karpal-profunctor

A complete profunctor hierarchy for Rust: Profunctor, Strong, Choice, and the canonical function-arrow instance.

What's inside

HKT2

Two-parameter type constructor for profunctors (defined in karpal-core, re-exported here):

use karpal_profunctor::HKT2;

// trait HKT2 { type P<A, B>; }

Profunctor

Contravariant in the first argument, covariant in the second:

use karpal_profunctor::{Profunctor, FnP};

let double: Box<dyn Fn(i32) -> i32> = Box::new(|x| x * 2);
let f = FnP::dimap(
    |s: &str| s.len() as i32,
    |n: i32| n.to_string(),
    double,
);
assert_eq!(f("hello"), "10"); // len=5, *2=10, to_string

Strong

Lifts P<A, B> into P<(A, C), (B, C)> — the key ingredient for lenses.

Choice

Lifts P<A, B> into P<Result<A, C>, Result<B, C>> — the key ingredient for prisms.

Traversing

Extends Strong + Choice with wander — the key ingredient for traversals.

ForgetF<R>

A profunctor that forgets the output: P<A, B> = Box<dyn Fn(A) -> R>. Implements Profunctor, Strong, and Choice + Traversing when R: Monoid. Used by Getter, Fold, and read-only optics.

TaggedF

A profunctor that ignores the input: P<A, B> = B. Implements Profunctor and Choice but not Strong (enforces write-only). Used by Review.

FnP

The canonical profunctor instance: Box<dyn Fn(A) -> B>. Implements Profunctor, Strong, Choice, and Traversing. Gated behind the alloc feature.

Features

License

AGPL-3.0-or-later