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//! By-reference variant of [`Foldable`](crate::classes::Foldable).
//!
//! **User story:** "I want to fold over a memoized value without consuming it."
//!
//! This trait is for types like [`Lazy`](crate::types::Lazy) where the container
//! holds a cached value accessible by reference. The closures receive `&A` instead
//! of `A`, avoiding unnecessary cloning.
//!
//! ### Examples
//!
//! ```
//! use fp_library::{
//! brands::*,
//! functions::explicit::*,
//! types::*,
//! };
//!
//! let lazy = RcLazy::new(|| 10);
//! let result =
//! fold_map::<RcFnBrand, LazyBrand<RcLazyConfig>, _, _, _, _>(|a: &i32| a.to_string(), &lazy);
//! assert_eq!(result, "10");
//! ```
#[fp_macros::document_module]
mod inner {
use {
crate::{
classes::*,
kinds::*,
types::Endofunction,
},
fp_macros::*,
};
/// By-reference folding over a structure.
///
/// Similar to [`Foldable`], but closures receive `&A` instead of `A`.
/// This is the honest interface for memoized types like [`Lazy`](crate::types::Lazy)
/// that internally hold a cached `&A` and would otherwise force a clone
/// to satisfy the by-value `Foldable` signature.
///
/// All three methods (`ref_fold_map`, `ref_fold_right`, `ref_fold_left`)
/// have default implementations in terms of each other, so implementors
/// only need to provide one.
#[kind(type Of<'a, A: 'a>: 'a;)]
pub trait RefFoldable {
/// Maps values to a monoid by reference and combines them.
#[document_signature]
///
#[document_type_parameters(
"The lifetime of the elements.",
"The brand of the cloneable function to use.",
"The type of the elements in the structure.",
"The monoid type."
)]
///
#[document_parameters(
"The function to map each element reference to a monoid.",
"The structure to fold."
)]
///
#[document_returns("The combined monoid value.")]
#[document_examples]
///
/// ```
/// use fp_library::{
/// brands::*,
/// functions::explicit::*,
/// types::*,
/// };
///
/// let lazy = RcLazy::new(|| 5);
/// let result =
/// fold_map::<RcFnBrand, LazyBrand<RcLazyConfig>, _, _, _, _>(|a: &i32| a.to_string(), &lazy);
/// assert_eq!(result, "5");
/// ```
fn ref_fold_map<'a, FnBrand, A: 'a + Clone, M>(
func: impl Fn(&A) -> M + 'a,
fa: &Apply!(<Self as Kind!( type Of<'a, T: 'a>: 'a; )>::Of<'a, A>),
) -> M
where
FnBrand: LiftFn + 'a,
M: Monoid + 'a, {
Self::ref_fold_right::<FnBrand, A, M>(
move |a: &A, acc| Semigroup::append(func(a), acc),
Monoid::empty(),
fa,
)
}
/// Folds the structure from the right by reference.
#[document_signature]
///
#[document_type_parameters(
"The lifetime of the elements.",
"The brand of the cloneable function to use.",
"The type of the elements in the structure.",
"The type of the accumulator."
)]
///
#[document_parameters(
"The function to apply to each element reference and the accumulator.",
"The initial value of the accumulator.",
"The structure to fold."
)]
///
#[document_returns("The final accumulator value.")]
#[document_examples]
///
/// ```
/// use fp_library::{
/// brands::*,
/// functions::explicit::*,
/// types::*,
/// };
///
/// let lazy = RcLazy::new(|| 10);
/// let result =
/// fold_right::<RcFnBrand, LazyBrand<RcLazyConfig>, _, _, _, _>(|a: &i32, b| *a + b, 5, &lazy);
/// assert_eq!(result, 15);
/// ```
fn ref_fold_right<'a, FnBrand, A: 'a + Clone, B: 'a>(
func: impl Fn(&A, B) -> B + 'a,
initial: B,
fa: &Apply!(<Self as Kind!( type Of<'a, T: 'a>: 'a; )>::Of<'a, A>),
) -> B
where
FnBrand: LiftFn + 'a, {
let f = <FnBrand as LiftFn>::new(move |(a, b): (A, B)| func(&a, b));
let m = Self::ref_fold_map::<FnBrand, A, Endofunction<FnBrand, B>>(
move |a: &A| {
let a = a.clone();
let f = f.clone();
Endofunction::<FnBrand, B>::new(<FnBrand as LiftFn>::new(move |b| {
f((a.clone(), b))
}))
},
fa,
);
m.0(initial)
}
/// Folds the structure from the left by reference.
#[document_signature]
///
#[document_type_parameters(
"The lifetime of the elements.",
"The brand of the cloneable function to use.",
"The type of the elements in the structure.",
"The type of the accumulator."
)]
///
#[document_parameters(
"The function to apply to the accumulator and each element reference.",
"The initial value of the accumulator.",
"The structure to fold."
)]
///
#[document_returns("The final accumulator value.")]
#[document_examples]
///
/// ```
/// use fp_library::{
/// brands::*,
/// functions::explicit::*,
/// types::*,
/// };
///
/// let lazy = RcLazy::new(|| 10);
/// let result =
/// fold_left::<RcFnBrand, LazyBrand<RcLazyConfig>, _, _, _, _>(|b, a: &i32| b + *a, 5, &lazy);
/// assert_eq!(result, 15);
/// ```
fn ref_fold_left<'a, FnBrand, A: 'a + Clone, B: 'a>(
func: impl Fn(B, &A) -> B + 'a,
initial: B,
fa: &Apply!(<Self as Kind!( type Of<'a, T: 'a>: 'a; )>::Of<'a, A>),
) -> B
where
FnBrand: LiftFn + 'a, {
let f = <FnBrand as LiftFn>::new(move |(b, a): (B, A)| func(b, &a));
let m = Self::ref_fold_right::<FnBrand, A, Endofunction<FnBrand, B>>(
move |a: &A, k: Endofunction<'a, FnBrand, B>| {
let a = a.clone();
let f = f.clone();
let current =
Endofunction::<FnBrand, B>::new(<FnBrand as LiftFn>::new(move |b| {
f((b, a.clone()))
}));
Semigroup::append(k, current)
},
Endofunction::<FnBrand, B>::empty(),
fa,
);
m.0(initial)
}
}
}
pub use inner::*;