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//! This file defines the Functor trait.
//!
//! This is the basis of the recursion scheme.
//!
//! More precisely, this file provides two versions of functor traits:
//! one whose `map` function consumes `self`, and one whose `map` does
//! not.
/// A functor can map over its generic parameter.
///
/// It can map from Functor(T) to Functor(S).
pub trait Functor<T> {
/// The target of the map.
///
/// Since Rust has no higher-kinded polymorphism, we have to
/// express this type explicitly.
///
/// # Note
///
/// This is a generic associated type, so we need a minimal Rust
/// version of 1.65, when this feature was first introduced to
/// stable Rust.
type Target<S>: Functor<S>;
/// Map from Functor(T) to Functor(S).
///
/// # Note
///
/// This consumes `self`. If one wants not to consume `self`,
/// then consider the trait [`FunctorRef`].
fn fmap<S>(self, f: impl FnMut(T) -> S) -> Self::Target<S>;
}
impl<T> Functor<T> for Vec<T> {
type Target<S> = Vec<S>;
fn fmap<S>(self, f: impl FnMut(T) -> S) -> Self::Target<S> {
self.into_iter().map(f).collect()
}
}
/// A functor can map over its generic type parameter.
///
/// It can map from Functor(T) to Functor(S).
///
/// This is similar to [`Functor`], but the
/// [`fmap`][FunctorRef<T>::fmap_ref] method takes a reference and
/// does not consume `self`.
pub trait FunctorRef<T> {
/// The target of the map.
///
/// Since Rust has no higher-kinded polymorphism, we have to
/// express this type explicitly.
///
/// # Note
///
/// This is a generic associated type, so we need a minimal Rust
/// version of 1.65, when this feature was first introduced to
/// stable Rust.
type Target<S>: Functor<S>;
/// Map from Functor(T) to Functor(S).
///
/// # Note
///
/// This does notconsume `self`. If one wants to consume `self`,
/// then consider the trait [`Functor`].
///
/// To avoid having to specify the trait when calling the method,
/// we give it a distinct name.
fn fmap_ref<S>(&self, f: impl FnMut(T) -> S) -> Self::Target<S>;
}
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