dart 中的词法 Scope 是什么？

Question

Basically I am going through the definition of closure functions which says -

A function that can be referenced with access to the variables in its lexical scope is called a closure

So I want to know this term lexical scope .

Answer 1

Lexical scope

lexical scoped variable/closure etc can only be accessed within the block of code in which it is defined.

Dart is a lexically scoped language. With lexical scoping, descendant scopes will access the most recently declared variable of the same name. The innermost scope is searched first, followed by a search outward through other enclosing scopes.

You can “follow the curly braces outwards” to see if a variable is in scope.

See the following example.

main() { //a new scope
  String language = "Dart";

  void outer()  {
    //curly bracket opens a child scope with inherited variables

    String level = 'one';
    String example = "scope";

    void inner() { //another child scope with inherited variables
      //the next 'level' variable has priority over previous
      //named variable in the outer scope with the same named identifier
      Map level = {'count': "Two"};
      //prints example: scope, level:two
      print('example: $example, level: $level');
      //inherited from the outermost scope: main
      print('What Language: $language');
    } //end inner scope

    inner();

    //prints example: scope, level:one
    print('example: $example, level: $level');
  } //end outer scope
  outer();
} //end main scope

Lexical closures

A closure is a function object that has access to variables in its lexical scope, even when the function is used outside of its original scope.

 /// Returns a function that adds [addBy] to the
/// function's argument.
Function makeAdder(num addBy) {
  return (num i) => addBy + i;
}

void main() {
  // Create a function that adds 2.
  var add2 = makeAdder(2);
    
  // Create a function that adds 4.
  var add4 = makeAdder(4);
    
  assert(add2(3) == 5);
  assert(add4(3) == 7);
}

You can read more from here .

Answer 2

Dart is a lexically scoped language, which means that the scope of variables is determined statically, simply by the layout of the code. You can “follow the curly braces outwards” to see if a variable is in scope.

Here is an example of nested functions with variables at each scope level:

String topLevel = 'Hello';

void firstFunction() {
  String secondLevel = 'Hi';
  print(topLevel);
  nestedFunction() {
    String thirdLevel = 'Howdy';
    print(topLevel);
    print(secondLevel);
    innerNestedFunction() {
      print(topLevel);
      print(secondLevel);
      print(thirdLevel);
    }
  }
  print(thirdLeve);
}

void main() => firstFunction();

This is a valid function, until the last print statement. The third-level variable is defined outside the scope of the nested function, because scope is limited to its own block or the blocks above it.