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use std::ops::Deref;
/// A mapper of keys to values, that memoises the results and re-evaluates when necessary.
///
/// This mapping can be:
/// * Storage or access of externally-modifiable values like files or resources shared with other threads (cells)
/// * Computations that are pure, except for their use of cells above (thunks)
///
/// Mappers should be mainly responsible for tracking and re-performing new computations as appropriate, using the API in [`crate::deps`].
pub trait Mapper {
/// The key or input type
type Key;
/// The result
type Value;
/// A wrapper around the result.
/// This allows things like read guards or Arcs to be returned, so long as they dereference to the correct type.
type Wrapper<'a>: Deref<Target = Self::Value> + 'a
where
Self::Value: 'a;
/// Get a value by the given key, calculating it if necessary.
async fn get<'a>(&'a self, key: &Self::Key) -> Self::Wrapper<'a>;
}
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