rsmonad 0.2.4

Haskell-style monads for idiomatic Rust.
Documentation 
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/*
 * This Source Code Form is subject to the terms of the Mozilla Public
 * License, v. 2.0. If a copy of the MPL was not distributed with this
 * file, You can obtain one at https://mozilla.org/MPL/2.0/.
 */

//! `Hazard` monad.

use crate::prelude::*;

/// Type alias for default failure info depending on `no_std` or not (`String` is a pain in the ass).
#[cfg(feature = "std")]
type DefaultErr = String;
/// Type alias for default failure info depending on `no_std` or not (`String` is a pain in the ass).
#[cfg(not(feature = "std"))]
type DefaultErr = ();

/// Encodes the possibility of failure with a reason.
/// # Use
/// ```rust
/// use rsmonad::prelude::*;
/// # #[cfg(feature = "std")]
/// # {
/// fn successor(x: u8) -> Hazard<u8, &'static str> {
///     x.checked_add(1).map_or_else(|| Failure("Overflow!"), Success)
/// }
/// assert_eq!(
///     Success(3_u8) >> successor,
///     Success(4),
/// );
/// assert_eq!(
///     Failure("No value provided") >> successor,
///     Failure("No value provided"),
/// );
/// assert_eq!(
///     Success(255_u8) >> successor,
///     Failure("Overflow!"),
/// );
/// # }
/// ```
#[derive(Clone, Copy, Debug, Eq, Hash, Ord, PartialEq, PartialOrd, QuickCheck)]
pub enum Hazard<A: Clone, E: Clone = DefaultErr> {
    /// Failure with information. Invoking `>>` will immediately return this failure as well.
    Failure(E),
    /// A value that hasn't failed (yet). Invoking `>>` on some function `f` will call `f` with that value as its argument.
    Success(A),
}
pub use Hazard::{Failure, Success};

monad! {
    Hazard<A, E: Clone>:

    fn consume(b) {
        Success(b)
    }

    fn bind(self, f) {
        match self {
            Success(a) => f(a),
            Failure(e) => Failure(e),
        }
    }
}

/// Convenience (D.R.Y.).
#[cfg(feature = "nightly")]
type Residual<E> = Hazard<core::convert::Infallible, E>;

#[cfg(feature = "nightly")]
impl<A: Clone, E: Clone> core::ops::Try for Hazard<A, E> {
    type Output = A;
    type Residual = Residual<E>;
    #[inline]
    fn from_output(a: A) -> Self {
        consume(a)
    }
    #[inline]
    fn branch(self) -> core::ops::ControlFlow<Residual<E>, A> {
        match self {
            Success(a) => core::ops::ControlFlow::Continue(a),
            Failure(s) => core::ops::ControlFlow::Break(Failure(s)),
        }
    }
}

#[cfg(feature = "nightly")]
impl<A: Clone, E: Clone> core::ops::FromResidual<Residual<E>> for Hazard<A, E> {
    #[inline]
    #[track_caller]
    fn from_residual(r: Residual<E>) -> Self {
        match r {
            Failure(s) => Failure(s),
            // SAFETY:
            // Type is literally uninstantiable. If we somehow hit this branch, there were much bigger problems upstream.
            Success(_) => unsafe { core::hint::unreachable_unchecked() },
        }
    }
}

#[cfg(all(test, feature = "nightly"))]
mod nightly_tests {
    use super::*;

    fn should_short_circuit() -> Hazard<()> {
        Failure("Intentional short-circuit".to_owned())?;
        Success(())
    }

    #[test]
    fn hazard_question_mark() {
        match should_short_circuit() {
            Success(()) => panic!("Didn't exit early!"),
            Failure(s) => assert_eq!(s, "Intentional short-circuit"),
        }
    }
}