diff options
Diffstat (limited to 'receme/src/hylo.rs')
-rw-r--r-- | receme/src/hylo.rs | 41 |
1 files changed, 41 insertions, 0 deletions
diff --git a/receme/src/hylo.rs b/receme/src/hylo.rs new file mode 100644 index 0000000..8d26c19 --- /dev/null +++ b/receme/src/hylo.rs @@ -0,0 +1,41 @@ +//! This file defines the behaviour of hylomorphisms. +//! +//! A hylomorphism is a composition of an anamorphism with a +//! catamorphism. But it is separated into a distinct trait as we can +//! implement hylomorphisms more efficiently by short-circuiting +//! during the expansion by the anamorphism. + +use crate::{ + algebra::Algebra, + catana::{Ana, Cata}, + coalgebra::Coalgebra, + functor::Functor, +}; + +/// A type implementing Hylo is able to expand from a value of type +/// `U` into a recursive structure holding values of type `G`, and +/// also to collapse from that recursive structure to a value of type +/// `T`. +/// +/// Types which implement [`Cata`] and [`Ana`] compatibly can +/// automatically implement [`Hylo`] as follows. +/// +/// But of course this is not efficient, and types should implement in +/// a more efficient manner, if available. +/// +/// ```ignore +/// Inter::ana(value, coalg).cata(alg) +/// ``` +pub trait Hylo<T, S, U, F, G, H, A, C>: Cata<T, F, A> + Ana<U, H, C> +where + F: Functor<T>, + G: Functor<S, Target<T> = F>, + H: Functor<U, Target<S> = G>, + A: Algebra<T, F>, + C: Coalgebra<U, H>, +{ + /// Expand from `value` to an intermediate recursive structure, by + /// means of a coalgebra, and then collapse into the result value, + /// by means of an algebra. + fn hylo(value: U, alg: A, coalg: C) -> T; +} |