ERROR HANDLING

• Rust doesn't have exceptions like other languages.
• Instead it has type Result<T, E>  for recoverable error and panic macro that stops the execution when the program encounters an unrecoverable error.

• fn main(){
    let value: Result<&str, Box<dyn std::error::Error>> = Ok("Hello");
  }

• Box is a reference to a memory in heap, in this case an error.
• You can access the values ftom result using pattern matching like given below

• match value {
    Ok(value: &str) => println!("{}", value),
    Err(error: Box<dyn Error>) => println!("Error {}", error)
  }

• In the below code, the result will either be string or empty error, this is used if the error itself is not important:

• fn main(){
    let value: Result<&str, ()> = Ok("Hello");
    match value {
       Ok(value: &str) => println!("{}", value),
       Err(_) => println!("Error Occured") 
    }
  } 

• Or you can use custom error like given below:

• fn get_username() -> Result<String, ()>{
    Ok("John".to_string())
  }
  
  fn main(){
    let username: String = get_username().expect(msg: "something went wrong");
  } 

• Can check if the result is an error or an ok using below methods:

• fn get_username() -> Result<String, ()>{
    Ok("John".to_string())
  }
  
  fn main(){
    let is_okay: bool = get_username().is_ok();
    let is_okay: bool = get_username().is_err();
  } 

• You can exit early from an error like below

• fn get_first_name() -> Result<String, ()>{
    Ok("Sanu".to_string())
  }
  
  fn get_last_name() -> Result<String, ()>{
    Ok("Shilshad".to_string())
  }
  
  fn get_full_name() -> Result<String, ()>{
    let first_name: String = get_first_name()?;
    let last_name: String = get_last_name()?;
    Ok(format!("{} {}", first_name, last_name)) 
  }
  
  fn main(){
    let name: Result<&str, ()> = get_full_name();
    match name{
      Ok(value: &str) => println!("{}", value),
      Err(_) => println!("Error occured")  } 
  } 

• In the above example in case an error occurs in get_last_name function, it propagates into the get_full_name function without breaking anything. The ? unwraps the value, only if the return type of the function is Result.

• You can perform map on Result like below:

• 
  fn main(){
    let name: Result<&str, ()> = get_full_name();
    let length: usize = name.map(|x: String| x.len()).unwrap_or_default();
  } 

• You can map error like below:

• fn get_first_name() -> Result<String, String>{
    Err("Error".to_string())
  }
  
  fn get_last_name() -> Result<String, String>{
    Ok("Shilshad".to_string())
  }
  
  fn get_full_name() -> Result<String, String>{
    let first_name: String = get_first_name()?;
    let last_name: String = get_last_name()?;
    Ok(format!("{} {}", first_name, last_name)) 
  }
  
  fn main(){
    let name: Result<String, ()> = get_full_name();
    let error_length: Result<String, usize> = name.map_err(op: |e: String| e.len());
  }  

• You can use ? in main function to unwrap if you add Result as the return type:

• 
  fn main() -> Result<(), Box<dyn std::error::Error>> {
    let name: String = get_full_name()?;
  }