Add Macro example

Author: CoderSales

documentation2/B08-Macro.md

@@ -0,0 +1,59 @@
+# [Macro](https://www.programiz.com/rust/macro)
+
+A macro in Rust is a piece of code that generates another piece of code.
+
+Macros generate code based on input, simplify repetitive patterns, and make code more concise.
+
+Rust macro simply allows us to write code that writes more code which is also known as meta programming. Macros are used extensively in Rust.
+
+Some of the popular Rust macros are `println!`, `vec!` and `panic!`.
+
+____
+
+## Creating a Macro in Rust
+
+We can create a macro using the `macro_rules!` macro. It might be surprising but yes we use a macro to create a macro.
+
+The `macro_rules!` macro has a special syntax.
+
+```rust
+macro_rules! macro_name {
+    (...) => {...}
+    // more match rules
+}
+```
+
+Here, `() => {}` is the entry for a macro rule. We can have many rules to match for in a single macro.
+
+Let's look at an example of a simple macro that defines a new function to print "Hello, World!".
+
+```rust
+// A simple macro named `hello_world`
+macro_rules! hello_world {
+    // `()` indicates that the macro takes no argument
+    () => {
+        // The macro will expand into the contents of this block
+        println!("Hello, World!")
+    };
+}
+
+fn main() {
+    // Call the hello_world macro
+    // This call will expand into `println!("Hello, World!");`
+    hello_world!()
+}
+```
+
+```bash
+cargo build
+```
+
+```bash
+cargo run
+```
+
+### Output
+
+```bash
+Hello, World!
+```

programiz/attempt-02/hello_world/src/main.rs

@@ -1,17 +1,14 @@
-use std::fs::OpenOptions;
-use std::io::Write;
+// A simple macro named `hello_world`
+macro_rules! hello_world {
+    // `()` indicates that the macro takes no argument
+    () => {
+        // The macro will expand into the contents of this block
+        println!("Hello, World!")
+    };
+}
 
 fn main() {
-    // Open a file with append option
-    let mut data_file = OpenOptions::new()
-        .append(true)
-        .open("data.txt")
-        .expect("cannot open file");
-
-    // Write to a file
-    data_file
-        .write("I am learning Rust!".as_bytes())
-        .expect("write failed");
-
-    println!("Appended content to a file");
+    // Call the hello_world macro
+    // This call will expand into `println!("Hello, World!");`
+    hello_world!()
 }