[][src]Struct frunk_core::hlist::HNil

pub struct HNil;

Represents the right-most end of a heterogeneous list

Examples

let h = h_cons(1, HNil);
let h = h.head;
assert_eq!(h, 1);Run

Methods

impl HNil
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Returns the length of a given HList

Examples

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

Prepend an item to the current HList

Examples

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");Run

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.

Examples

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]);Run

Reverse the HList.

Examples

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

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

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

Examples

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

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

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

Examples

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

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

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;

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]);Run

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:

This example is not tested
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);
accRun

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

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
This example is not tested
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)Run

Trait Implementations

impl<Choices> CoproductSubsetter<CNil, HNil> for Choices
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impl CoproductEmbedder<CNil, HNil> for CNil
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impl<Head, Tail> CoproductEmbedder<Coproduct<Head, Tail>, HNil> for CNil where
    CNil: CoproductEmbedder<Tail, HNil>, 
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impl HList for HNil
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Deprecated since 0.1.30

: Please use len() or static_len() instead.

Returns the length of a given HList Read more

Prepends an item to the current HList Read more

impl<Source> Sculptor<HNil, HNil> for Source
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Implementation for when the target is an empty HList (HNil)

Index type is HNil because we don't need an index for finding HNil

impl<F> HMappable<F> for HNil
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impl<F, Init> HFoldRightable<F, Init> for HNil
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impl<F, Acc> HFoldLeftable<F, Acc> for HNil
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impl IntoUnlabelled for HNil
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Implementation for HNil

impl IntoValueLabelled for HNil
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impl Transmogrifier<HNil, HNil> for HNil
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Implementation of Transmogrifier for when the Target is empty and the Source is empty.

impl<SourceHead, SourceTail> Transmogrifier<HNil, HNil> for HCons<SourceHead, SourceTail>
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Implementation of Transmogrifier for when the Target is empty and the Source is non-empty.

impl<'a> ToRef<'a> for HNil
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impl<'a> ToMut<'a> for HNil
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impl IntoReverse for HNil
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impl<T> Into<Vec<T>> for HNil
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impl PartialOrd<HNil> for HNil
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This method tests less than (for self and other) and is used by the < operator. Read more

This method tests less than or equal to (for self and other) and is used by the <= operator. Read more

This method tests greater than (for self and other) and is used by the > operator. Read more

This method tests greater than or equal to (for self and other) and is used by the >= operator. Read more

impl Default for HNil
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impl Ord for HNil
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Compares and returns the maximum of two values. Read more

Compares and returns the minimum of two values. Read more

impl PartialEq<HNil> for HNil
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This method tests for !=.

impl From<()> for HNil
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impl Clone for HNil
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Performs copy-assignment from source. Read more

impl Copy for HNil
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impl Eq for HNil
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impl Debug for HNil
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impl Hash for HNil
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Feeds a slice of this type into the given [Hasher]. Read more

impl<RHS> Add<RHS> for HNil where
    RHS: HList
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The resulting type after applying the + operator.

Auto Trait Implementations

impl Send for HNil

impl Sync for HNil

Blanket Implementations

impl<Choices> CoproductSubsetter for Choices
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impl<Source> Sculptor for Source
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impl<T, U, I> LiftInto for T where
    U: LiftFrom<T, I>, 
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impl<T, U> Into for T where
    U: From<T>, 
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impl<T> ToOwned for T where
    T: Clone
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impl<T> From for T
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impl<T, U> TryFrom for T where
    T: From<U>, 
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🔬 This is a nightly-only experimental API. (try_from)

The type returned in the event of a conversion error.

impl<T> Borrow for T where
    T: ?Sized
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impl<T> Any for T where
    T: 'static + ?Sized
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impl<T, U> TryInto for T where
    U: TryFrom<T>, 
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🔬 This is a nightly-only experimental API. (try_from)

The type returned in the event of a conversion error.

impl<T> BorrowMut for T where
    T: ?Sized
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