monorepo/flow/src/lib.rs

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/*
Copyright 2023, Savanni D'Gerinel <savanni@luminescent-dreams.com>
This file is part of the Luminescent Dreams Tools.
Luminescent Dreams Tools is free software: you can redistribute it and/or modify it under the terms of the GNU General Public License as published by the Free Software Foundation, either version 3 of the License, or (at your option) any later version.
Luminescent Dreams Tools is distributed in the hope that it will be useful, but WITHOUT ANY WARRANTY; without even the implied warranty of MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the GNU General Public License for more details.
You should have received a copy of the GNU General Public License along with Lumeto. If not, see <https://www.gnu.org/licenses/>.
*/
//! Control Flow for Correctness-Critical applications
//!
//! https://sled.rs/errors.html
//!
//! Where the sled.rs library uses `Result<Result<A, Error>, FatalError>`, these are a little hard to
//! work with. This library works out a set of utility functions that allow us to work with the
//! nested errors in the same way as a regular Result.
use std::{error::Error, fmt};
/// Implement this trait for the application's fatal errors.
///
/// Fatal errors should be things that should trigger an application shutdown. Applications should
/// not try to recover from fatal errors, but simply bring the app to the safest shutdown point and
/// report the best possible information to the user.
///
/// Examples: database corruption, or the database is unavailable in an application that cannot
/// function without it. Graphics environment cannot be initialized in a GUI app.
///
/// Applications should generally have only one FatalError type. There are no handling utilities
/// for Fatal conditions, so Fatal conditions must be handled through an ordinary `match`
/// statement.
pub trait FatalError: Error {}
/// Flow<A, FE, E> represents a return value that might be a success, might be a fatal error, or
/// might be a normal handleable error.
pub enum Flow<A, FE, E> {
/// The operation was successful
Ok(A),
/// The operation encountered a fatal error. These should be bubbled up to a level that can
/// safely shut the application down.
Fatal(FE),
/// Ordinary errors. These should be handled and the application should recover gracefully.
Err(E),
}
impl<A, FE, E> Flow<A, FE, E> {
/// Apply an infallible function to a successful value.
pub fn map<B, O>(self, mapper: O) -> Flow<B, FE, E>
where
O: FnOnce(A) -> B,
{
match self {
Flow::Ok(val) => Flow::Ok(mapper(val)),
Flow::Fatal(err) => Flow::Fatal(err),
Flow::Err(err) => Flow::Err(err),
}
}
/// Apply a potentially fallible function to a successful value.
///
/// Like `Result.and_then`, the mapping function can itself fail.
pub fn and_then<B, O>(self, handler: O) -> Flow<B, FE, E>
where
O: FnOnce(A) -> Flow<B, FE, E>,
{
match self {
Flow::Ok(val) => handler(val),
Flow::Fatal(err) => Flow::Fatal(err),
Flow::Err(err) => Flow::Err(err),
}
}
/// Map a normal error from one type to another. This is useful for converting an error from
/// one type to another, especially in re-throwing an underlying error. `?` syntax does not
/// work with `Flow`, so you will likely need to use this a lot.
pub fn map_err<F, O>(self, mapper: O) -> Flow<A, FE, F>
where
O: FnOnce(E) -> F,
{
match self {
Flow::Ok(val) => Flow::Ok(val),
Flow::Fatal(err) => Flow::Fatal(err),
Flow::Err(err) => Flow::Err(mapper(err)),
}
}
/// Provide a function to use to recover from (or simply re-throw) an error.
pub fn or_else<O, F>(self, handler: O) -> Flow<A, FE, F>
where
O: FnOnce(E) -> Flow<A, FE, F>,
{
match self {
Flow::Ok(val) => Flow::Ok(val),
Flow::Fatal(err) => Flow::Fatal(err),
Flow::Err(err) => handler(err),
}
}
}
/// Convert from a normal `Result` type to a `Flow` type. The error condition for a `Result` will
/// be treated as `Flow::Err`, never `Flow::Fatal`.
impl<A, FE, E> From<Result<A, E>> for Flow<A, FE, E> {
fn from(r: Result<A, E>) -> Self {
match r {
Ok(val) => Flow::Ok(val),
Err(err) => Flow::Err(err),
}
}
}
impl<A, FE, E> fmt::Debug for Flow<A, FE, E>
where
A: fmt::Debug,
FE: fmt::Debug,
E: fmt::Debug,
{
fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
match self {
Flow::Ok(val) => f.write_fmt(format_args!("Flow::Ok {:?}", val)),
Flow::Err(err) => f.write_fmt(format_args!("Flow::Err {:?}", err)),
Flow::Fatal(err) => f.write_fmt(format_args!("Flow::Fatal {:?}", err)),
}
}
}
impl<A, FE, E> PartialEq for Flow<A, FE, E>
where
A: PartialEq,
FE: PartialEq,
E: PartialEq,
{
fn eq(&self, rhs: &Self) -> bool {
match (self, rhs) {
(Flow::Ok(val), Flow::Ok(rhs)) => val == rhs,
(Flow::Err(_), Flow::Err(_)) => true,
(Flow::Fatal(_), Flow::Fatal(_)) => true,
_ => false,
}
}
}
/// Convenience function to create an ok value.
pub fn ok<A, FE: FatalError, E: Error>(val: A) -> Flow<A, FE, E> {
Flow::Ok(val)
}
/// Convenience function to create an error value.
pub fn error<A, FE: FatalError, E: Error>(err: E) -> Flow<A, FE, E> {
Flow::Err(err)
}
/// Convenience function to create a fatal value.
pub fn fatal<A, FE: FatalError, E: Error>(err: FE) -> Flow<A, FE, E> {
Flow::Fatal(err)
}
/// Return early from the current function if the value is a fatal error.
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#[macro_export]
macro_rules! return_fatal {
($x:expr) => {
match $x {
Flow::Fatal(err) => return Flow::Fatal(err),
Flow::Err(err) => Err(err),
Flow::Ok(val) => Ok(val),
}
};
}
#[macro_export]
/// Return early from the current function is the value is an error.
macro_rules! return_error {
($x:expr) => {
match $x {
Flow::Ok(val) => val,
Flow::Err(err) => return Flow::Err(err),
Flow::Fatal(err) => return Flow::Fatal(err),
}
};
}
#[cfg(test)]
mod test {
use super::*;
use thiserror::Error;
#[derive(Debug, Error)]
enum FatalError {
#[error("A fatal error occurred")]
FatalError,
}
impl super::FatalError for FatalError {}
impl PartialEq for FatalError {
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fn eq(&self, _rhs: &Self) -> bool {
true
}
}
#[derive(Debug, Error)]
enum Error {
#[error("an error occurred")]
Error,
}
impl PartialEq for Error {
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fn eq(&self, _rhs: &Self) -> bool {
true
}
}
#[test]
fn it_can_map_things() {
let success: Flow<i32, FatalError, Error> = ok(15);
assert_eq!(ok(16), success.map(|v| v + 1));
}
#[test]
fn it_can_chain_success() {
let success: Flow<i32, FatalError, Error> = ok(15);
assert_eq!(ok(16), success.and_then(|v| ok(v + 1)));
}
#[test]
fn it_can_handle_an_error() {
let failure: Flow<i32, FatalError, Error> = error(Error::Error);
assert_eq!(
ok::<i32, FatalError, Error>(16),
failure.or_else(|_| ok(16))
);
}
#[test]
fn early_exit_on_fatal() {
fn ok_func() -> Flow<i32, FatalError, Error> {
let value = return_fatal!(ok::<i32, FatalError, Error>(15));
match value {
Ok(_) => ok(14),
Err(err) => error(err),
}
}
fn err_func() -> Flow<i32, FatalError, Error> {
let value = return_fatal!(error::<i32, FatalError, Error>(Error::Error));
match value {
Ok(_) => panic!("shouldn't have gotten here"),
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Err(_) => ok(0),
}
}
fn fatal_func() -> Flow<i32, FatalError, Error> {
return_fatal!(fatal::<i32, FatalError, Error>(FatalError::FatalError));
panic!("failed to bail");
}
fatal_func();
assert_eq!(ok_func(), ok(14));
assert_eq!(err_func(), ok(0));
}
#[test]
fn it_can_early_exit_on_all_errors() {
fn ok_func() -> Flow<i32, FatalError, Error> {
let value = return_error!(ok::<i32, FatalError, Error>(15));
assert_eq!(value, 15);
ok(14)
}
fn err_func() -> Flow<i32, FatalError, Error> {
return_error!(error::<i32, FatalError, Error>(Error::Error));
panic!("failed to bail");
}
fn fatal_func() -> Flow<i32, FatalError, Error> {
return_error!(fatal::<i32, FatalError, Error>(FatalError::FatalError));
panic!("failed to bail");
}
fatal_func();
assert_eq!(ok_func(), ok(14));
err_func();
}
}