Dart Anonymous Function

We have learned the Dart Function, which is defined by using a user-define name. Dart also provides the facility to specify a nameless function or function without a name. This type of function is known as an anonymous function, lambda, or closure. An anonymous function behaves the same as a regular function, but it does not have a name with it. It can have zero or any number of arguments with an optional type annotation.

We can assign the anonymous function to a variable, and then we can retrieve or access the value of the closure based on our requirement.

An Anonymous function contains an independent block of the code, and that can be passed around in our code as function parameters. The syntax is as follows.

Syntax:

1. (parameter_list) {  
2.    statement(s)  
3. }  

Let's consider the following example.

Example -

1. void main() {   
2.   var list = ["James","Patrick","Mathew","Tom"];  
3.   print("Example of anonymous function");  
4.   list.forEach((item) {  
5.       print('${list.indexOf(item)}: $item');  
6. });  
7. }  

Output:

Example of anonymous function
0: James
1: Patrick
2: Mathew
3: Tom

Explanation:

In the above example, we defined an anonymous function with an untype argument item. The function called for each item in the list and printed the strings with its specified index value.

If the function consists of one statement, then we can also write the above code in the following way.

1. list.forEach(  
2. (item) => print("${list.indexOf(item)}: $item"));  

It is equivalent to the previous code. You can verify it by paste in your dart pad and run.

Lexical Scope

As we have discussed in the Dart

introduction, it is a lexical scope language which means the variable's scope is decided at compile-time. The scope of the variable is determined when code is compiled. The variable behaves differently if they defined in the different curly braces. Let's understand the following example.

Example -

1. bool topVariable = true;  
2.   
3. void main() {  
4.   var inside_Main = true;  
5.  // Defining Nested Function   
6.    
7.  void myFunction() {  
8.     var inside_Function = true;  
9.      
10.  void nestedFunction() {  
11.       var inside_NestedFunction = true;  
12.       // This function is using all variable of the previous functions.  
13.       assert(topVariable);  
14.       assert(inside_Main);  
15.       assert(inside_Function);  
16.       assert(inside_NestedFunction);  
17.     }  
18.   }  
19. }  

Observe the above code, the nestedFunction() used the variables of the previous function.

Lexical Closure

A lexical closure is referred to as a closure, is a function object that has access to variables in its lexical scope even when the function is used of its original scope. In other words, it provides access to an outer function's scope from inner function. Let's understand the following example.

Example -

1. void main() {  
2.  String initial() {  
3.      var name = 'Will Smith'; // name is a local variable created by init  
4.     
5.      void disp_Name() { // displayName() is the inner function, a closure  
6.            print(name); // use variable declared in the parent function  
7.   }  
8.   disp_Name();  
9. }  
10. init();  

Output

Will Smith

Explanation:

In the above code, the initial() function created a local variable called name and function called disp_Name(). The disp_Name() function defined inside the initial() function and hence disp_Name() function has no local variable its own.

The inner function can access the variable of the outer functions. The function disp_Name() can access the name variable which is declared in the outer function, initial()