Struct frunk::hlist::HCons

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

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>

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

Returns the length of a given HList

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/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) -> <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 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).

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

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)

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.

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

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)

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

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

Borrow an element by type from an HList.

Examples

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!!"),
}

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

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]);

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

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])

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

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);

Trait Implementations§

impl<H, T, RHS> Add<RHS> for HCons<H, T>where
T: Add<RHS>,
RHS: HList,

type Output = HCons<H, <T as Add<RHS>>::Output>

The resulting type after applying the + operator.

fn add(self, rhs: RHS) -> <HCons<H, T> as Add<RHS>>::Output

Performs the + operation. Read more

impl<K, V, Tail> ByNameFieldPlucker<K, Here> for HCons<Field<K, V>, Tail>

Implementation when the pluck target key is in the head.

type TargetValue = V

type Remainder = Tail

fn pluck_by_name(
self
) -> (Field<K, <HCons<Field<K, V>, Tail> as ByNameFieldPlucker<K, Here>>::TargetValue>, <HCons<Field<K, V>, Tail> as ByNameFieldPlucker<K, Here>>::Remainder)

Returns a pair consisting of the value pointed to by the target key and the remainder.

impl<Head, Tail, K, TailIndex> ByNameFieldPlucker<K, There<TailIndex>> for HCons<Head, Tail>where
Tail: ByNameFieldPlucker<K, TailIndex>,

Implementation when the pluck target key is in the tail.

type TargetValue = <Tail as ByNameFieldPlucker<K, TailIndex>>::TargetValue

type Remainder = HCons<Head, <Tail as ByNameFieldPlucker<K, TailIndex>>::Remainder>

fn pluck_by_name(
self
) -> (Field<K, <HCons<Head, Tail> as ByNameFieldPlucker<K, There<TailIndex>>>::TargetValue>, <HCons<Head, Tail> as ByNameFieldPlucker<K, There<TailIndex>>>::Remainder)

Returns a pair consisting of the value pointed to by the target key and the remainder.

impl<H, T> Clone for HCons<H, T>where
H: Clone,
T: Clone,

fn clone(&self) -> HCons<H, T>

Returns a copy of the value. Read more

1.0.0 · source§

fn clone_from(&mut self, source: &Self)

Performs copy-assignment from source. Read more

impl<Head, Tail, Out, NHead, NTail> CoproductEmbedder<Out, HCons<NHead, NTail>> for Coproduct<Head, Tail>where
Out: CoprodInjector<Head, NHead>,
Tail: CoproductEmbedder<Out, NTail>,

fn embed(self) -> Out

Convert a coproduct into another that can hold its variants. Read more

impl<F, R, FTail, CH, CTail> CoproductFoldable<HCons<F, FTail>, R> for Coproduct<CH, CTail>where
F: FnOnce(CH) -> R,
CTail: CoproductFoldable<FTail, R>,

fn fold(self, f: HCons<F, FTail>) -> R

Use functions to fold a coproduct into a single value. Read more

impl<H, T> Debug for HCons<H, T>where
H: Debug,
T: Debug,

fn fmt(&self, f: &mut Formatter<'_>) -> Result<(), Error>

Formats the value using the given formatter. Read more

impl<T, Tail> Default for HCons<T, Tail>where
T: Default,
Tail: Default + HList,

fn default() -> HCons<T, Tail>

Returns the “default value” for a type. Read more