Struct frunk::hlist::HCons

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pub struct HCons<H, T> {
    pub head: H,
    pub tail: T,
}

Expand description

Represents the most basic non-empty HList. Its value is held in head while its tail is another HList.

Fields§

§head: H§tail: T

Implementations§

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impl<H, T> HCons<H, T>

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pub fn pop(self) -> (H, T)

Returns the head of the list and the tail of the list as a tuple2. The original list is consumed

impl<Head, Tail> HCons<Head, Tail>

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pub fn len(&self) -> usizewhere HCons<Head, Tail>: HList,

Returns the length of a given HList

pub fn is_empty(&self) -> boolwhere HCons<Head, Tail>: HList,

Returns whether a given HList is empty

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

Prepend an item to the current HList

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

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

sculpt allows us to extract/reshape/sculpt 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) -> <HCons<Head, Tail> as IntoReverse>::Outputwhere HCons<Head, Tail>: IntoReverse,

Reverse the HList.

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

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

pub fn to_mut<'a>(&'a mut self) -> <HCons<Head, Tail> as ToMut<'a>>::Outputwhere HCons<Head, Tail>: ToMut<'a>,

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

pub fn map<F>(self, mapper: F) -> <HCons<Head, Tail> as HMappable<F>>::Outputwhere HCons<Head, Tail>: 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 zip<Other>( self, other: Other ) -> <HCons<Head, Tail> as HZippable<Other>>::Zippedwhere HCons<Head, Tail>: HZippable<Other>,

Zip two HLists together.

This zips a HList![A1, B1, ..., C1] with a HList![A2, B2, ..., C2] to make a HList![(A1, A2), (B1, B2), ..., (C1, C2)]

pub fn foldl<Folder, Acc>( self, folder: Folder, acc: Acc ) -> <HCons<Head, Tail> as HFoldLeftable<Folder, Acc>>::Outputwhere HCons<Head, Tail>: 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).
A single Poly.

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

use frunk_core::hlist;

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

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pub fn foldr<Folder, Init>( self, folder: Folder, init: Init ) -> <HCons<Head, Tail> as HFoldRightable<Folder, Init>>::Outputwhere HCons<Head, Tail>: 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.
A single Poly.

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)...)))

Examples

use frunk_core::hlist;

let nil = hlist![];

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

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

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

assert_eq!(9001, folded)

Run

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pub fn extend<Other>( self, other: Other ) -> <HCons<Head, Tail> as Add<Other>>::Outputwhere HCons<Head, Tail>: Add<Other>, Other: HList,

Extend the contents of this HList with another HList

This exactly the same as the Add impl.

Examples

use frunk_core::hlist;

let first = hlist![0u8, 1u16];
let second = hlist![2u32, 3u64];

assert_eq!(first.extend(second), hlist![0u8, 1u16, 2u32, 3u64]);

Run

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impl<Head, Tail> HCons<Head, Tail>

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pub fn get<T, Index>(&self) -> &Twhere HCons<Head, Tail>: Selector<T, Index>,

Borrow an element by type from an HList.

Examples

use frunk_core::hlist;

let h = hlist![1i32, 2u32, "hello", true, 42f32];

// Often, type inference can figure out the type you want.
// You can help guide type inference when necessary by
// using type annotations.
let b: &bool = h.get();
if !b { panic!("no way!") };

// If space is tight, you can also use turbofish syntax.
// The Index is still left to type inference by using `_`.
match *h.get::<u32, _>() {
    2 => { }
    _ => panic!("it can't be!!"),
}

Run

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pub fn get_mut<T, Index>(&mut self) -> &mut Twhere HCons<Head, Tail>: Selector<T, Index>,

Mutably borrow an element by type from an HList.

Examples

use frunk_core::hlist;

let mut h = hlist![1i32, true];

// Type inference ensures we fetch the correct type.
*h.get_mut() = false;
*h.get_mut() = 2;
// *h.get_mut() = "neigh";  // Won't compile.

assert_eq!(h, hlist![2i32, false]);

Run

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pub fn pluck<T, Index>( self ) -> (T, <HCons<Head, Tail> as Plucker<T, Index>>::Remainder)where HCons<Head, Tail>: Plucker<T, Index>,

Remove an element by type from an HList.

The remaining elements are returned along with it.

Examples

use frunk_core::hlist;

let list = hlist![1, "hello", true, 42f32];

// Often, type inference can figure out the target type.
let (b, list): (bool, _) = list.pluck();
assert!(b);

// When type inference will not suffice, you can use a turbofish.
// The Index is still left to type inference by using `_`.
let (s, list) = list.pluck::<i32, _>();

// Each time we plucked, we got back a remainder.
// Let's check what's left:
assert_eq!(list, hlist!["hello", 42.0])

Run

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pub fn into_tuple2( self ) -> (<HCons<Head, Tail> as IntoTuple2>::HeadType, <HCons<Head, Tail> as IntoTuple2>::TailOutput)where HCons<Head, Tail>: IntoTuple2,

Turns an HList into nested Tuple2s, which are less troublesome to pattern match and have a nicer type signature.

Examples

use frunk_core::hlist;

let h = hlist![1, "hello", true, 42f32];

// We now have a much nicer pattern matching experience
let (first,(second,(third, fourth))) = h.into_tuple2();

assert_eq!(first ,       1);
assert_eq!(second, "hello");
assert_eq!(third ,    true);
assert_eq!(fourth,   42f32);