Enum SpanError 

#[non_exhaustive]
pub enum SpanError {
    Build {
        source: BuildError,
    },
    Apex {
        source: SchemaError,
    },
    Stopped {
        limit: LimitKind,
    },
    Iso {
        source: IsoError,
    },
    EpicIsNotASpanProperty,
    ContractingRightLeg,
}

Why a span search could not produce a span.

None of these variants means “no morphism exists”. The span search is total: leaving every source vertex out of the apex is always feasible, so the absence of a match is reported as an empty apex rather than as an error. What is reported here is a search that could not be posed or a result that is not a schema, both of which are defects rather than answers.

Variants (Non-exhaustive)

Non-exhaustive enums could have additional variants added in future. Therefore, when matching against variants of non-exhaustive enums, an extra wildcard arm must be added to account for any future variants.

Build

The cost function network could not be built from the schema pair.

Fields

source: BuildError

What the network builder refused.

Apex

The apex is not a well-formed sub-schema of the source.

This carries whatever validating the induced apex against the protocol reported. Dangling references are one cause and the one the network guards against, by forbidding the assignments whose apex would carry one, so a dangling reference here does mean a hard constraint is missing. It is not the only cause: validation also checks vertex kinds, edge rules and constraint sorts, none of which the network models, and a sub-schema of a source the protocol already rejects inherits the parent’s findings. The common case is therefore an invalid input rather than a missing constraint, and the two are told apart by running validate on the source: if it reports the same findings, the apex only surfaced them.

Fields

source: SchemaError

What inducing the apex reported.

Stopped

The total-morphism search stopped before reaching any complete assignment, so whether one exists is unknown.

Distinct from Ok(vec![]), which is the search finishing and finding nothing. Branch and bound reaches complete assignments as it dives, so a budget spent before the first leaf leaves it with no incumbent at all, and the empty answer that would report is a claim the search never established. A stop after a leaf is not reported here: that incumbent is a genuine total morphism, only not a proven-optimal one.

Fields

limit: LimitKind

Which budget ran out.

Iso

The maximum common sub-schema search refused the network.

Its reward frame has preconditions the network must meet, and it refuses rather than silently optimising a different objective when one is broken.

Fields

source: IsoError

The precondition of the reward frame the network broke.

EpicIsNotASpanProperty

Surjectivity was asked of a span.

SearchOptions::epic is a property of a total morphism and the span search cannot promise it: a span’s right leg is deliberately partial, the empty apex is always feasible, and find_span is documented never to refuse for want of a match. Enforcing surjectivity would make it refuse, and ignoring the flag would answer a different question than the one asked, so the combination is rejected instead.

ContractingRightLeg

The span’s right leg identifies two apex vertices, so it has no pushout.

A merge along the apex has to commute: an apex vertex must reach the same merged vertex through either leg. A right leg that sends two apex vertices to one target vertex makes that impossible, and the square that comes back is not a cocone over the span it was asked about. Set SearchOptions::iso, which is what discover_overlap does, to search for a span whose right leg is an embedding.

Trait Implementations

impl Debug for SpanError

fn fmt(&self, f: &mut Formatter<'_>) -> Result

Formats the value using the given formatter.

impl Display for SpanError

fn fmt(&self, __formatter: &mut Formatter<'_>) -> Result

Formats the value using the given formatter.

impl Error for SpanError

fn source(&self) -> Option<&(dyn Error + 'static)>

Returns the lower-level source of this error, if any.

fn description(&self) -> &str

👎Deprecated since 1.42.0:

use the Display impl or to_string()

fn cause(&self) -> Option<&dyn Error>

👎Deprecated since 1.33.0:

replaced by Error::source, which can support downcasting

fn provide<'a>(&'a self, request: &mut Request<'a>)

🔬This is a nightly-only experimental API. (error_generic_member_access)

Provides type-based access to context intended for error reports.

impl From<BuildError> for SpanError

fn from(source: BuildError) -> Self

Converts to this type from the input type.

impl From<IsoError> for SpanError

fn from(source: IsoError) -> Self

Converts to this type from the input type.

impl From<SchemaError> for SpanError

fn from(source: SchemaError) -> Self

Converts to this type from the input type.

impl From<SpanError> for MigError

fn from(source: SpanError) -> Self

Converts to this type from the input type.