Trait Alt

Source

pub trait Alt: Functor {
    // Required method
    fn alt<'a, A: 'a>(
        fa1: <Self as Kind_cdc7cd43dac7585f>::Of<'a, A>,
        fa2: <Self as Kind_cdc7cd43dac7585f>::Of<'a, A>,
    ) -> <Self as Kind_cdc7cd43dac7585f>::Of<'a, A>;
}

Expand description

A type class for associative choice on type constructors.

Alt is similar to Semigroup, except that it applies to types of kind * -> * (like Option or Vec) rather than concrete types (like String or i32).

A common use case is to select the first “valid” item, or, if all items are “invalid”, fall back to the last item.

§Laws

Alt instances must satisfy the following laws:

Associativity: alt(alt(x, y), z) = alt(x, alt(y, z)).
Distributivity: map(f, alt(x, y)) = alt(map(f, x), map(f, y)).

§Examples

Alt laws for Option:

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

// Associativity: alt(alt(x, y), z) = alt(x, alt(y, z))
let x: Option<i32> = None;
let y = Some(1);
let z = Some(2);
assert_eq!(
	alt::<OptionBrand, _, _, _>(alt::<OptionBrand, _, _, _>(x, y), z),
	alt::<OptionBrand, _, _, _>(x, alt::<OptionBrand, _, _, _>(y, z)),
);

// Distributivity: map(f, alt(x, y)) = alt(map(f, x), map(f, y))
let f = |i: i32| i * 2;
let x = Some(3);
let y: Option<i32> = None;
assert_eq!(
	map::<OptionBrand, _, _, _, _>(f, alt::<OptionBrand, _, _, _>(x, y)),
	alt::<OptionBrand, _, _, _>(
		map::<OptionBrand, _, _, _, _>(f, x),
		map::<OptionBrand, _, _, _, _>(f, y)
	),
);

Required Methods§

Source

fn alt<'a, A: 'a>( fa1: <Self as Kind_cdc7cd43dac7585f>::Of<'a, A>, fa2: <Self as Kind_cdc7cd43dac7585f>::Of<'a, A>, ) -> <Self as Kind_cdc7cd43dac7585f>::Of<'a, A>

Chooses between two values in a context.

This method provides an associative binary operation on type constructors. For Option, this returns the first Some value. For Vec, this concatenates the two vectors.

§Type Signature

forall A. (Self A, Self A) -> Self A

§Type Parameters

'a: The lifetime of the values.
A: The type of the value inside the context.

§Parameters

fa1: The first value.
fa2: The second value.

§Returns

The chosen/combined value.

§Examples

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

let x: Option<i32> = None;
let y = Some(5);
let z = alt::<OptionBrand, _, _, _>(x, y);
assert_eq!(z, Some(5));