问题
Could someone please give me a brief introduction to lexical scoping?
回答1:
I understand them through examples. :)
First, Lexical Scope (also called Static Scope), in C-like syntax:
void fun()
{
int x = 5;
void fun2()
{
printf("%d", x);
}
}
Every inner level can access its outer levels.
There is another way, called Dynamic Scope used by first implementation of Lisp, again in C-like Syntax:
void fun()
{
printf("%d", x);
}
void dummy1()
{
int x = 5;
fun();
}
void dummy2()
{
int x = 10;
fun();
}
Here fun can either access x in dummy1 or dummy2, or any x in any function that call fun with x declared in it.
dummy1();
will print 5,
dummy2();
will print 10.
The first one is called static because it can be deduced at compile-time, the second is called dynamic because the outer scope is dynamic and depends on the chain call of the functions.
I find static scoping easier for the eye. Most languages went this way eventually even Lisp(can do both, right?). Dynamic scoping is like passing references of all variables to the called function.
An example of why the compiler can not deduce the outer dynamic scope of a function, consider our last example, if we write something like this:
if(/* some condition */)
dummy1();
else
dummy2();
The call chain depends on a run time condition. If it is true, then the call chain looks like:
dummy1 --> fun()
If the condition is false:
dummy2 --> fun()
The outer scope of fun in both cases is the caller plus the caller of the caller and so on.
Just to mention that C language does not allow nested functions nor dynamic scoping.
回答2:
Lets try the shortest possible definition:
Lexical Scoping defines how variable names are resolved in nested functions: inner functions contain the scope of parent functions even if the parent function has returned.
That is all there is to it!
回答3:
var scope = "I am global";
function whatismyscope(){
var scope = "I am just a local";
function func() {return scope;}
return func;
}
whatismyscope()()
The above code will return "I am just a local". It will not return "I am a global". Because the function func() counts where is was originally defined which is under the scope of function whatismyscope.
It will not bother from whatever it is being called(the global scope/from within another function even), that's why global scope value I am global will not be printed.
This is called lexical scoping where "functions are executed using the scope chain that was in effect when they were defined" - according to JavaScript Definition Guide.
Lexical scope is a very very powerful concept.
Hope this helps..:)
回答4:
Scope defines the area, where functions, variables and such are available. The availability of a variable for example is defined within its the context, let's say the function, file, or object, they are defined in. We usually call these local variables.
The lexical part means that you can derive the scope from reading the source code.
Lexical scope is also known as static scope.
Dynamic scope defines global variables that can be called or referenced from anywhere after being defined. Sometimes they are called global variables, even though global variables in most programmin languages are of lexical scope. This means, it can be derived from reading the code that the variable is available in this context. Maybe one has to follow a uses or includes clause to find the instatiation or definition, but the code/compiler knows about the variable in this place.
In dynamic scoping, by contrast, you search in the local function first, then you search in the function that called the local function, then you search in the function that called that function, and so on, up the call stack. "Dynamic" refers to change, in that the call stack can be different every time a given function is called, and so the function might hit different variables depending on where it is called from. (see here)
To see an interesting example for dynamic scope see here.
For further details see here and here.
Some examples in Delphi/Object Pascal
Delphi has lexical scope.
unit Main;
uses aUnit; // makes available all variables in interface section of aUnit
interface
var aGlobal: string; // global in the scope of all units that use Main;
type
TmyClass = class
strict private aPrivateVar: Integer; // only known by objects of this class type
// lexical: within class definition,
// reserved word private
public aPublicVar: double; // known to everyboday that has access to a
// object of this class type
end;
implementation
var aLocalGlobal: string; // known to all functions following
// the definition in this unit
end.
The closest Delphi gets to dynamic scope is the RegisterClass()/GetClass() function pair. For its use see here.
Let's say that the time RegisterClass([TmyClass]) is called to register a certain class cannot be predicted by reading the code (it gets called in a button click method called by the user), code calling GetClass('TmyClass') will get a result or not. The call to RegisterClass() does not have to be in the lexical scope of the unit using GetClass();
Another possibility for dynamic scope are anonymous methods (closures) in Delphi 2009, as they know the variables of their calling function. It does not follow the calling path from there recursively and therefore is not fully dynamic.
回答5:
Lexical (AKA static) scoping refers to determining a variable's scope based solely on its position within the textual corpus of code. A variable always refers to its top-level environment. It's good to understand it in relation to dynamic scope.
回答6:
I love the fully featured, language-agnostic answers from folks like @Arak. Since this question was tagged JavaScript though, I'd like to chip in some notes very specific to this language.
In javascript our choices for scoping are:
- as-is (no scope adjustment)
- lexical
var _this = this; function callback(){ console.log(_this); } - bound
callback.bind(this)
It's worth noting, I think, that JavaScript doesn't really have dynamic scoping. .bind adjusts the this keyword, and that's close, but not technically the same.
Here is an example demonstrating both approaches. You do this every time you make a decision about how to scope callbacks so this applies to promises, event handlers, and more.
Lexical
Here is what you might term Lexical Scoping of callbacks in JavaScript:
var downloadManager = {
initialize: function() {
var _this = this; // Set up `_this` for lexical access
$('.downloadLink').on('click', function () {
_this.startDownload();
});
},
startDownload: function(){
this.thinking = true;
// request the file from the server and bind more callbacks for when it returns success or failure
}
//...
};
Bound
Another way to scope is to use Function.prototype.bind:
var downloadManager = {
initialize: function() {
$('.downloadLink').on('click', function () {
this.startDownload();
}.bind(this)); // create a function object bound to `this`
}
//...
These methods are, as far as I know, behaviorally equivalent.
回答7:
Lexical scoping: Variables declared outside of a function are global variables and are visible everywhere in a JavaScript program. Variables declared inside a function have function scope and are visible only to code that appears inside that function.
回答8:
IBM defines it as:
The portion of a program or segment unit in which a declaration applies. An identifier declared in a routine is known within that routine and within all nested routines. If a nested routine declares an item with the same name, the outer item is not available in the nested routine.
Example 1:
function x() {
/*
Variable 'a' is only available to function 'x' and function 'y'.
In other words the area defined by 'x' is the lexical scope of
variable 'a'
*/
var a = "I am a";
function y() {
console.log( a )
}
y();
}
// outputs 'I am a'
x();
Example 2:
function x() {
var a = "I am a";
function y() {
/*
If a nested routine declares an item with the same name,
the outer item is not available in the nested routine.
*/
var a = 'I am inner a';
console.log( a )
}
y();
}
// outputs 'I am inner a'
x();
回答9:
There is an important part of the conversation surrounding Lexical and Dynamic Scoping that is missing: a plain explanation of the lifetime of the scoped variable - or when the variable can be accessed.
Dynamic scoping only very loosely corresponds to "global" scoping in the way that we traditionally think about it (the reason I bring up the comparison between the two is that it has already been mentioned - and I don't particularly like the linked article's explanation); it is probably best we don't make the comparison between global and dynamic - though supposedly, according to the linked article, "...[it] is useful as a substitute for globally scoped variables."
So, in plain English, what's the important distinction between the two scoping mechanisms?
Lexical scoping has been defined very well throughout the answers above: lexically scoped variables are available - or, accessible - at the local level of the function in which it was defined.
However - as it is not the focus of the OP - dynamic scoping has not received a great deal of attention and the attention it has received means it probably needs a bit more (that's not a criticism of other answers, but rather a "oh, that answer made we wish there was a bit more"). So, here's a little bit more:
Dynamic scoping means that a variable is accessible to the larger program during the lifetime of the function call - or, while the function is executing. Really, Wikipedia actually does a nice job with the explanation of the difference between the two. So as not to obfuscate it, here is the text that describes dynamic scoping:
...[I]n dynamic scoping (or dynamic scope), if a variable name's scope is a certain function, then its scope is the time-period during which the function is executing: while the function is running, the variable name exists, and is bound to its variable, but after the function returns, the variable name does not exist.
回答10:
In simple language, lexical scope is a variable defined outside your scope or upper scope is automatically available inside your scope which means you don't need to pass it there.
Ex:
let str="JavaScript";
const myFun = () => {
console.log(str);
}
myFun();
// Output: JavaScript
回答11:
Lexical scope means that a function looks up variables in the context where it was defined, and not in the scope immediately around it.
Look at how lexical scope works in Lisp if you want more detail. The selected answer by Kyle Cronin in Dynamic and Lexical variables in Common Lisp is a lot clearer than the answers here.
Coincidentally I only learned about this in a Lisp class, and it happens to apply in JS as well.
I ran this code in chrome's console.
// javascript equivalent Lisp
var x = 5; //(setf x 5)
console.debug(x); //(print x)
function print_x(){ //(defun print-x ()
console.debug(x); // (print x)
} //)
(function(){ //(let
var x = 10; // ((x 10))
console.debug(x); // (print x)
print_x(); // (print-x)
})(); //)
output:
5
10
5
回答12:
A lexical scope in Javascript means that a variable defined outside a function can be accessible inside another function defined after the variable declaration. But the opposite is not true, the variables defined inside a function will not be accessible outside that function.
This concept is heavily used in closures in Javascript.
Let say we have the below code.
var x = 2;
var add = function() {
var y = 1;
return x + y;
};
Now, when you call add() --> this will print 3.
So, add() function is accessing the global variable x which is defined before method function add. This is called due to lexical scoping in javascript.
回答13:
Lexical Scope means that in a nested group of functions, the inner functions have access to the variables and other resources of their parent scope. This means that the child functions are lexically bound to the execution context of their parents. Lexical scope is sometimes also referred to as Static Scope.
function grandfather() {
var name = 'Hammad';
// likes is not accessible here
function parent() {
// name is accessible here
// likes is not accessible here
function child() {
// Innermost level of the scope chain
// name is also accessible here
var likes = 'Coding';
}
}
}
The thing you will notice about lexical scope is that it works forward, meaning name can be accessed by its children's execution contexts. But it doesn't work backward to its parents, meaning that the variable likes cannot be accessed by its parents. This also tells us that variables having the same name in different execution contexts gain precedence from top to bottom of the execution stack. A variable, having a name similar to another variable, in the innermost function (topmost context of the execution stack) will have higher precedence.
Note this is taken from here
回答14:
Lexical scope refers to the lexicon of identifiers (e.g., variables, functions, etc.) visible from the current position in the execution stack.
- global execution context
- foo
- bar
- function1 execution context
- foo2
- bar2
- function2 execution context
- foo3
- bar3
foo and bar are always within the lexicon of available identifiers because they are global.
When function1 is executed, it has access to a lexicon of foo2, bar2, foo, and bar.
When function2 is executed, it has access to a lexicon of foo3, bar3, foo2, bar2, foo, and bar.
The reason global and/or outer functions do not have access to an inner functions identifiers is because the execution of that function has not occurred yet and therefore, none of its identifiers have been allocated to memory. What’s more, once that inner context executes, it is removed from the execution stack, meaning that all of it’s identifiers have been garbage collected and are no longer available.
Finally, this is why a nested execution context can ALWAYS access it’s ancestors execution context and thus why it has access to a greater lexicon of identifiers.
See:
- https://tylermcginnis.com/ultimate-guide-to-execution-contexts-hoisting-scopes-and-closures-in-javascript/
- https://developer.mozilla.org/en-US/docs/Glossary/Identifier
Special thanks to @robr3rd for help simplifying the above definition.
回答15:
Here's a different angle on this question that we can get by taking a step back and looking at the role of scoping in the larger framework of interpretation (running a program). In other words, imagine that you were building an interpreter (or compiler) for a language and were responsible for computing the output, given a program and some input to it.
Interpretation involves keeping track of three things:
1) State - namely, variables and referenced memory locations on the heap and stack.
2) Operations on that state - namely, every line of code in your program
3) The Environment in which a given Operation runs - namely, the projection of State on an Operation.
An interpreter starts at the first line of code in a program, computes its environment, runs the line in that environment and captures its effect on the program's state. It then follows the program's control flow to execute the next line of code, and repeats the process till the program ends.
The way you compute the environment for any operation is through a formal set of rules defined by the programming language. The term "binding" is frequently used to describe the mapping of the overall state of the program to a value in the environment. Note that by "overall state" we do not mean global state, but rather the sum total of every reachable definition, at any point in the execution)
This is the framework in which the scoping problem is defined. Now to the next part of what our options are.
- As the implementor of the interpreter, you could simplify your task by making the environment as close as possible to the program's state. Accordingly, the environment of a line of code would simply be defined by environment of the previous line of code with the effects of that operation applied to it, regardless of whether the previous line was an assignment, a function call, return from a function, or a control structure such as a while loop.
This is the gist of dynamic scoping, wherein the environment that any code runs in is bound to the state of the program as defined by its execution context.
- Or, you could think of a programmer using your language and simplify his or her task of keeping track of the values a variable can take. There are way too many paths and too much complexity involved in reasoning about the outcome the totality of past execution. Lexical Scoping helps do this by restricting the current environment to the portion of state defined in the current block, function or other unit of scope, and its parent (i.e. the block enclosing the current clock, or the function that called the present function).
In other words, with Lexical Scope the environment that any code sees is bound to state associated with a scope defined explicitly in the language, such as a block or a function.
回答16:
I normally learn by example, here's a little something:
const lives = 0;
function catCircus () {
this.lives = 1;
const lives = 2;
const cat1 = {
lives: 5,
jumps: () => {
console.log(this.lives);
}
};
cat1.jumps(); // 1
console.log(cat1); // { lives: 5, jumps: [Function: jumps] }
const cat2 = {
lives: 5,
jumps: () => {
console.log(lives);
}
};
cat2.jumps(); // 2
console.log(cat2); // { lives: 5, jumps: [Function: jumps] }
const cat3 = {
lives: 5,
jumps: () => {
const lives = 3;
console.log(lives);
}
};
cat3.jumps(); // 3
console.log(cat3); // { lives: 5, jumps: [Function: jumps] }
const cat4 = {
lives: 5,
jumps: function () {
console.log(lives);
}
};
cat4.jumps(); // 2
console.log(cat4); // { lives: 5, jumps: [Function: jumps] }
const cat5 = {
lives: 5,
jumps: function () {
var lives = 4;
console.log(lives);
}
};
cat5.jumps(); // 4
console.log(cat5); // { lives: 5, jumps: [Function: jumps] }
const cat6 = {
lives: 5,
jumps: function () {
console.log(this.lives);
}
};
cat6.jumps(); // 5
console.log(cat6); // { lives: 5, jumps: [Function: jumps] }
const cat7 = {
lives: 5,
jumps: function thrownOutOfWindow () {
console.log(this.lives);
}
};
cat7.jumps(); // 5
console.log(cat7); // { lives: 5, jumps: [Function: thrownOutOfWindow] }
}
catCircus();
来源：https://stackoverflow.com/questions/1047454/what-is-lexical-scope