update match-iflet.md

src/pattern-match/match-iflet.md

@@ -32,8 +32,8 @@ fn main() {
 
     // fill the blank with an match expression:
     //
-    // boolean = true, binary = 1
-    // boolean = false, binary = 0
+    // boolean = true => binary = 1
+    // boolean = false =>  binary = 0
     let binary = __;
 
     assert_eq!(binary, 1);
@@ -116,4 +116,80 @@ fn main() {
 }
 ```
 
-### if let
+### if let
+For some cases, when matching enums, `match` is too heavy, we can use `if let` instead.
+
+🌟 
+```rust,editable
+
+fn main() {
+    let o = Some(7);
+
+    // remove the whole `match` block, using `if let` instead 
+    match o {
+        Some(i) => {
+            println!("This is a really long string and `{:?}`", i);
+        }
+        _ => {}
+    };
+}
+```
+
+🌟🌟
+```rust,editable
+
+// fill in the blank
+enum Foo {
+    Bar(u8)
+}
+
+fn main() {
+    let a = Foo::Bar(1);
+
+    __ {
+        println!("foobar holds the value: {}", i);
+    }
+}
+```
+
+🌟🌟
+```rust,editable
+
+enum Foo {
+    Bar,
+    Baz,
+    Qux(u32)
+}
+
+fn main() {
+    let a = Foo::Qux(10);
+
+    // remove the codes below, using `match` instead 
+    if let Foo::Bar = a {
+        println!("matches foo::bar")
+    } else if let Foo::Baz = a {
+        println!("matches foo::baz")
+    } else {
+        println!("matches others")
+    }
+}
+```
+
+### Shadowing
+🌟🌟
+```rust,editable
+
+// fix the errors in-place
+fn main() {
+    let age = Some(30);
+    if let Some(age) = age { // create a new variable with the same name as previous `age`
+       assert_eq!(age, Some(30));
+    } // the new variable `age` goes out of scope here
+    
+    match age {
+        // match can also introduce a new shadowed variable
+        Some(age) =>  println!("age is a new variable, it's value is {}",age),
+        _ => ()
+    }
+ }
+ ```