no predefined semantic, just term which used to describe programs.
callback function is term related to program execution semantic. In some kind callback function is function call with scope, which pass as parameter to another function or method, some parameters of callback function can be free variables and callback function executed generally after method or function in which it was passed.
Related
I know by definition higher order functions are functions that receive a function as a parameter or return a function.
I want to know if a nested function is considered a higher order function since there are functions defined inside of it, and if it is the case are nested functions that return a dispatch dictionary considered higher order as well?(they may not receive a function but they do return a dictionary which can activate a function but its not a function up until activation with a correct key).
No, a nested function is not higher-order. As you noted, "higher order functions are functions that receive a function as a parameter or return a function." It is "higher-order" because it is a function that operates on functions. Nesting a function does not do that. It simply modifies its scope. Similarly, a closure (which in most languages has many similarities to a nested function), is not itself a higher-order function.
Is there a way in a where clause in Rust to check if a function passed as a parameter takes specified arguments?
If you're defining a function that takes a function argument, that argument has a very specific type associated with it that dictates the arguments. You cannot call that function with something that doesn't match, it just won't compile.
If you're thinking in terms of dynamic languages where the arguments are somewhat subjective, you're assuming you can make a mistake here and call it incorrectly. You can't. It's strictly disallowed.
Here is a simplified code
func MyHandler(a int) http.Handler {
return http.HandlerFunc(func(w http.ResponseWriter, r *http.Request) {
w.WriteCode(a)
})
}
Whenever a http request comes MyHandler will be called, and it will return a function which will be used to handle the request. So whenever a http request comes a new function object will be created. Function is taken as the first class in Go. I'm trying to understand what actually happened when you return a function from memory's perspective. When you return a value, for example an integer, it will occupy 4 bytes in stack. So how about return a function and lots of things inside the function body? Is it an efficient way to do so? What's shortcomings?
If you're not used to closures, they may seem a bit magic. They are, however, easy to implement in compilers:
The compiler finds any variables that must be captured by the closure. It puts them into a working area that will be allocated and remain allocated as long as the closure itself exists.
The compiler then generates the inner function with a secret extra parameter, or some other runtime trickery,1 such that calling the function activates the closure.
Because the returned function accesses its closure variables through the compile-time arrangement, there's nothing special needed. And since Go is a garbage-collected language, there's nothing else needed either: the pointer to the closure keeps the closure data alive until the pointer is gone because the function cannot be called any more, at which point the closure data evaporates (well, at the next GC).
1GCC sometimes uses trampolines to do this for C, where trampolines are executable code generated at runtime. The executable code may set a register or pass an extra parameter or some such. This can be expensive since something treated as data at runtime (generated code) must be turned into executable code at runtime (possibly requiring a system call and potentially requiring that some supervisory code "vet" the resulting runtime code).
Go does not need any of this because the language was defined with closures in mind, so implementors don't, er, "close off" any easy ways to make this all work. Some runtime ABIs are defined with closures in mind as well, e.g., register r1 is reserved as the closure-variables pointer in all pointer-to-function types, or some such.
Actual function size is irrelevant. When you return a function like this, memory will be allocated for the closure, that is, any variables in the scope that the function uses. In this case, a pointer will be returned containing the address of the function and a pointer to the closure, which will contain a reference to the variable a.
Is there a way to pass a function(which can be generic) to another function?
I know that with known input types and return types we can pass a function but a generic approach is needed
When reading the Go2 proposal on generic: "Type Parameters - Draft Design", I am not sure you would be able to pass as parameter a generic function without explicitly specify the type used by said function
See "Instantiating a function"
Go normally permits you to refer to a function without passing any arguments, producing a value of function type.
You may not do this with a function that has type parameters; all type arguments must be known at compile time.
That said, you can instantiate the function, by passing type arguments, but you don't have to call the instantiation. This will produce a function value with no type parameters.
// PrintInts is type func([]int).
var PrintInts = Print[int]
I'm quite new to Kotlin. I hit this part while I was going over the docs:
"a lambda cannot return from the enclosing function" (unless it's inlined).
So, this doesn't work;
fun foo() {
ordinaryFunction {
return // ERROR: cannot make `foo` return here
}
}
I wonder why it works that way?
The only thing I can think of it's dangerous since there might be some extra stuff the enclosing function might be doing after the lambda execution. But I'm not sure that's the reason because you can overcome this by using qualified returns or using inline keyword. So, that kind of implies there's a technical reason behind it (apart from any usability/safety reasons) like the compiler cannot figure out where to return unless it's labeled or inlined.
Any help would be great!
The problem is here that non-local returns can't be done on the JVM.
If you want to return from lambda (local return) you can add label #ordinaryFunction:
fun foo() {
ordinaryFunction {
return#ordinaryFunction
}
}
Docs say:
If we need to return from a lambda expression, we have to label it and qualify the return. Oftentimes it is more convenient to use implicit labels: such a label has the same name as the function to which the lambda is passed. In our case it is #ordinaryFunction.
Someone else can probably explain this better but in pretty much any programming language, when you call a function, a new entry is created on top of the stack. The stack keeps information about the arguments that the function was called with and the place you should return to when the function completes.
Kotlin doesn't have a feature that lets you return from multiple function calls in one return, so you have to return from each function call manually.
When you inline a function the machine code that would normally execute in a separate subroutine is now copy pasted to the function call site instead. That's why return from an inline function actually returns from the function that called the inlined lambda.
there might be some extra stuff the enclosing function might be doing after the lambda execution.
The problem is the other way around: the lambda can "escape" from the enclosing function's scope and end up executing after the function returns. E.g. consider
fun foo() {
Thread(Runnable {
Thread.sleep(1000)
return
})
}
Or just
fun foo() = // lambda
In either case it makes no sense for the lambda to return from foo, does it? And the compiler doesn't know if your ordinaryFunction lets the lambda escape foo's scope unless it's inline.
you can overcome this by using qualified returns
That's not really overcoming, that's just not returning from the enclosing function.