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//! Types that can be mapped over by receiving or returning references to their contents.
//!
//! ### Examples
//!
//! ```
//! use fp_library::{
//! brands::*,
//! functions::explicit::*,
//! types::*,
//! };
//!
//! let memo = Lazy::<_, RcLazyConfig>::new(|| 10);
//! let mapped = map::<LazyBrand<RcLazyConfig>, _, _, _, _>(|x: &i32| *x * 2, &memo);
//! assert_eq!(*mapped.evaluate(), 20);
//! ```
#[fp_macros::document_module]
mod inner {
use {
crate::kinds::*,
fp_macros::*,
};
/// A type class for types that can be mapped over, returning references.
///
/// This is a variant of `Functor` for types where `map` receives/returns references.
/// This is required for types like `Lazy` where `get()` returns `&A`, not `A`.
///
/// `RefFunctor` is intentionally independent from
/// [`SendRefFunctor`](crate::classes::SendRefFunctor). Although one might
/// expect `SendRefFunctor` to be a subtrait of `RefFunctor`, this is not the case because
/// `ArcLazy::new` requires `Send` on the closure, which a generic `RefFunctor` cannot
/// guarantee. As a result, `ArcLazy` implements only `SendRefFunctor`, not `RefFunctor`,
/// and `RcLazy` implements only `RefFunctor`, not `SendRefFunctor`.
///
/// ### Laws
///
/// `RefFunctor` instances must satisfy the following laws:
///
/// **Identity:** `ref_map(|x| x.clone(), fa)` is equivalent to `fa`, given `A: Clone`.
/// The `Clone` requirement arises because the mapping function receives `&A` but must
/// produce a value of type `A` to satisfy the identity law.
///
/// **Composition:** `ref_map(|x| g(&f(x)), fa)` is equivalent to
/// `ref_map(g, ref_map(f, fa))`.
#[document_examples]
///
/// RefFunctor laws for [`Lazy`](crate::types::Lazy):
///
/// ```
/// use fp_library::{
/// brands::*,
/// functions::explicit::*,
/// types::*,
/// };
///
/// // Identity: ref_map(|x| x.clone(), fa) evaluates to the same value as fa.
/// let fa = RcLazy::pure(5);
/// let mapped = map::<LazyBrand<RcLazyConfig>, _, _, _, _>(|x: &i32| *x, &fa);
/// assert_eq!(*mapped.evaluate(), *fa.evaluate());
///
/// // Composition: ref_map(|x| g(&f(x)), fa) = ref_map(g, ref_map(f, fa))
/// let f = |x: &i32| *x * 2;
/// let g = |x: &i32| x + 1;
/// let fa = RcLazy::pure(5);
/// let composed = map::<LazyBrand<RcLazyConfig>, _, _, _, _>(|x: &i32| g(&f(x)), &fa);
/// let sequential = map::<LazyBrand<RcLazyConfig>, _, _, _, _>(
/// g,
/// &map::<LazyBrand<RcLazyConfig>, _, _, _, _>(f, &fa),
/// );
/// assert_eq!(*composed.evaluate(), *sequential.evaluate());
/// ```
///
/// # Cache chain behavior
///
/// Chaining `ref_map` calls on memoized types like [`Lazy`](crate::types::Lazy) creates
/// a linked list of `Rc`/`Arc`-referenced cells. Each mapped value retains a reference to
/// its predecessor, so the entire chain of predecessor cells stays alive as long as any
/// downstream mapped value is reachable. Be aware that long chains can accumulate memory
/// that is only freed when the final value in the chain is dropped.
///
/// # Why `Fn` (not `FnOnce`)?
///
/// The `func` parameter uses `Fn` rather than `FnOnce` because multi-element
/// containers like `Vec` call the closure once per element. `FnOnce` would
/// restrict `RefFunctor` to single-element containers. Closures that move
/// out of their captures (`FnOnce` but not `Fn`) cannot be used with
/// `ref_map`; these are rare and can be restructured by extracting the
/// move into a surrounding scope.
#[kind(type Of<'a, A: 'a>: 'a;)]
pub trait RefFunctor {
/// Maps a function over the values in the functor context, where the function takes a reference.
#[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::*,
/// classes::*,
/// types::*,
/// };
///
/// let memo = Lazy::<_, RcLazyConfig>::new(|| 10);
/// let mapped = LazyBrand::<RcLazyConfig>::ref_map(|x: &i32| *x * 2, &memo);
/// assert_eq!(*mapped.evaluate(), 20);
/// ```
fn ref_map<'a, A: 'a, B: 'a>(
func: 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>);
}
}
pub use inner::*;
#[cfg(test)]
mod tests {
use {
crate::{
brands::*,
functions::explicit,
types::*,
},
quickcheck_macros::quickcheck,
};
/// RefFunctor identity law: map(Clone::clone, lazy) evaluates to the same value as lazy.
#[quickcheck]
fn prop_ref_functor_identity(x: i32) -> bool {
let lazy = RcLazy::pure(x);
let mapped = explicit::map::<LazyBrand<RcLazyConfig>, _, _, _, _>(|v: &i32| *v, &lazy);
*mapped.evaluate() == *lazy.evaluate()
}
/// RefFunctor composition law: map(|x| g(&f(x)), lazy) == map(g, map(f, lazy)).
#[quickcheck]
fn prop_ref_functor_composition(x: i32) -> bool {
let f = |v: &i32| v.wrapping_mul(2);
let g = |v: &i32| v.wrapping_add(1);
let lazy1 = RcLazy::pure(x);
let lazy2 = RcLazy::pure(x);
let composed =
explicit::map::<LazyBrand<RcLazyConfig>, _, _, _, _>(|v: &i32| g(&f(v)), &lazy1);
let sequential = explicit::map::<LazyBrand<RcLazyConfig>, _, _, _, _>(
g,
&explicit::map::<LazyBrand<RcLazyConfig>, _, _, _, _>(f, &lazy2),
);
*composed.evaluate() == *sequential.evaluate()
}
}