errlanes/

result_ext.rs

//! `ResultExt` — the one trait a consumer imports to move a `Result` from one
//! error signature to another. `lift`, `into_fault`, `into_fail`, the three
//! `narrow_*`, `rejected`, `classify`, and (under `tracing`) `record` all
//! relocate a `Result`'s error, so they live on one blanket-implemented trait
//! rather than one import per verb.

use std::convert::Infallible;

use crate::{
    carrier::{Carrier, IntoLanes, fail_into_fault, kind},
    classify::Classify,
    fail::{Fail, Fault, Laned, Lift, Rejection, UnmappedInto},
    lane::{Denied, Fatal},
    profile::{LaneProfile, NarrowDenied, WithoutDenied},
};

/// Sealed. The error-level engine for [`ResultExt::narrow_rejected`] — keyed
/// on the source shape: a `Fail` narrows to
/// the `Fault` of its own profile, a bare `Rejection` to the bare `Fatal`, a
/// carrier through its built-in. Both outputs are associated types, read off
/// the source, so `.narrow_rejected()?` never needs a destination named at the
/// call site.
///
/// One impl over `S: IntoLanes`, handing off to [`NarrowRejectedBy`] on the
/// concrete shape type. A separate `C: Carrier` impl would overlap the
/// `R: Rejection` one (generic against generic).
#[doc(hidden)]
#[diagnostic::on_unimplemented(
    message = "`{Self}` has no rejected lane to narrow",
    note = "`narrow_rejected` is for a `Fail<D, L>`, or a bare `Rejection`, with no \
            caller left to correct the rejection; a `Fault<L>` has no rejected lane at all"
)]
pub trait NarrowRejectedLane: narrow_sealed::Sealed {
    type Narrowed;
    fn narrow_rejected(self) -> Self::Narrowed;
}

mod narrow_sealed {
    use crate::carrier::IntoLanes;
    pub trait Sealed {}
    impl<S: IntoLanes> Sealed for S {}
}

impl<S: IntoLanes> NarrowRejectedLane for S
where
    S::Shape: NarrowRejectedBy<S>,
{
    type Narrowed = <S::Shape as NarrowRejectedBy<S>>::Narrowed;
    fn narrow_rejected(self) -> Self::Narrowed {
        <S::Shape as NarrowRejectedBy<S>>::narrow(self)
    }
}

#[doc(hidden)]
pub trait NarrowRejectedBy<S> {
    type Narrowed;
    fn narrow(s: S) -> Self::Narrowed;
}

impl<D: Rejection, L: LaneProfile<Fatal = Fatal>> NarrowRejectedBy<Fail<D, L>> for kind::Fail {
    type Narrowed = Fault<L>;
    fn narrow(s: Fail<D, L>) -> Fault<L> {
        Fail::narrow_rejected(s)
    }
}

/// A bare rejection has no profile to keep, so narrowing it yields the bare
/// payload: `Fatal(Invariant)` with the rejection as its (opaque) source. `?`
/// then carries that into any `Fault<L>` or `Fail<D, L>` whose `Fatal` lane
/// is enabled — the destination profile is read off the function signature,
/// never named at the call site.
impl<R: Rejection> NarrowRejectedBy<R> for kind::Source {
    type Narrowed = Fatal;
    fn narrow(s: R) -> Fatal {
        crate::fail::invariant_from_rejection(s)
    }
}

impl<C: Carrier> NarrowRejectedBy<C> for kind::CarrierShape
where
    C::Repr: NarrowRejectedLane,
{
    type Narrowed = <C::Repr as NarrowRejectedLane>::Narrowed;
    fn narrow(s: C) -> Self::Narrowed {
        s.into_repr().narrow_rejected()
    }
}

/// Sealed. The error-level engine for [`ResultExt::narrow_denied`], shared by
/// both carriers — `Fail` and `Fault` each still have a denied lane to
/// narrow.
#[doc(hidden)]
#[diagnostic::on_unimplemented(
    message = "`{Self}` has no denied lane to narrow",
    note = "`narrow_denied` is only available on a profile whose `Denied` lane is enabled"
)]
pub trait NarrowDeniedLane: crate::fail::sealed::Sealed {
    type Narrowed;
    fn narrow_denied(self) -> Self::Narrowed;
}

// `L: LaneProfile<Denied = Denied>` (the concrete marker, not the generic
// `Slot<Denied>`) is load-bearing: `NarrowDenied<F>` also has a blanket impl
// for `Infallible` (a disabled slot), so bounding on `NarrowDenied` alone
// would make this engine — and so `ResultExt::narrow_denied` — callable even
// when the profile has no `Denied` lane at all, turning narrowing into a
// silent identity on an already-absent lane instead of the compile error
// decision 2 requires. The inherent `Fault::narrow_denied`/`Fail::narrow_denied`
// keep the wider bound; only this engine is deliberately narrower.
impl<D: Rejection, L> NarrowDeniedLane for Fail<D, L>
where
    L: LaneProfile<Denied = Denied>,
    L::Denied: NarrowDenied<L::Fatal>,
{
    type Narrowed = Fail<D, WithoutDenied<L>>;
    fn narrow_denied(self) -> Self::Narrowed {
        Fail::narrow_denied(self)
    }
}

impl<L> NarrowDeniedLane for Fault<L>
where
    L: LaneProfile<Denied = Denied>,
    L::Denied: NarrowDenied<L::Fatal>,
{
    type Narrowed = Fault<WithoutDenied<L>>;
    fn narrow_denied(self) -> Self::Narrowed {
        Fault::narrow_denied(self)
    }
}

/// A carrier narrows through its built-in, so its own lane set governs
/// availability: a carrier with no `Denied` lane has a `Repr` with a disabled
/// `Denied` slot, and `narrow_denied` is not callable on it. The result is
/// the narrowed *built-in*, which `?` carries on.
impl<C: Carrier> NarrowDeniedLane for C
where
    C::Repr: NarrowDeniedLane,
{
    type Narrowed = <C::Repr as NarrowDeniedLane>::Narrowed;
    fn narrow_denied(self) -> Self::Narrowed {
        self.into_repr().narrow_denied()
    }
}

/// A source that can enter its own `Fault` without losing a rejection.
#[doc(hidden)]
#[diagnostic::on_unimplemented(
    message = "`{Self}` can reject and cannot enter `Fault`",
    note = "handle the rejection, call `.narrow_rejected()` when justified, or use `.into_fail()`"
)]
pub trait FaultSource: IntoLanes {
    fn own_fault(self) -> Fault<Self::Lanes>;
}
impl<W: IntoLanes<Rejected = Infallible>> FaultSource for W {
    fn own_fault(self) -> Fault<Self::Lanes> {
        fail_into_fault(self.into_lanes())
    }
}

/// A source whose built-in has a rejected lane.
#[doc(hidden)]
#[diagnostic::on_unimplemented(
    message = "`{Self}` never rejects",
    note = "use `.into_fault()` instead"
)]
pub trait FailSource: IntoLanes {
    fn own_fail(self) -> Fail<Self::Rejected, Self::Lanes>;
}
impl<W: IntoLanes> FailSource for W
where
    W::Rejected: Rejection,
{
    fn own_fail(self) -> Fail<Self::Rejected, Self::Lanes> {
        self.into_lanes()
    }
}

/// The one trait a consumer imports to move a `Result` between error
/// signatures: lift its rejection, narrow away a lane that is no
/// longer live at this boundary, hand the rejection to the caller as a value,
/// classify a foreign error into a local wrapper, or (under `tracing`) record
/// it onto the current span.
pub trait ResultExt<T, E>: Sized {
    /// Lift a rejection into a new vocabulary and enable `Fatal` for any
    /// unmapped case. `?` subsequently expands the resulting lanes.
    fn lift<P>(self) -> Result<T, Fail<P, <E::Lanes as LaneProfile>::WithFatal>>
    where
        E: IntoLanes,
        E::Rejected: Rejection,
        P: Rejection + Lift<E::Rejected>,
        <E::Lanes as LaneProfile>::Fatal: Into<Fatal>,
        P::Unmapped: UnmappedInto<Fatal>;

    /// Narrows away the `Rejected` lane: a rejection with no caller left to
    /// correct it becomes `Fatal(Invariant)`, carrying the rejection as its
    /// source. On a `Fail<D, L>` the result is `Fault<L>`. On a *bare*
    /// `Rejection` — a public method that returns just `R` because its
    /// caller can act on it, consumed by an internal frame that already
    /// proved the precondition — the result is the bare `Fatal`, and `?`
    /// carries it into whatever `Fault`/`Fail` the function returns, so the
    /// destination profile is never named at the call site:
    ///
    /// ```
    /// use errlanes::{Fault, ResultExt, lanes};
    ///
    /// #[derive(Debug, errlanes::Rejection)]
    /// #[rejection(code = "LANE_DISABLED")]
    /// struct LaneDisabled;
    ///
    /// fn listen() -> Result<(), LaneDisabled> {
    ///     Err(LaneDisabled)
    /// }
    ///
    /// // The lane was required at registration: by now, off is an invariant.
    /// fn run() -> Result<(), Fault<lanes!(Transient, Fatal)>> {
    ///     listen().narrow_rejected()?;
    ///     Ok(())
    /// }
    ///
    /// assert!(matches!(run(), Err(Fault::Fatal(_))));
    /// ```
    ///
    /// A `Fault` has no rejected lane to narrow, and that is a compile
    /// error, not an identity:
    ///
    /// ```compile_fail
    /// use errlanes::{Fault, ResultExt, lanes};
    /// fn narrow(value: Result<(), Fault<lanes!(Fatal)>>)
    ///     -> Result<(), Fault<lanes!(Fatal)>>
    /// {
    ///     value.narrow_rejected()
    /// }
    /// ```
    fn narrow_rejected(self) -> Result<T, E::Narrowed>
    where
        E: NarrowRejectedLane;

    /// Narrows away the `Transient` lane: a caller-owned retry loop hands
    /// back the narrowed profile once it stops retrying, so it stops
    /// offering `Transient` to its own callers. `attempts` is what the loop
    /// counted; an exhausted transient becomes `Fatal(Exhausted)` with the
    /// last transient as its source.
    fn narrow_transient(self, attempts: u32) -> Result<T, E::WithoutTransient>
    where
        E: Laned;

    /// Narrows away the `Denied` lane: a denial at a boundary with no
    /// subject (code running as the system) becomes `Fatal(Denied)`. Only
    /// available where `E`'s profile enables `Denied`:
    ///
    /// ```compile_fail
    /// use errlanes::{Fault, ResultExt, lanes};
    /// fn narrow(value: Result<(), Fault<lanes!(Transient, Fatal)>>)
    ///     -> Result<(), Fault<lanes!(Transient, Fatal)>>
    /// {
    ///     value.narrow_denied()
    /// }
    /// ```
    fn narrow_denied(self) -> Result<T, E::Narrowed>
    where
        E: NarrowDeniedLane;

    /// Hands the rejection to the caller as a value and keeps the faults
    /// propagating: `Result<T, Fail<D, L>>` becomes
    /// `Result<Result<T, D>, Fault<L>>`, the `Result`-level form of
    /// [`Fail::rejected`]. The outer `?` carries the faults on into any
    /// enclosing carrier; the inner `Result` is the domain outcome, with the
    /// rejection as its `Err`, matched right where it occurred:
    ///
    /// ```
    /// use errlanes::{Fail, Fault, ResultExt, lanes};
    ///
    /// #[derive(Debug, errlanes::Rejection)]
    /// #[rejection(code = "TIMED_OUT")]
    /// struct TimedOut;
    ///
    /// fn await_completion() -> Result<u64, Fail<TimedOut, lanes!(Transient, Fatal)>> {
    ///     Err(Fail::Rejected(TimedOut))
    /// }
    ///
    /// fn poll_once() -> Result<Option<u64>, Fault<lanes!(Transient, Fatal)>> {
    ///     match await_completion().rejected()? {
    ///         Ok(outcome) => Ok(Some(outcome)),
    ///         Err(TimedOut) => Ok(None),
    ///     }
    /// }
    ///
    /// assert!(poll_once().unwrap().is_none());
    /// ```
    ///
    /// This is the dual of [`narrow_rejected`](Self::narrow_rejected): that
    /// one is for a boundary with no caller left to correct the rejection,
    /// this one for the call site that is going to. There is deliberately no
    /// `Option`-returning accessor on a `Result`: an `as_rejected()` that
    /// answered `None` for both `Ok` and a fault would be the one place a
    /// lane could be dropped without naming it. Only available where `E`
    /// carries a rejected lane; a `Fault` has none:
    ///
    /// ```compile_fail
    /// use errlanes::{Fault, ResultExt, lanes};
    /// fn split(value: Result<(), Fault<lanes!(Fatal)>>) {
    ///     let _ = value.rejected();
    /// }
    /// ```
    fn rejected(self) -> Result<Result<T, E::Rejected>, Fault<E::Lanes>>
    where
        E: IntoLanes,
        E::Rejected: Rejection;

    /// Maps the rejection with a closure and leaves every other lane as it
    /// is: the `Result`-level form of [`Fail::map_rejected`]. For a
    /// type-level remap use [`lift`](Self::lift); this is for enriching a
    /// rejection with data only the call site has, such as the input that
    /// was attempted: `repo.create(new).await.map_rejected(|r| r.with_attempted(id))?`.
    fn map_rejected<D2>(self, f: impl FnOnce(E::Rejected) -> D2) -> Result<T, Fail<D2, E::Lanes>>
    where
        E: IntoLanes,
        E::Rejected: Rejection;

    /// Convert a never-rejecting source to its own `Fault` at a box or carrier boundary.
    fn into_fault(self) -> Result<T, Fault<E::Lanes>>
    where
        E: FaultSource;

    /// Convert a rejecting source to its own `Fail` at a foreign carrier boundary.
    fn into_fail(self) -> Result<T, Fail<E::Rejected, E::Lanes>>
    where
        E: FailSource;

    /// `.classify::<W>()` — the verb that turns a foreign error into a local
    /// [`Classify`] wrapper at a one-off call site, so a function that does
    /// not itself return `W` can still enter the lanes through it:
    /// `conn.query(..).await.classify::<DbWrite>()?`.
    fn classify<W: Classify + From<E>>(self) -> Result<T, W>;

    /// Records onto the current span, then hands the result straight back —
    /// for a call site that must record and keep going rather than
    /// propagate (`?`), such as a batch dispatcher writing its own
    /// `conclusion` after the fact.
    #[cfg(feature = "tracing")]
    fn record(self) -> Self
    where
        E: Laned;
}

impl<T, E> ResultExt<T, E> for Result<T, E> {
    fn lift<P>(self) -> Result<T, Fail<P, <E::Lanes as LaneProfile>::WithFatal>>
    where
        E: IntoLanes,
        E::Rejected: Rejection,
        P: Rejection + Lift<E::Rejected>,
        <E::Lanes as LaneProfile>::Fatal: Into<Fatal>,
        P::Unmapped: UnmappedInto<Fatal>,
    {
        self.map_err(|e| e.into_lanes().lift())
    }

    fn narrow_rejected(self) -> Result<T, E::Narrowed>
    where
        E: NarrowRejectedLane,
    {
        self.map_err(NarrowRejectedLane::narrow_rejected)
    }

    fn narrow_transient(self, attempts: u32) -> Result<T, E::WithoutTransient>
    where
        E: Laned,
    {
        self.map_err(|e| e.narrow_transient(attempts))
    }

    fn narrow_denied(self) -> Result<T, E::Narrowed>
    where
        E: NarrowDeniedLane,
    {
        self.map_err(NarrowDeniedLane::narrow_denied)
    }

    fn rejected(self) -> Result<Result<T, E::Rejected>, Fault<E::Lanes>>
    where
        E: IntoLanes,
        E::Rejected: Rejection,
    {
        match self {
            Ok(value) => Ok(Ok(value)),
            Err(e) => e.into_lanes().rejected().map(Err),
        }
    }

    fn map_rejected<D2>(self, f: impl FnOnce(E::Rejected) -> D2) -> Result<T, Fail<D2, E::Lanes>>
    where
        E: IntoLanes,
        E::Rejected: Rejection,
    {
        self.map_err(|e| e.into_lanes().map_rejected(f))
    }

    fn into_fault(self) -> Result<T, Fault<E::Lanes>>
    where
        E: FaultSource,
    {
        self.map_err(FaultSource::own_fault)
    }

    fn into_fail(self) -> Result<T, Fail<E::Rejected, E::Lanes>>
    where
        E: FailSource,
    {
        self.map_err(FailSource::own_fail)
    }

    fn classify<W: Classify + From<E>>(self) -> Result<T, W> {
        self.map_err(W::from)
    }

    #[cfg(feature = "tracing")]
    fn record(self) -> Self
    where
        E: Laned,
    {
        if let Err(e) = &self {
            e.record(&tracing::Span::current());
        }
        self
    }
}