What causes "Could not construct an EvenCore" in bevyengine/bevy?

Match on the variant to get the real cause: NotEnoughSamples { samples } vs UnboundedDomain.

bevyengine/bevy · error · EvenCoreError

Could not construct an EvenCore

Error message

Could not construct an EvenCore

What it means

EvenCoreError is the umbrella error returned by EvenCore::new (and APIs that build evenly-sampled curves on top of it, like EvenSampleCurve/SampleCurve constructors). The enum-level message appears when the error is displayed without matching a variant — e.g. logged via {} or converted to a string — while the concrete cause (too few samples or unbounded domain) is in the variant. Check the variants NotEnoughSamples and UnboundedDomain for specifics.

Source

Thrown at crates/bevy_math/src/curve/cores.rs:140

#[cfg_attr(feature = "bevy_reflect", derive(Reflect))]
pub struct EvenCore<T> {
    /// The domain over which the samples are taken, which corresponds to the domain of the curve
    /// formed by interpolating them.
    ///
    /// # Invariants
    /// This must always be a bounded interval; i.e. its endpoints must be finite.
    pub domain: Interval,

    /// The samples that are interpolated to extract values.
    ///
    /// # Invariants
    /// This must always have a length of at least 2.
    pub samples: Vec<T>,
}

/// An error indicating that an [`EvenCore`] could not be constructed.
#[derive(Debug, Error)]
#[error("Could not construct an EvenCore")]
pub enum EvenCoreError {
    /// Not enough samples were provided.
    #[error("Need at least two samples to create an EvenCore, but {samples} were provided")]
    NotEnoughSamples {
        /// The number of samples that were provided.
        samples: usize,
    },

    /// Unbounded domains are not compatible with `EvenCore`.
    #[error("Cannot create an EvenCore over an unbounded domain")]
    UnboundedDomain,
}

#[cfg(feature = "alloc")]
impl<T> EvenCore<T> {
    /// Create a new [`EvenCore`] from the specified `domain` and `samples`. The samples are
    /// regarded to be evenly spaced within the given domain interval, so that the outermost
    /// samples form the boundary of that interval. An error is returned if there are not at

View on GitHub (pinned to 396ca72708)

Solutions

Match on the variant to get the real cause: NotEnoughSamples { samples } vs UnboundedDomain.
Use {:?} (Debug) in logs — it prints the variant and its payload.
Fix the underlying condition per the variant's documentation (see the sibling error entries).

Example fix

// before
let curve = EvenCore::new(domain, samples).map_err(|e| format!("curve failed: {e}"))?; // "Could not construct an EvenCore"

// after
let core = match EvenCore::new(domain, samples) {
    Ok(core) => core,
    Err(EvenCoreError::NotEnoughSamples { samples }) => {
        return Err(format!("need >= 2 samples, got {samples}").into());
    }
    Err(EvenCoreError::UnboundedDomain) => {
        return Err("domain must be bounded".into());
    }
};

Defensive patterns

Strategy: try-catch

Try / catch

match EvenCore::new(domain, samples) {
    Ok(core) => core,
    Err(e @ (EvenCoreError::NotEnoughSamples { .. } | EvenCoreError::UnboundedDomain)) => {
        warn!("even core rejected: {e:?}");
        return Ok(());
    }
}

Prevention

Always match EvenCoreError variants instead of displaying the enum, so the real cause is never hidden.
In logs use {:?} for these error enums.
Wrap curve construction in one place with a single, well-logged match so every failure mode is observable.

When it happens

Trigger: Any call to EvenCore::new(domain, samples), or higher-level constructors that wrap it, failing for either reason; then formatting the error with {} instead of {:?} or matching variants, so only the umbrella text is visible.

Common situations: Logging curve-construction failures in a plugin and only seeing the generic message; chaining errors with anyhow/eyre where the enum Display is what gets printed.

Related errors

AI-assisted analysis of bevyengine/bevy@396ca72708 (2026-08-20). Data as JSON: /api/errors/47fe973b2cbf5724. Report an issue: GitHub.