Struct frunk::hlist::HCons[][src]

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: Htail: T

Implementations

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

Examples

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

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!!"),
}
Run

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]);
Run

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])
Run

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);
Run

Returns the length of a given HList

Examples

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

Returns whether a given HList is empty

Examples

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

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

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;

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),
]);
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).
  • 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

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.
  • 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

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

Trait Implementations

The resulting type after applying the + operator.

Performs the + operation. Read more

Implementation when the pluck target key is in the head.

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

Implementation when the pluck target key is in the tail.

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

Returns a copy of the value. Read more

Performs copy-assignment from source. Read more

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

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

Attempt to extract a value from a subset of the types.

Formats the value using the given formatter. Read more

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

Performs the conversion.

Performs the conversion.

Performs the conversion.

Performs the conversion.

Performs the conversion.

Performs the conversion.

Performs the conversion.

Performs the conversion.

Performs the conversion.

Performs the conversion.

Performs the conversion.

Performs the conversion.

Performs the conversion.

Performs the conversion.

Performs the conversion.

Performs the conversion.

Performs the conversion.

Performs the conversion.

Performs the conversion.

Performs the conversion.

Performs the conversion.

Performs the conversion.

Performs the conversion.

Performs the conversion.

Performs the conversion.

Performs the conversion.

Performs the conversion.

Performs the conversion.

Performs the conversion.

Performs the conversion.

Performs the conversion.

Performs the conversion.

Performs the conversion.

Performs the conversion.

Performs the conversion.

Performs the conversion.

Performs the conversion.

Performs the conversion.

Performs the conversion.

Performs the conversion.

Performs the conversion.

Performs the conversion.

Performs the conversion.

Performs the conversion.

Performs the conversion.

Performs the conversion.

Performs the conversion.

Performs the conversion.

Performs the conversion.

Performs the conversion.

Performs the conversion.

Performs the conversion.

Performs the conversion.

Performs the conversion.

Performs the conversion.

Performs the conversion.

Performs the conversion.

Performs the conversion.

Performs the conversion.

Performs the conversion.

Performs the conversion.

Performs the conversion.

Performs the conversion.

Performs the conversion.

Performs the conversion.

Performs the conversion.

Performs the conversion.

Performs the conversion.

Performs the conversion.

Performs the conversion.

Performs the conversion.

Performs the conversion.

Performs the conversion.

Performs the conversion.

Performs the conversion.

Performs the conversion.

Performs the conversion.

Performs the conversion.

Performs the conversion.

Performs the conversion.

Performs the conversion.

Performs the conversion.

Performs the conversion.

Performs the conversion.

Performs the conversion.

Performs the conversion.

Performs the conversion.

Performs the conversion.

Performs the conversion.

Performs the conversion.

Performs the conversion.

Performs the conversion.

Performs the conversion.

Performs the conversion.

Performs the conversion.

Performs the conversion.

Performs the conversion.

Performs the conversion.

Performs the conversion.

Performs the conversion.

Performs the conversion.

Performs the conversion.

Performs the conversion.

Performs the conversion.

Performs the conversion.