Module frunk::hlist

Expand description

Module that holds HList data structures, implementations, and typeclasses.

Typically, you would want to use the hlist! macro to make it easier for you to use HList.


use frunk_core::{hlist, HList, poly_fn};

let h = hlist![1, "hi"];
assert_eq!(h.len(), 2);
let (a, b) = h.into_tuple2();
assert_eq!(a, 1);
assert_eq!(b, "hi");

// Reverse
let h1 = hlist![true, "hi"];
assert_eq!(h1.into_reverse(), hlist!["hi", true]);

// foldr (foldl also available)
let h2 = hlist![1, false, 42f32];
let folded = h2.foldr(
            hlist![|acc, i| i + acc,
                   |acc, _| if acc > 42f32 { 9000 } else { 0 },
                   |acc, f| f + acc],
assert_eq!(folded, 9001);

let h3 = hlist![9000, "joe", 41f32];
// Mapping over an HList with a polymorphic function,
// declared using the poly_fn! macro (you can choose to impl
// it manually)
let mapped =
    |f: f32|   -> f32 { f + 1f32 },
    |i: isize| -> isize { i + 1 },
    ['a] |s: &'a str| -> &'a str { s }
assert_eq!(mapped, hlist![9001, "joe", 42f32]);

// Plucking a value out by type
let h4 = hlist![1, "hello", true, 42f32];
let (t, remainder): (bool, _) = h4.pluck();
assert_eq!(remainder, hlist![1, "hello", 42f32]);

// Resculpting an HList
let h5 = hlist![9000, "joe", 41f32, true];
let (reshaped, remainder2): (HList![f32, i32, &str], _) = h5.sculpt();
assert_eq!(reshaped, hlist![41f32, 9000, "joe"]);
assert_eq!(remainder2, hlist![true]);



  • Represents the most basic non-empty HList. Its value is held in head while its tail is another HList.
  • Represents the right-most end of a heterogeneous list


  • Trait for performing a left fold over an HList
  • Trait for performing a right fold over an HList
  • Typeclass for HList-y behaviour
  • Trait for mapping over an HList
  • Trait for zipping HLists
  • Trait for transforming an HList into a nested tuple.
  • Indexed type conversions of T -> Self with index I. This is a generalized version of From which for example allows the caller to use default values for parts of Self and thus “fill in the blanks”.
  • An indexed conversion that consumes self, and produces a T. To produce T, the index I may be used to for example “fill in the blanks”. LiftInto is the reciprocal of LiftFrom.
  • Trait defining extraction from a given HList
  • Trait for pulling out some subset of an HList, using type inference.
  • Trait for borrowing an HList element by type


  • Takes an element and an Hlist and returns another one with the element prepended to the original list. The original list is consumed
  • Free function version of LiftFrom::lift_from.