I have a paramterised module in Erlang in which I wish to call a function A from within function B of the same parameterised module. How can I do this?
From this paper:
in every function of an abstract module, the variable THIS
is always implicitly bound to the current module instance
So you can simply write inside a function B:
THIS:A().
Just to recapitulate in an answer. You don't have to do anything special to call functions within a parametrised module, just write the code as you normally would. It is only when want to make an "remote" call to an exported function from within the module you need THIS:a(). Externally you need the parametrised module reference.
Though I agree with #Christian, stay away from them, you don't really need them.
Related
Lua's require(<name>) function, if called on a <name>.dll, will look for a function called luaopen_<name>.
What should I do if I want to say require("folder1.folder2.library")? It's not like I can name a function luaopen_folder1.folder2.library.
I am looking for a way to do this that doesn't involve changing Lua's module path—i.e. a way to do this that scales with the complexity of a project.
Name the function luaopen_folder1_folder2_library.
Just wondering, since closure is a function that has references to variables/methods outside it's definition.
Every function closes over program's global variables (basically in every mainstream language, be it javascript/python/c/c+/whatever).
So, consequently, every function is a closure?
Edit: let me reemphasize, I'm not talking only about closures in javascript but in a more general context
Yes, exactly. As you've identified, every function in JavaScript is a closure over at least one context: The global context. That's how/why global variables work in JavaScript.
We don't normally call them closures unless they close over some other context and actually make use of the fact that they do, but you're quite right that at a technical level, they all are.
Every function closes over program's global variables (basically in every mainstream language, be it javascript/c/c+/whatever).
I wouldn't generalize that far, no. Different languages have different ways of implementing global variables. Whether functions in those languages are all "closures" is probably open for debate, so I've restricted my answer above to JavaScript.
closure is a function that has references to variables/methods outside its definition
No, this is a "function with free variables", not a "closure".
To quote wikipedia
...a closure is only distinct from a function with free variables when outside of the scope of the non-local variables, otherwise the defining environment and the execution environment coincide and there is nothing to distinguish these (static and dynamic binding can't be distinguished because the names resolve to the same values).
In other words, in some context, a closure is a reference to a function that binds variables from another context. Otherwise, it wouldn't make sense to call it a "closure".
What does external mean in Dart? For example: external DateTime._now();
I'm new to Dart, I can't find documentation for external, so can you give an example to help explain？
9.4 External Functions
An external function is a function whose body is provided separately from its
declaration. An external function may be a top-level function (17), a method
The body of the function is defined somewhere else.
As far as I know this is used to fix different implementations for Dart VM in the browser and Dart VM on the Server.
When we make an external function inside a class like toString()
external String toString();
means this method is abstract and the child of the parent class will add the function body, that's because in Dart we only can make an abstract class.
Summary:
external function = abstract function in not abstract classes
I don't think external keyword is meant to be used to mark methods as abstract, even if that's possible
It's enough to leave a method with no implementation to set it abstract, inside an abstract class
It's the equivalent of declare in TypeScript, and extern in C#, those are used for interoperability with other runtimes, which means you're telling the compiler "Don't worry about this method's implementation, I promise it will exist at runtime", the runtime may be in C or Javascript or whatever
In case, if you are wondering why or where should I even use external keyword, here is a one more example for Flutter.
class MyStruct extends Struct {
#Int32()
external int a;
#Float()
external double b;
external Pointer<Void> c;
}
Sometimes, but not often when you play with native libraries, in this case with Struct to access native struct's field in memory. Under Struct We must declare all fields as external because it will be external fields from dart:ffi (C / C++).
So, external is more than just way to declare "abstract method".
9.4 External Functions
An external function is a function whose
body is provided separately from its
declaration.
What this does mean is that you define the function, but without implementation. It's exactly how you define the abstract method, but the only difference is that with external you don't implement the method in dart but in C or something else.
Something like String class it can be considered as external functions except that the String class it marked with #pragma('vm:entry-point') which make the entire class use native code.
See the following example to understand:
This dart's side.
https://github.com/dart-lang/sdk/blob/main/sdk/lib/core/string.dart#L711
This the implementation in C++.
https://github.com/dart-lang/sdk/blob/main/runtime/lib/string.cc#L467-#L472
In my opinion it is an equivalent of Java native keyword. For example, since current time milliseconds is implemented differently on Android, iOS, Linux etc, DateTime.now().millisecondsSinceEpoch will be linked to different implementations at runtime. So it is not initially known how this method will look like. For this reason it is marked as external meaning it is platform dependent.
What is the difference between the following Lua scripts in terms of function scope. How would it effect the need to require 'calculator' in some other Lua script. And how would it be referenced in say a LuaState.getGlobal(function_name). What would be its proper function name? Also any comment on advantages/disadvantages of the declaration approaches.
A) Calculator.lua
function foo(n)
return n+1;
end
B) Calculator.lua
calc= {}
function calc.foo(n)
return n+1;
end
C) Calculator.lua
function foo(n)
return n+1;
end
function calculator()
calc = {}
calc.foo=foo
return calc
end
I don't know what it is you mean exactly by the "scope" of these scripts. Exactly what these scripts do depends on how they're being executed. You could give them a different environment, thus changing what they think of as "globals".
So, I will explain what each of these scripts does based on the assumption that you are loading them with dofile, dostring, or something of the like. That is, you're applying them to the global environment.
A)
This creates a single global variable, foo, which is a function.
B)
This creates a single global variable, calc, which is a table. This table has a single entry, with the key foo, who's value is a function.
C)
This creates two global variables. foo is a function. calculator is also a function. Each call to calculator will cause the global variable calc to be overwritten. The new value of calc will be a table that has a single entry, with the key foo, who's value is a copy of what is stored in the global variable foo.
It's hard to say what the "advantages" of method C are, since it makes no sense. It creates two functions instead of one, and that second function keeps creating new tables.
B is just a "namespace" scoped version of A. The general expectation with Lua modules is that including them will usually create some table that will contain all of the functions in that module, to avoid name conflicts with existing global state. In that regard, B may be better. But it depends mostly on what this script will be used for.
Personally, for simple shell scripts and utility scripts, I don't bother with proper module scoping. When I do make a proper module, I generally use the module Lua function.
Not exactly an answer, but a comment on semantics - in Lua, you do not declare a function, you create it. The function becomes available at the time the function(...) ... end is executed. How it is available depends on where you store it.
You have to remember that:
function myfun(foo) print('foo') end
is just syntactic sugar for:
myfun = function(foo) print('foo') end
If you do not do anything special, myfun becomes a global variable stored in the global state (accessible through _G). If you say local myfun before actually calling function myfun() end, the function (actually a closure) will be stored in the local variable and available only to the scope of the local variable.
If you use require, it just finds the file and loads it once. It doesn't do anything fancy with the module like hiding globals, etc. So if you write function foo() end in your module Calculator.lua, then calling require 'Calculator' will create a function in global foo, which you can access using LuaState.getGlobal("foo"). If you create a table of functions (step B), you have to use 2 steps:
L.getGlobal("calc") -- pushes the "calc" global (a table) to the stack
L.getField(-1, "foo") -- gets the "foo" field from the table
I inserted this line into my init() of my BootStrap class
Integer.metaClass.minutes = { 60000L * delegate }
I was then not able to use it from a Job class (Quartz plugin). Do I put this line of code somewhere else to make it a global modification?
I was also wondering the best way to make a function available inside all classes in Grails.
Like a global function. Would it be to extend the Object metaclass? or is there a better way?
Do I put this line of code somewhere else to make it a global modification?
Use the DelegatingMetaClass
I was also wondering the best way to make a function available inside all classes in Grails. Like a global function. Would it be to extend the Object metaclass? or is there a better way?
If you want the function to be an instance method of all classes, then you must add it to the metaClass of Object (see above). If not, simply add the function as a static method of a class, i.e. the same way you make functions globally accessible in Java.