Enum QueryError 

#[non_exhaustive]
pub enum QueryError {
    Cycle,
}

The error returned from Database::get and System::compute when a query cannot be resolved.

The engine resolves a derived query by running its compute, which in turn reads other queries. That recursion terminates only when the dependency graph is acyclic. If a query asks — directly or through a chain of other queries — for a result that is itself still being computed, there is no value to return and no way to make progress: the queries form a cycle. Rather than recurse without bound or panic, the engine unwinds the whole chain with QueryError::Cycle.

The type is non_exhaustive: resolution has exactly one failure mode today, and new engine-level failure modes can be added later without breaking a match that already handles Cycle.

Examples

A query that reads itself cannot resolve:

use query_lang::{Database, System, QueryError};

struct SelfReferential;
impl System for SelfReferential {
    type Key = u32;
    type Value = u32;
    fn compute(&self, db: &Database<Self>, key: &u32) -> Result<u32, QueryError> {
        // Asking for the very key being computed closes a cycle.
        db.get(key)
    }
}

let db = Database::new(SelfReferential);
assert_eq!(db.get(&1), Err(QueryError::Cycle));

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.

Cycle

A query depended on itself, directly or through a chain of other queries.

The dependency graph must be acyclic. When it is not, the query that closed the cycle — and every query waiting on it — resolves to this error. The fix is in the query definitions: break the cyclic dependency, usually by splitting the offending query so the two halves no longer wait on each other.

Trait Implementations

impl Clone for QueryError

fn clone(&self) -> QueryError

Returns a duplicate of the value.

fn clone_from(&mut self, source: &Self)

Performs copy-assignment from source.

impl Copy for QueryError

impl Debug for QueryError

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

Formats the value using the given formatter.

impl Display for QueryError

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

Formats the value using the given formatter.

impl Eq for QueryError

impl Error for QueryError

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 Hash for QueryError

fn hash<__H: Hasher>(&self, state: &mut __H)

Feeds this value into the given Hasher.

fn hash_slice<H>(data: &[Self], state: &mut H)

where H: Hasher, Self: Sized,

Feeds a slice of this type into the given Hasher.

impl PartialEq for QueryError

fn eq(&self, other: &QueryError) -> bool

Equality operator ==.

fn ne(&self, other: &Rhs) -> bool

Inequality operator !=.

impl StructuralPartialEq for QueryError