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Extract

fp_library::classes::extract

Trait Extract 

Source 
pub trait Extract: Kind_cdc7cd43dac7585f {
    // Required method
    fn extract<'a, A: 'a>(fa: <Self as Kind_cdc7cd43dac7585f>::Of<'a, A>) -> A;
}
Expand description

A type containing exactly one extractable value, providing a natural transformation F ~> Id.

This trait witnesses that a type always holds a single value that can be extracted by running its effect. It is used by Free::evaluate to execute the effects in a Free monad.

Extract is the inverse of Deferrable: Deferrable constructs lazy values from thunks, while Extract forces and extracts them. For types whose brand implements Extract (e.g., ThunkBrand), the round-trip law extract(defer(|| x)) == x holds.

Implemented by types that always contain exactly one value and can surrender ownership of it. Lazy cannot implement this trait because forcing it returns &A (a reference), not an owned A. Trampoline does not have a brand and therefore cannot participate in HKT traits.

§Laws

Pure-extract: extracting a pure value returns the original value. For any x: A:

extract(pure(x)) == x

This law states that wrapping a value in the type and immediately extracting it is the identity.

Required Methods§

Source

fn extract<'a, A: 'a>(fa: <Self as Kind_cdc7cd43dac7585f>::Of<'a, A>) -> A

Extracts the inner value.

§Type Signature

forall A. Self A -> A

§Type Parameters
  • 'a: The lifetime of the value.
  • A: The type of the value inside the container.
§Parameters
  • fa: The container to extract from.
§Returns

The inner value.

§Examples
use fp_library::{
	brands::*,
	functions::*,
	types::*,
};

let thunk = Thunk::new(|| 42);
assert_eq!(extract::<ThunkBrand, _>(thunk), 42);

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§