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Result and Enum

  • What Is Result?

Result is a built-in enum in the Rust standard library. It has two variants Ok(T) and Err(E).

  • Variants:
    • Ok(T), returns the success statement of type T
    • Err, returns the error statement of type E.
  • When to Use Result?
    • Result should be used as a return type for a function that can encounter error situations. Such functions can return an Ok value in case of success or an Err value in case of an error.
  • Result and Function
    • Using Result as a function return type can be used to return various kinds of success and error codes to let the calling function decode the execution state of the called function.

Example 1

The following code has a function file_found which takes a number i and returns a Result of type i32, in case of variant Ok and bool, in case of Err.

fn main() {
   println!("{:?}",file_found(true)); // invoke function by passing true 
   println!("{:?}",file_found(false)); // invoke function by passing false
}
fn file_found(i:bool) -> Result<i32,bool> {
   if i { // if true
      Ok(200) // return Ok(200)
   } else { // if false
      Err(false) // return Err(false)
   }
}

output

Ok(200)
Err(false)

Example 2 #

The following code has a function divisible_by_3 which takes a number i and returns a Result of type String in case of both variants Ok and Err. If i is divisible by 3 Ok(Given number is divisible by 3) is returned and Err(Given number is not divisible by 3).

fn main() {
   println!("{:?}", divisible_by_3(6)); // invoke function by passing a number 6
   println!("{:?}", divisible_by_3(2)); // invoke function by passing a number 2
}
fn divisible_by_3(i:i32)->Result<String,String> {
   if i % 3 == 0 { // if number mod 3 equals 0
      Ok("Given number is divisible by 3".to_string()) // return this statement
   } else { // if if number mod 3 is not equals 0
      Err("Given number is not divisible by 3".to_string()) // return this statement
   }
}


output

Ok("Given number is divisible by 3")
Err("Given number is not divisible by 3")

is_ok(), is_err() Functions

Rust helps you to check whether the variable of type Result is set to Ok or Err.

fn main() {
   let check1 = divisible_by_3(6);
   if check1.is_ok(){ // check if the function returns ok
      println!("The number is divisible by 3");
   }
   else{
      println!("The number is not divisible by 3");

   }
   let check2 = divisible_by_3(2);
   if check2.is_err(){ // check if the function returns error
      println!("The number is not divisible by 3");
   }
   else{
      println!("The number is divisible by 3");

   }
}
fn divisible_by_3(i:i32)->Result<String,String> {
   if i % 3 == 0 { // check i modulus 3
      Ok("Given number is divisible by 3".to_string())
   } else {
      Err("Given number is not divisible by 3".to_string())
   }
}


output

The number is divisible by 3
The number is not divisible by 3

Example 2

The following example uses the assert_eq! macro to check whether the variable value of type Result is set to Ok or Err.

fn main() {
   let check1 = divisible_by_3(6);
   assert_eq!(check1.is_ok(), true);  // left is true and right is true so the assertion passes
   let check2 = divisible_by_3(2);
   assert_eq!(check2.is_err(), true); // left is true and right is true so the assertion passes
}
fn divisible_by_3(i:i32)->Result<String,String> {
   if i % 3 == 0 {
      Ok("Given number is divisible by 3".to_string())
   } else {
      Err("Given number is not divisible by 3".to_string())
   }
}


Note: The assertion passes since the expression evaluates to true.