Added error handling

Author: hitesh-bhatnagar

examples/05_error_handling.rs

@@ -1,25 +1,26 @@
-use::std::fs;
+use std::fs;
 use std::io::{self, Read};
 use std::num::ParseIntError;
 
 // FUNCTIONS defined outside main()
-// 
-fn divide(a: f64, b: f64) -> Result<f64, String> {  // Returns result --> caller must handle Ok or Err
-    if b == 0.0{
+//
+fn divide(a: f64, b: f64) -> Result<f64, String> {
+    // Returns result --> caller must handle Ok or Err
+    if b == 0.0 {
         Err(String::from("Can't divide by zero"))
-    }else{
-        Ok(a/b)
+    } else {
+        Ok(a / b)
     }
 }
 
 fn read_file(path: &str) -> Result<String, io::Error> {
-    let contents = fs::read_to_string(path)?;       // returns Err if file missing
+    let contents = fs::read_to_string(path)?; // returns Err if file missing
     Ok(contents)
 }
 
 // Using ? with multiple operations
 fn read_and_parse(path: &str) -> Result<i32, Box<dyn std::error::Error>> {
-    let contents = fs::read_to_string(path)?;   // could fail -> file missing
+    let contents = fs::read_to_string(path)?; // could fail -> file missing
     let number: i32 = contents.trim().parse()?; // could fail -> not a number
     Ok(number)
 }
@@ -44,83 +45,81 @@ impl std::fmt::Display for AppError {
 }
 
 fn safe_divide(a: f64, b: f64) -> Result<f64, AppError> {
-    if  b == 0.0 {
+    if b == 0.0 {
         Err(AppError::DivisionByZero)
-    }else {
-        Ok(a/b)
+    } else {
+        Ok(a / b)
     }
 }
 
-fn main(){
-    let result = "42".parse::<i32>().unwrap();      // unwrap() => give me the value, crash if Err
+fn main() {
+    let result = "42".parse::<i32>().unwrap(); // unwrap() => give me the value, crash if Err
     println!("parsed: {}", result);
-    
+
     // PART - 2: match - full control over Ok and Err
-    
-    match divide (10.0, 2.0){
+
+    match divide(10.0, 2.0) {
         Ok(result) => println!("10/2 = {}", result),
         Err(e) => println!("Error: {}", e),
     }
-    
-    match divide(10.0, 0.0){
+
+    match divide(10.0, 0.0) {
         Ok(result) => println!("10/ 0 = {}", result),
         Err(e) => println!("Error: {}", e),
     }
-    
-    
-    match read_file("hello.txt"){
+
+    match read_file("hello.txt") {
         Ok(contents) => println!("File contents: {}", contents),
         Err(e) => println!("Could not read file: {}", e),
     }
-    
+
     // PART 4: unwrap_or == safe default value
-    
-    // if Err, use the default value instead of crashing 
-    let name = std::env::var("USERNAME")
-        .unwrap_or(String::from("guest"));
+
+    // if Err, use the default value instead of crashing
+    let name = std::env::var("USERNAME").unwrap_or(String::from("guest"));
     println!("Hello, {}!", name);
-    
+
     let number = "abc".parse::<i32>().unwrap_or(0);
     println!("parsed number (or 0): {}", number);
-    
+
     // PART 5: if let -- handle just the success case
-    
+
     // if let Ok -- only runs if result is Ok
     if let Ok(n) = "99".parse::<i32>() {
         println!("Successfully parsed: {}", n);
-    } 
-    
+    }
+
     // uf let Some -- only runs if Option has a value
-    let numbers = vec![1,2,3];
+    let numbers = vec![1, 2, 3];
     if let Some(first) = numbers.first() {
         println!("First number: {}", first);
     }
-    
+
     // Part 6: Custom Error Types
-    
-    match safe_divide(10.0, 2.0){
+
+    match safe_divide(10.0, 2.0) {
         Ok(result) => println!("Result: {}", result),
         Err(e) => println!("AppError: {}", e),
     }
-    
-    match safe_divide(10.0, 0.0){
+
+    match safe_divide(10.0, 0.0) {
         Ok(result) => println!("Result: {}", result),
         Err(e) => println!("AppError: {}", e),
     }
-    
+
     // PART 7: Three levels of Error Handling -- side by side
-    
+
     // level 1 -- unwrap (crash on error)
     let a = "10".parse::<i32>().unwrap();
-    
+
     // level 2 -- unwrap_or (default on error)
     let b = "bad".parse::<i32>().unwrap_or(0);
-    
+
     // level 3 -- match (full control)
-    let c= match "42".parse::<i32>() {
+    let c = match "42".parse::<i32>() {
         Ok(n) => n,
         Err(_) => -1,
     };
-    
+
     println!("a = {}, b = {}, c = {}", a, b, c)
-}
+}