option_or_result.rs

// abstract from std
{
  enum Result <T, E> {
    Ok(()),
    Err(())
  }

  enum Option <T> {
    Some(()),
    None
  }
}

// 1. Option
// ✅ one exact expectation (exists or none)
HashMap::get() -> Option <&V>;

// ❌ uneccesary declaration of Error
HashMap::get() -> Result <&V, NotExistError>;

// 2. Result
enum Utf8Error {WeirdByte, UnpadedByte};
// ✅ one expectation but possible reason for not getting right expectation
std::str::from_utf8() -> Result<&str, Utf8Error>;

// ❌ just like above, but using Option is not "verbose" to the user
std::str::from_utf8() -> Option<&str>;

// 3. ? operator - postfix

match operand {
    Ok(ok) => ok,
    Err(bad) => Err(From::from(bad)), // Err() = ! = never
}

do_fallible_stuff()?;

Ok((do_fallible_stuff()?, do_fallible_stuff()?))