Karpal

Higher-Kinded Types and algebraic structures for Rust

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Features

Type-Safe Abstractions

HKT encoding via GATs lets you write functions generic over Option, Result, Vec, and any container that supports mapping, sequencing, or folding.

Complete Hierarchy

Functor through Monad, Alt through Alternative, Foldable, Traversable, Comonad, and contravariant duals — all with property-based law verification.

Profunctor Optics

Lens, Prism, and composition powered by the profunctor hierarchy. Build reusable, first-class field accessors and pattern matchers.

Ergonomic Macros

do_! flattens nested .and_then() chains. ado_! combines independent computations. Both work with any Monad or Applicative.

Proof & Verification

karpal-proof provides law witnesses and refinement types, while karpal-verify exports obligations to SMT and Lean with explicit trust boundaries.

Quick Example

Flatten nested error handling with do_!:

#![allow(unused)]
fn main() {
use karpal_std::prelude::*;

// Without do_! — rightward drift with every step
fn process(input: &str) -> Option<String> {
    parse_id(input).and_then(|id| {
        lookup_user(id).and_then(|user| {
            check_permissions(&user).and_then(|role| {
                Some(format!("{} logged in as {:?}", user.name, role))
            })
        })
    })
}

// With do_! — reads top-to-bottom
fn process(input: &str) -> Option<String> {
    do_! { OptionF;
        id = parse_id(input);
        user = lookup_user(id);
        role = check_permissions(&user);
        Some(format!("{} logged in as {:?}", user.name, role))
    }
}
}

Workspace

karpal-core, karpal-profunctor, karpal-arrow, karpal-free, karpal-recursion, karpal-algebra, karpal-effect, karpal-proof, and the modeling/export portions of karpal-verify are no_std compatible with optional std/alloc feature gates.

Documentation Map

Karpal is licensed under Apache-2.0 + CLA. View on GitHub.