domani/src/error/unexpected.rs

use std::fmt::Write;
use std::{error, fmt, result};

#[derive(Debug, Clone, PartialEq)]
/// Error is a String which implements the Error trait. It is intended to be used in
/// situations where the caller is being given an error they can't really handle, except to pass it
/// along or log it.
///
/// The error is intended to also implement Send + Sync + Clone, such that it is easy to use in
/// async situations.
pub struct Error(String);

pub type Result<T> = result::Result<T, Error>;

impl Error {
    fn from_displays<D1, D2, D3>(prefix: Option<D1>, body: &D2, source: Option<D3>) -> Error
    where
        D1: fmt::Display,
        D2: fmt::Display + ?Sized,
        D3: fmt::Display,
    {
        let mut w = String::new();

        if let Some(prefix) = prefix {
            write!(w, "{prefix}: ").expect("error writing prefix");
        }

        write!(w, "{body}").expect("error formatting body");

        if let Some(source) = source {
            write!(w, " [{source}]").expect("error formatting source");
        }

        Error(w.to_string())
    }

    fn from_err<E, D>(prefix: Option<D>, e: E) -> Error
    where
        E: error::Error,
        D: fmt::Display,
    {
        return Error::from_displays(prefix, &e, e.source());
    }

    pub fn from<S>(s: S) -> Error
    where
        S: AsRef<str>,
    {
        Error::from_displays(None::<String>, s.as_ref(), None::<String>)
    }
}

impl fmt::Display for Error {
    fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
        write!(f, "{}", self.0)
    }
}

impl error::Error for Error {}

pub trait Mappable<T> {
    /// or_unexpected returns an Err(Error) wrapping self's Err, or the Ok value of self.
    fn or_unexpected(self) -> Result<T>;

    /// or_unexpected_while is like or_unexpected, but will prefix the error message. The prefix
    /// should be worded as if it started with the word "while", e.g.: `opening file {path}`.
    fn or_unexpected_while<D: fmt::Display>(self, prefix: D) -> Result<T>;

    /// map_unexpected_while is like or_unexpected_while, but uses a closure to produce the error
    /// prefix.
    fn map_unexpected_while<F, D>(self, f: F) -> Result<T>
    where
        F: FnOnce() -> D,
        D: fmt::Display;
}

fn map_unexpected_maybe_while<T, E, F, D>(
    res: result::Result<T, E>,
    prefix_fn: Option<F>,
) -> Result<T>
where
    E: error::Error,
    F: FnOnce() -> D,
    D: std::fmt::Display,
{
    match res {
        Ok(res) => Ok(res),
        Err(err) => Err(Error::from_err(prefix_fn.map(|prefix_fn| prefix_fn()), err)),
    }
}

impl<T, E: error::Error> Mappable<T> for result::Result<T, E> {
    fn or_unexpected(self) -> Result<T> {
        let no_fn = None::<Box<dyn FnOnce() -> Box<dyn fmt::Display>>>; // lol, good job rust
        map_unexpected_maybe_while(self, no_fn)
    }

    fn or_unexpected_while<D: fmt::Display>(self, prefix: D) -> Result<T> {
        map_unexpected_maybe_while(self, Some(|| prefix))
    }

    fn map_unexpected_while<F, D>(self, f: F) -> Result<T>
    where
        F: FnOnce() -> D,
        D: fmt::Display,
    {
        map_unexpected_maybe_while(self, Some(f))
    }
}

#[derive(thiserror::Error, Debug)]
enum OptionError {
    #[error("required but not given")]
    None,
}

impl<T> Mappable<T> for Option<T> {
    fn or_unexpected(self) -> Result<T> {
        self.ok_or(OptionError::None).or_unexpected()
    }

    fn or_unexpected_while<D: fmt::Display>(self, prefix: D) -> Result<T> {
        self.ok_or(OptionError::None).or_unexpected_while(prefix)
    }

    fn map_unexpected_while<F, D>(self, f: F) -> Result<T>
    where
        F: FnOnce() -> D,
        D: fmt::Display,
    {
        self.ok_or(OptionError::None).map_unexpected_while(f)
    }
}

pub trait Intoable {
    fn into_unexpected(self) -> Error;

    /// into_unexpected_while is like to_unexpected, but will prefix the Error error. The
    /// prefix should be worded as if it started with the word "while", e.g.: `opening file
    /// {path}`.
    fn into_unexpected_while<D>(self, prefix: D) -> Error
    where
        D: fmt::Display;
}

impl<E: error::Error> Intoable for E {
    fn into_unexpected(self) -> Error {
        Error::from_err(None::<String>, self)
    }

    fn into_unexpected_while<D>(self, prefix: D) -> Error
    where
        D: fmt::Display,
    {
        Error::from_err(Some(prefix), self)
    }
}