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//! Types that can be mapped over, allowing functions to be applied to values within a context.
//!
//! ### Examples
//!
//! ```
//! use fp_library::{
//! brands::*,
//! functions::*,
//! };
//!
//! let x = Some(5);
//! let y = map::<OptionBrand, _, _>(|i| i * 2, x);
//! assert_eq!(y, Some(10));
//! ```
#[fp_macros::document_module]
mod inner {
use {
crate::kinds::*,
fp_macros::*,
};
/// A type class for types that can be mapped over.
///
/// A `Functor` represents a context or container that allows functions to be applied
/// to values within that context without altering the structure of the context itself.
///
/// ### Hierarchy Unification
///
/// This trait inherits from [`Kind!(type Of<'a, A: 'a>: 'a;)`](crate::kinds::Kind_cdc7cd43dac7585f), ensuring that all functor
/// contexts satisfy the strict lifetime requirements where the type argument must
/// outlive the context's application lifetime.
///
/// By explicitly requiring that the type parameter outlives the application lifetime `'a`,
/// we provide the compiler with the necessary guarantees to handle trait objects
/// (like `dyn Fn`) commonly used in functor implementations. This resolves potential
/// E0310 errors where the compiler cannot otherwise prove that captured variables in
/// closures satisfy the required lifetime bounds.
///
/// ### Laws
///
/// `Functor` instances must satisfy the following laws:
/// * Identity: `map(identity, fa) = fa`.
/// * Composition: `map(compose(f, g), fa) = map(f, map(g, fa))`.
#[document_examples]
///
/// Functor laws for [`Option`]:
///
/// ```
/// use fp_library::{
/// brands::*,
/// functions::*,
/// };
///
/// // Identity: map(identity, fa) = fa
/// assert_eq!(map::<OptionBrand, _, _>(identity, Some(5)), Some(5));
/// assert_eq!(map::<OptionBrand, _, _>(identity, None::<i32>), None);
///
/// // Composition: map(compose(f, g), fa) = map(f, map(g, fa))
/// let f = |x: i32| x + 1;
/// let g = |x: i32| x * 2;
/// assert_eq!(
/// map::<OptionBrand, _, _>(compose(f, g), Some(5)),
/// map::<OptionBrand, _, _>(f, map::<OptionBrand, _, _>(g, Some(5))),
/// );
/// ```
///
/// Functor laws for [`Vec`]:
///
/// ```
/// use fp_library::{
/// brands::*,
/// functions::*,
/// };
///
/// // Identity: map(identity, fa) = fa
/// assert_eq!(map::<VecBrand, _, _>(identity, vec![1, 2, 3]), vec![1, 2, 3]);
///
/// // Composition: map(compose(f, g), fa) = map(f, map(g, fa))
/// let f = |x: i32| x + 1;
/// let g = |x: i32| x * 2;
/// assert_eq!(
/// map::<VecBrand, _, _>(compose(f, g), vec![1, 2, 3]),
/// map::<VecBrand, _, _>(f, map::<VecBrand, _, _>(g, vec![1, 2, 3])),
/// );
/// ```
#[kind(type Of<'a, A: 'a>: 'a;)]
pub trait Functor {
/// Maps a function over the values in the functor context.
///
/// This method applies a function to the value(s) inside the functor context, producing a new functor context with the transformed value(s).
#[document_signature]
///
#[document_type_parameters(
"The lifetime of the values.",
"The type of the value(s) inside the functor.",
"The type of the result(s) of applying the function."
)]
///
#[document_parameters(
"The function to apply to the value(s) inside the functor.",
"The functor instance containing the value(s)."
)]
///
#[document_returns(
"A new functor instance containing the result(s) of applying the function."
)]
///
#[document_examples]
///
/// ```
/// use fp_library::{
/// brands::*,
/// functions::*,
/// };
///
/// let x = Some(5);
/// let y = map::<OptionBrand, _, _>(|i| i * 2, x);
/// assert_eq!(y, Some(10));
/// ```
fn map<'a, A: 'a, B: 'a>(
f: impl Fn(A) -> B + 'a,
fa: Apply!(<Self as Kind!( type Of<'a, T: 'a>: 'a; )>::Of<'a, A>),
) -> Apply!(<Self as Kind!( type Of<'a, T: 'a>: 'a; )>::Of<'a, B>);
}
/// Maps a function over the values in the functor context.
///
/// Free function version that dispatches to [the type class' associated function][`Functor::map`].
#[document_signature]
///
#[document_type_parameters(
"The lifetime of the values.",
"The brand of the functor.",
"The type of the value(s) inside the functor.",
"The type of the result(s) of applying the function."
)]
///
#[document_parameters(
"The function to apply to the value(s) inside the functor.",
"The functor instance containing the value(s)."
)]
///
#[document_returns("A new functor instance containing the result(s) of applying the function.")]
///
#[document_examples]
///
/// ```
/// use fp_library::{
/// brands::*,
/// functions::*,
/// };
///
/// let x = Some(5);
/// let y = map::<OptionBrand, _, _>(|i| i * 2, x);
/// assert_eq!(y, Some(10));
/// ```
pub fn map<'a, Brand: Functor, A: 'a, B: 'a>(
f: impl Fn(A) -> B + 'a,
fa: Apply!(<Brand as Kind!( type Of<'a, T: 'a>: 'a; )>::Of<'a, A>),
) -> Apply!(<Brand as Kind!( type Of<'a, T: 'a>: 'a; )>::Of<'a, B>) {
Brand::map(f, fa)
}
}
pub use inner::*;