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//! Support for collapsing and expanding recursive structures by
//! repeatedly expanding or collapsing it one layer at a time.
//!
#[cfg(any(test, feature = "experimental"))]
use std::ops::ControlFlow;

use futures::future::BoxFuture;

/// Support for collapsing a structure into a single value, one layer at a time
pub trait Collapse<A, Wrapped> {
    fn collapse_layers<F: FnMut(Wrapped) -> A>(self, collapse_layer: F) -> A;
}

/// Support for expanding a structure from a seed value, one layer at a time
pub trait Expand<A, Wrapped> {
    fn expand_layers<F: Fn(A) -> Wrapped>(a: A, expand_layer: F) -> Self;
}

#[cfg(any(test, feature = "experimental"))]
pub trait ExpandCF<Seed, Break, Wrapped> {
    fn expand_layers_cf<F: Fn(Seed) -> ControlFlow<Break, Wrapped>>(
        a: Seed,
        expand_layer: F,
    ) -> ControlFlow<Break, Self>
    where
        Self: Sized;
}

/// Support for asynchronously expanding a structure from a seed value, one layer at a time.
pub trait ExpandAsync<A, Wrapped> {
    fn expand_layers_async<
        'a,
        E: Send + 'a,
        F: Fn(A) -> BoxFuture<'a, Result<Wrapped, E>> + Send + Sync + 'a,
    >(
        a: A,
        expand_layer: F,
    ) -> BoxFuture<'a, Result<Self, E>>
    where
        Self: Sized,
        A: Send + 'a;
}