Trait MonadPlus 

pub trait MonadPlus: Monad + Alternative { }

A type class for monads that also support choice, combining Monad and Alternative.

class (Monad m, Alternative m) => MonadPlus m

MonadPlus has no members of its own. It specifies that the type constructor supports both monadic sequencing (bind, pure) and choice with an identity element (alt, empty).

Laws

A lawful MonadPlus must satisfy:

• Distributivity: binding over a choice distributes:

  bind(alt(x, y), f) == alt(bind(x, f), bind(y, f))

The following property also holds for any Monad + Alternative:

• Left zero: binding on the identity element yields the identity element:

  bind(empty(), f) == empty()

Examples

Distributivity law for Vec:

use fp_library::{
	brands::*,
	functions::explicit::*,
};

let x: Vec<i32> = vec![1, 2];
let y: Vec<i32> = vec![3, 4];
let f = |n: i32| vec![n * 10, n * 100];

assert_eq!(
	bind::<VecBrand, _, _, _, _>(alt::<VecBrand, _, _, _>(x.clone(), y.clone()), f),
	alt::<VecBrand, _, _, _>(
		bind::<VecBrand, _, _, _, _>(x, f),
		bind::<VecBrand, _, _, _, _>(y, f)
	),
);

Left-zero law for Vec:

use fp_library::{
	brands::*,
	functions::{
		explicit::bind,
		*,
	},
};

let f = |n: i32| vec![n * 2];

assert_eq!(
	bind::<VecBrand, _, _, _, _>(plus_empty::<VecBrand, i32>(), f),
	plus_empty::<VecBrand, i32>(),
);

Implementors

The blanket implementation applies to every brand with both Monad and Alternative. The following brands are known to be lawful (satisfying distributivity and left zero):

• VecBrand
• CatListBrand

Note that OptionBrand acquires the trait via the blanket impl but does not satisfy the distributivity law, because alt for Option picks the first Some and discards the second branch entirely.

Dyn Compatibility

This trait is not dyn compatible.

In older versions of Rust, dyn compatibility was called "object safety", so this trait is not object safe.

Implementors

impl<Brand> MonadPlus for Brand

where Brand: Monad + Alternative,

Blanket implementation of MonadPlus.

Type Parameters

• Brand: The brand type.