Multi-branch early-return code examples

multi_branch_early_return.rs

#![allow(unused_variables, dead_code)]

use std::fmt;
use std::error::Error;

fn main()
{
    match do_work()
    {
        Ok(v) => assert!(v == 4),
        Err(e) => println!("Error: {}", e.description()),
    }
}

fn do_work() -> Result<i32, MultiStageError>
{
    Ok(foo4(foo1()?, foo2()?, foo3()?)?)
}

fn foo1() -> Result<i32, MultiStageError> { Ok(1) }
fn foo2() -> Result<i32, MultiStageError> { Ok(2) }
fn foo3() -> Result<i32, MultiStageError> { Ok(3) }
fn foo4(a: i32, b: i32, c: i32) -> Result<i32, MultiStageError> { Ok(4) }

// ****************************************************************************
// Below is the impementation for the custom error types to make the above run
// ****************************************************************************
#[derive(Debug, PartialEq, Eq)]
enum MultiStageError
{
    Stage1,
    Stage2,
    Stage3,
    Stage4,
}

impl std::error::Error for MultiStageError
{
    fn description(&self) -> &str
    {
        match *self
        {
            MultiStageError::Stage1 => "Stage 1 error",
            MultiStageError::Stage2 => "Stage 2 error",
            MultiStageError::Stage3 => "Stage 3 error",
            MultiStageError::Stage4 => "Stage 4 error",
        }
    }

    fn cause(&self) -> Option<&std::error::Error> { None }
}

impl fmt::Display for MultiStageError
{
    fn fmt(&self, f: &mut fmt::Formatter) -> Result<(), fmt::Error>
    {
        match *self
        {
            MultiStageError::Stage1 => write!(f, "{}", "Stage 1 error"),
            MultiStageError::Stage2 => write!(f, "{}", "Stage 2 error"),
            MultiStageError::Stage3 => write!(f, "{}", "Stage 3 error"),
            MultiStageError::Stage4 => write!(f, "{}", "Stage 4 error"),
        }
    }
}