feat: add unit error

foundations/src/batcher/dataloader.rs

@@ -7,6 +7,24 @@ use super::{BatchOperation, Batcher, BatcherConfig, BatcherDataloader, BatcherEr
 #[allow(type_alias_bounds)]
 pub type LoaderOutput<L: Loader<S>, S: BuildHasher = RandomState> = Result<HashMap<L::Key, L::Value, S>, L::Error>;
 
+
+#[derive(Debug)]
+pub struct UnitError;
+
+impl std::error::Error for UnitError {}
+
+impl std::fmt::Display for UnitError {
+	fn fmt(&self, f: &mut std::fmt::Formatter<'_>) -> std::fmt::Result {
+		write!(f, "unknown")
+	}
+}
+
+impl From<()> for UnitError {
+	fn from(_: ()) -> Self {
+		Self
+	}
+}
+
 pub trait Loader<S: BuildHasher + Default = RandomState> {
 	type Key: Clone + Eq + std::hash::Hash + Send + Sync;
 	type Value: Clone + Send + Sync;