knowhow 0.1.0

Error with context and with known source error
Documentation 
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use std::{
    error::Error,
    fmt::{Debug, Display},
};

pub trait Context<T, E>
where
    E: Error + 'static,
{
    fn context<V, RE>(self, value: V) -> Result<T, Contextual<RE>>
    where
        V: Into<ContextValue>,
        RE: From<E> + Error + 'static;
}

#[derive(Debug)]
pub struct Contextual<E: Error> {
    inner: E,
    context: Vec<ContextValue>,
}

impl<E: Error> Contextual<E> {
    pub fn source(&self) -> &E {
        &self.inner
    }
}

#[derive(Debug)]
pub enum ContextValue {
    Str(&'static str),
}

impl From<&'static str> for ContextValue {
    fn from(value: &'static str) -> Self {
        Self::Str(value)
    }
}

impl<T, E> Context<T, E> for Result<T, E>
where
    E: Error + 'static,
{
    fn context<V, RE>(self, value: V) -> Result<T, Contextual<RE>>
    where
        V: Into<ContextValue>,
        RE: From<E> + Error + 'static,
    {
        match self {
            Ok(v) => Ok(v),
            Err(e) => Err(Contextual {
                inner: e.into(),
                context: vec![value.into()],
            }),
        }
    }
}

impl<T, E> Context<T, E> for Result<T, Contextual<E>>
where
    E: Error + 'static,
{
    fn context<V, RE>(self, value: V) -> Result<T, Contextual<RE>>
    where
        V: Into<ContextValue>,
        RE: From<E> + Error + 'static,
    {
        match self {
            Ok(v) => Ok(v),
            Err(e) => {
                let mut context = e.context;
                context.push(value.into());
                Err(Contextual {
                    inner: e.inner.into(),
                    context,
                })
            }
        }
    }
}

impl<E, R> From<E> for Contextual<R>
where
    E: Error + 'static,
    R: From<E> + Error + 'static,
{
    fn from(value: E) -> Self {
        Contextual {
            inner: value.into(),
            context: vec![],
        }
    }
}

impl<E> Display for Contextual<E>
where
    E: Error,
{
    fn fmt(&self, f: &mut std::fmt::Formatter<'_>) -> std::fmt::Result {
        const CAUSED_INDENT: &str = "  ";

        let Some(e) = self.context.last() else {
            write!(f, "Error: {}", self.inner)?;
            return Ok(());
        };
        writeln!(f, "Error: {e}")?;
        writeln!(f, "Caused by:")?;
        for c in self.context.iter().rev().skip(1) {
            writeln!(f, "{CAUSED_INDENT}{c}")?;
        }
        write!(f, "{CAUSED_INDENT}{}", self.inner)?;
        Ok(())
    }
}

impl Display for ContextValue {
    fn fmt(&self, f: &mut std::fmt::Formatter<'_>) -> std::fmt::Result {
        match self {
            Self::Str(s) => write!(f, "{s}"),
        }
    }
}

#[cfg(test)]
mod tests {
    use super::*;

    #[test]
    fn test_display() {
        #[derive(Debug, thiserror::Error)]
        enum Error {
            #[error("test: {0}")]
            Test(&'static str),
        }

        #[expect(clippy::needless_question_mark)]
        fn a() -> Result<(), Contextual<Error>> {
            Ok(source().context("failed to test")?)
        }
        fn b() -> Result<(), Contextual<Error>> {
            source().context("failed to test")
        }
        fn source() -> Result<(), Error> {
            Err(Error::Test("value"))
        }

        fn ab() -> Result<(), Contextual<Error>> {
            a().context("failed to call a")
        }

        assert_eq!(
            a().unwrap_err().to_string(),
            "Error: failed to test\nCaused by:\n  test: value"
        );
        assert_eq!(
            b().unwrap_err().to_string(),
            "Error: failed to test\nCaused by:\n  test: value"
        );
        assert_eq!(
            ab().unwrap_err().to_string(),
            "Error: failed to call a\nCaused by:\n  failed to test\n  test: value"
        );
    }
}