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//! By-reference variant of [`FoldableWithIndex`](crate::classes::FoldableWithIndex).
//!
//! **User story:** "I want to fold over a memoized value by reference, with access to the index."
//!
//! All three methods (`ref_fold_map_with_index`, `ref_fold_right_with_index`,
//! `ref_fold_left_with_index`) have default implementations in terms of each other,
//! so implementors only need to provide one.
//!
//! ### Examples
//!
//! ```
//! use fp_library::{
//! brands::*,
//! classes::ref_foldable_with_index::RefFoldableWithIndex,
//! types::*,
//! };
//!
//! let lazy = RcLazy::new(|| 42);
//! let result = <LazyBrand<RcLazyConfig> as RefFoldableWithIndex>::ref_fold_map_with_index::<
//! RcFnBrand,
//! _,
//! _,
//! >(|_, x: &i32| x.to_string(), &lazy);
//! assert_eq!(result, "42");
//! ```
#[fp_macros::document_module]
mod inner {
use {
crate::{
classes::*,
kinds::*,
types::{
Dual,
Endofunction,
},
},
fp_macros::*,
};
/// By-reference folding with index over a structure.
///
/// Similar to [`FoldableWithIndex`], but the closure receives `&A` instead of `A`.
/// This is the honest interface for memoized types like [`Lazy`](crate::types::Lazy)
/// that internally hold a cached `&A`.
///
/// All three methods (`ref_fold_map_with_index`, `ref_fold_right_with_index`,
/// `ref_fold_left_with_index`) have default implementations in terms of each other,
/// so implementors only need to provide one.
#[kind(type Of<'a, A: 'a>: 'a;)]
pub trait RefFoldableWithIndex: RefFoldable + WithIndex {
/// Maps each element of the structure to a monoid by reference,
/// providing the index, and combines the results.
#[document_signature]
#[document_type_parameters(
"The lifetime of the values.",
"The brand of the cloneable function to use.",
"The type of the elements.",
"The monoid type."
)]
#[document_parameters(
"The function to apply to each element's index and reference.",
"The structure to fold over."
)]
#[document_returns("The combined result.")]
#[document_examples]
///
/// ```
/// use fp_library::{
/// brands::*,
/// classes::ref_foldable_with_index::RefFoldableWithIndex,
/// types::*,
/// };
///
/// let lazy = RcLazy::new(|| 42);
/// let result = <LazyBrand<RcLazyConfig> as RefFoldableWithIndex>::ref_fold_map_with_index::<
/// RcFnBrand,
/// _,
/// _,
/// >(|_, x: &i32| x.to_string(), &lazy);
/// assert_eq!(result, "42");
/// ```
fn ref_fold_map_with_index<'a, FnBrand, A: 'a + Clone, R: Monoid + 'a>(
f: impl Fn(Self::Index, &A) -> R + 'a,
fa: &Apply!(<Self as Kind!( type Of<'a, T: 'a>: 'a; )>::Of<'a, A>),
) -> R
where
FnBrand: LiftFn + 'a,
Self::Index: 'a, {
Self::ref_fold_right_with_index::<FnBrand, A, R>(
move |i, a: &A, acc| Semigroup::append(f(i, a), acc),
Monoid::empty(),
fa,
)
}
/// Folds the structure from the right by reference, providing the index.
#[document_signature]
#[document_type_parameters(
"The lifetime of the values.",
"The brand of the cloneable function to use.",
"The type of the elements.",
"The type of the accumulator."
)]
#[document_parameters(
"The function to apply to each element's index, reference, and accumulator.",
"The initial value of the accumulator.",
"The structure to fold over."
)]
#[document_returns("The final accumulator value.")]
#[document_examples]
///
/// ```
/// use fp_library::{
/// brands::*,
/// classes::ref_foldable_with_index::RefFoldableWithIndex,
/// types::*,
/// };
///
/// let lazy = RcLazy::new(|| 10);
/// let result = <LazyBrand<RcLazyConfig> as RefFoldableWithIndex>::ref_fold_right_with_index::<
/// RcFnBrand,
/// _,
/// _,
/// >(|_, x: &i32, acc: i32| acc + *x, 0, &lazy);
/// assert_eq!(result, 10);
/// ```
fn ref_fold_right_with_index<'a, FnBrand, A: 'a + Clone, B: 'a>(
func: impl Fn(Self::Index, &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,
Self::Index: 'a, {
let f = <FnBrand as LiftFn>::new(move |(i, a, b): (Self::Index, A, B)| func(i, &a, b));
let m = Self::ref_fold_map_with_index::<FnBrand, A, Endofunction<FnBrand, B>>(
move |i, a: &A| {
let a = a.clone();
let f = f.clone();
Endofunction::<FnBrand, B>::new(<FnBrand as LiftFn>::new(move |b| {
let a = a.clone();
let i = i.clone();
f((i, a, b))
}))
},
fa,
);
m.0(initial)
}
/// Folds the structure from the left by reference, providing the index.
#[document_signature]
#[document_type_parameters(
"The lifetime of the values.",
"The brand of the cloneable function to use.",
"The type of the elements.",
"The type of the accumulator."
)]
#[document_parameters(
"The function to apply to the accumulator, each element's index, and reference.",
"The initial value of the accumulator.",
"The structure to fold over."
)]
#[document_returns("The final accumulator value.")]
#[document_examples]
///
/// ```
/// use fp_library::{
/// brands::*,
/// classes::ref_foldable_with_index::RefFoldableWithIndex,
/// types::*,
/// };
///
/// let lazy = RcLazy::new(|| 10);
/// let result = <LazyBrand<RcLazyConfig> as RefFoldableWithIndex>::ref_fold_left_with_index::<
/// RcFnBrand,
/// _,
/// _,
/// >(|_, acc: i32, x: &i32| acc + *x, 0, &lazy);
/// assert_eq!(result, 10);
/// ```
fn ref_fold_left_with_index<'a, FnBrand, A: 'a + Clone, B: 'a>(
func: impl Fn(Self::Index, 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,
Self::Index: 'a, {
let f = <FnBrand as LiftFn>::new(move |(i, b, a): (Self::Index, B, A)| func(i, b, &a));
let m = Self::ref_fold_map_with_index::<FnBrand, A, Dual<Endofunction<FnBrand, B>>>(
move |i, a: &A| {
let a = a.clone();
let f = f.clone();
Dual(Endofunction::<FnBrand, B>::new(<FnBrand as LiftFn>::new(move |b| {
let a = a.clone();
let i = i.clone();
f((i, b, a))
})))
},
fa,
);
(m.0).0(initial)
}
}
/// Maps each element to a monoid by reference with its index and combines the results.
///
/// Free function version that dispatches to [the type class' associated function][`RefFoldableWithIndex::ref_fold_map_with_index`].
#[document_signature]
#[document_type_parameters(
"The lifetime of the values.",
"The brand of the cloneable function to use.",
"The brand of the structure.",
"The type of the elements.",
"The monoid type."
)]
#[document_parameters(
"The function to apply to each element's index and reference.",
"The structure to fold over."
)]
#[document_returns("The combined result.")]
#[document_examples]
///
/// ```
/// use fp_library::{
/// brands::*,
/// functions::explicit::*,
/// types::*,
/// };
///
/// let lazy = RcLazy::new(|| 42);
/// let result = fold_map_with_index::<RcFnBrand, LazyBrand<RcLazyConfig>, _, _, _, _>(
/// |_, x: &i32| x.to_string(),
/// &lazy,
/// );
/// assert_eq!(result, "42");
/// ```
pub fn ref_fold_map_with_index<
'a,
FnBrand: LiftFn + 'a,
Brand: RefFoldableWithIndex,
A: 'a + Clone,
R: Monoid + 'a,
>(
f: impl Fn(Brand::Index, &A) -> R + 'a,
fa: &Apply!(<Brand as Kind!( type Of<'a, T: 'a>: 'a; )>::Of<'a, A>),
) -> R
where
Brand::Index: 'a, {
Brand::ref_fold_map_with_index::<FnBrand, A, R>(f, fa)
}
}
pub use inner::*;