Struct frunk_core::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

§Examples

use frunk_core::hlist;

let h = hlist!("hi");
let (h, tail) = h.pop();
assert_eq!(h, "hi");
assert_eq!(tail, hlist![]);

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

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

Returns the length of a given HList

§Examples

use frunk_core::hlist;

let h = hlist![1, "hi"];
assert_eq!(h.len(), 2);

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pub fn is_empty(&self) -> bool
where Self: HList,

Returns whether a given HList is empty

§Examples

use frunk_core::hlist;

let h = hlist![];
assert!(h.is_empty());

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

Prepend an item to the current HList

§Examples

use frunk_core::hlist;

let h1 = hlist![1, "hi"];
let h2 = h1.prepend(true);
let (a, (b, c)) = h2.into_tuple2();
assert_eq!(a, true);
assert_eq!(b, 1);
assert_eq!(c, "hi");

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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/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.

§Examples

use frunk_core::{hlist, HList};

let h = hlist![9000, "joe", 41f32, true];
let (reshaped, remainder): (HList![f32, i32, &str], _) = h.sculpt();
assert_eq!(reshaped, hlist![41f32, 9000, "joe"]);
assert_eq!(remainder, hlist![true]);

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

Reverse the HList.

§Examples

use frunk_core::hlist;

assert_eq!(hlist![].into_reverse(), hlist![]);

assert_eq!(
    hlist![1, "hello", true, 42f32].into_reverse(),
    hlist![42f32, true, "hello", 1],
)

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

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

§Examples

use frunk_core::hlist;

assert_eq!(hlist![].to_ref(), hlist![]);

assert_eq!(hlist![1, true].to_ref(), hlist![&1, &true]);

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pub fn to_mut<'a>(&'a mut self) -> <Self as ToMut<'a>>::Output
where Self: ToMut<'a>,

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

§Examples

use frunk_core::hlist;

assert_eq!(hlist![].to_mut(), hlist![]);

assert_eq!(hlist![1, true].to_mut(), hlist![&mut 1, &mut true]);

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

§Examples

use frunk::HNil;
use frunk_core::hlist;

assert_eq!(HNil.map(HNil), HNil);

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

// Sadly we need to help the compiler understand the bool type in our mapper

let mapped = h.to_ref().map(hlist![
    |&n| n + 1,
    |b: &bool| !b,
    |&f| f + 1f32]);
assert_eq!(mapped, hlist![2, true, 43f32]);

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

let mapped2 = h.map(hlist![
    |n| n + 3,
    |b: bool| !b,
    |f| f + 8959f32]);
assert_eq!(mapped2, hlist![4, true, 9001f32]);

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pub fn zip<Other>(self, other: Other) -> <Self as HZippable<Other>>::Zipped
where Self: 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)]

§Example

use frunk::HNil;
use frunk_core::hlist;

assert_eq!(HNil.zip(HNil), HNil);

let h1 = hlist![1, false, 42f32];
let h2 = hlist![true, "foo", 2];

let zipped = h1.zip(h2);
assert_eq!(zipped, hlist![
    (1, true),
    (false, "foo"),
    (42f32, 2),
]);

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

§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)

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

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pub fn extend<Other>(self, other: Other) -> <Self as Add<Other>>::Output
where Self: 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]);

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

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pub fn get<T, Index>(&self) -> &T
where Self: 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!!"),
}

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pub fn get_mut<T, Index>(&mut self) -> &mut T
where Self: 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]);

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pub fn pluck<T, Index>(self) -> (T, <Self as Plucker<T, Index>>::Remainder)
where Self: 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])

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pub fn into_tuple2( self, ) -> (<Self as IntoTuple2>::HeadType, <Self as IntoTuple2>::TailOutput)
where Self: 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);

Trait Implementations§

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

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

The resulting type after applying the + operator.

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fn add(self, rhs: RHS) -> Self::Output

Performs the + operation. Read more

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

Implementation when the pluck target key is in the head.

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type TargetValue = V

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type Remainder = Tail

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fn pluck_by_name(self) -> (Field<K, Self::TargetValue>, Self::Remainder)

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

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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.

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

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

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fn pluck_by_name(self) -> (Field<K, Self::TargetValue>, Self::Remainder)

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

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

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

Returns a copy of the value. Read more

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

Performs copy-assignment from source. Read more

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

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fn embed(self) -> Out

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

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

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

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

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impl<'a, F, R, MapperTail, CH, CTail> CoproductMappable<&'a HCons<F, MapperTail>> for Coproduct<CH, CTail>
where F: Fn(CH) -> R, CTail: CoproductMappable<&'a MapperTail>,

Implementation for mapping a Coproduct using a &hlist!.

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type Output = Coproduct<R, <CTail as CoproductMappable<&'a MapperTail>>::Output>

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fn map(self, mapper: &'a HCons<F, MapperTail>) -> Self::Output

Use functions to map each variant of a coproduct. Read more

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impl<'a, F, R, MapperTail, CH, CTail> CoproductMappable<&'a mut HCons<F, MapperTail>> for Coproduct<CH, CTail>
where F: FnMut(CH) -> R, CTail: CoproductMappable<&'a mut MapperTail>,

Implementation for mapping a Coproduct using a &mut hlist!.

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type Output = Coproduct<R, <CTail as CoproductMappable<&'a mut MapperTail>>::Output>

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fn map(self, mapper: &'a mut HCons<F, MapperTail>) -> Self::Output

Use functions to map each variant of a coproduct. Read more

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

Implementation for mapping a Coproduct using an hlist!.

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type Output = Coproduct<R, <CTail as CoproductMappable<MapperTail>>::Output>

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fn map(self, mapper: HCons<F, MapperTail>) -> Self::Output

Use functions to map each variant of a coproduct. Read more

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

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

Formats the value using the given formatter. Read more

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

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fn default() -> Self

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

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