Struct frunk_core::hlist::HNil

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pub struct HNil;

Expand description

Represents the right-most end of a heterogeneous list

Implementations

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impl HNil

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pub fn len(&self) -> usizewhere
Self: HList,

Returns the length of a given HList

pub fn prepend<H>(self, h: H) -> HCons<H, Self>where
Self: HList,

Prepend an item to the current HList

pub fn sculpt<Ts, Indices>(
self
) -> (Ts, <Self as Sculptor<Ts, Indices>>::Remainder)where
Self: Sculptor<Ts, Indices>,

Consume the current HList and return an HList with the requested shape.

sculpt allows us to extract/reshape/scult the current HList into another shape, provided that the requested shape’s types are are contained within the current HList.

The Indices type parameter allows the compiler to figure out that Ts and Self can be morphed into each other.

pub fn into_reverse(self) -> <Self as IntoReverse>::Outputwhere
Self: IntoReverse,

Reverse the HList.

pub fn to_ref<'a>(&'a self) -> <Self as ToRef<'a>>::Outputwhere
Self: ToRef<'a>,

Return an HList where the contents are references to the original HList on which this method was called.

pub fn map<F>(self, mapper: F) -> <Self as HMappable<F>>::Outputwhere
Self: HMappable<F>,

Apply a function to each element of an HList.

This transforms some Hlist![A, B, C, ..., E] into some Hlist![T, U, V, ..., Z]. A variety of types are supported for the folder argument:

An hlist![] of closures (one for each element).
A single closure (for mapping an HList that is homogenous).
A single Poly.

pub fn foldl<Folder, Acc>(
 self,
 folder: Folder,
 acc: Acc
) -> <Self as HFoldLeftable<Folder, Acc>>::Outputwhere
 Self: HFoldLeftable<Folder, Acc>,

Perform a left fold over an HList.

This transforms some Hlist![A, B, C, ..., E] into a single value by visiting all of the elements in left-to-right order. A variety of types are supported for the mapper argument:

An hlist![] of closures (one for each element).
A single closure (for folding an HList that is homogenous).

The accumulator can freely change type over the course of the call. When called with a list of N functions, an expanded form of the implementation with type annotations might look something like this:

let acc: Acc0 = init_value;
let acc: Acc1 = f1(acc, x1);
let acc: Acc2 = f2(acc, x2);
let acc: Acc3 = f3(acc, x3);
...
let acc: AccN = fN(acc, xN);
acc

Run

Examples

let nil = hlist![];

assert_eq!(nil.foldl(hlist![], 0), 0);

let h = hlist![1, false, 42f32];

let folded = h.to_ref().foldl(
    hlist![
        |acc, &i| i + acc,
        |acc, b: &bool| if !b && acc > 42 { 9000f32 } else { 0f32 },
        |acc, &f| f + acc
    ],
    1
);

assert_eq!(42f32, folded);

// There is also a value-consuming version that passes values to your folding
// functions instead of just references:

let folded2 = h.foldl(
    hlist![
        |acc, i| i + acc,
        |acc, b: bool| if !b && acc > 42 { 9000f32 } else { 0f32 },
        |acc, f| f + acc
    ],
    8918
);

assert_eq!(9042f32, folded2)

Run

source

pub fn foldr<Folder, Init>(
 self,
 folder: Folder,
 init: Init
) -> <Self as HFoldRightable<Folder, Init>>::Outputwhere
 Self: HFoldRightable<Folder, Init>,

Perform a right fold over an HList.

This transforms some Hlist![A, B, C, ..., E] into a single value by visiting all of the elements in reverse order. A variety of types are supported for the mapper argument:

An hlist![] of closures (one for each element).
A single closure (for folding an HList that is homogenous), taken by reference.

The accumulator can freely change type over the course of the call.

Comparison to `foldl`

While the order of element traversal in foldl may seem more natural, foldr does have its use cases, in particular when it is used to build something that reflects the structure of the original HList (such as folding an HList of Options into an Option of an HList). An implementation of such a function using foldl will tend to reverse the list, while foldr will tend to preserve its order.

The reason for this is because foldr performs what is known as “structural induction;” it can be understood as follows:

Write out the HList in terms of h_cons and HNil.
Substitute each h_cons with a function, and substitute HNil with init

the list:
    h_cons(x1, h_cons(x2, h_cons(x3, ...h_cons(xN, HNil))...)))

becomes:
       f1( x1,    f2( x2,    f3( x3, ...   fN( xN, init))...)))

Run

Examples

let nil = hlist![];

assert_eq!(nil.foldr(hlist![], 0), 0);

let h = hlist![1, false, 42f32];

let folded = h.foldr(
    hlist![
        |i, acc| i + acc,
        |b: bool, acc| if !b && acc > 42f32 { 9000 } else { 0 },
        |f, acc| f + acc
    ],
    1f32
);

assert_eq!(9001, folded)