What is the Lexical Scope in dart?

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);
}

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