This crate provides abstractions for creating type witnesses.

The inciting motivation for this crate is emulating trait polymorphism in const fn (as of 2023-04-30 it's not possible to call trait methods in const contexts on stable).

What are type witnesses

Type witnesses are enums that allow coercing between a type parameter and a range of possible types (one per variant).

The simplest type witness is TypeEq<L, R>, which only allows coercing between L and R.

Most type witnesses are enums with TypeEq fields, which can coerce between a type parameter and as many types as there are variants.

Examples

Polymorphic function

This demonstrates how one can write a return-type-polymorphic const fn (as of 2023-04-30, trait methods can't be called in const fns)

use typewit::{MakeTypeWitness, TypeEq};

assert_eq!(returnal::<u8>(), 3);
assert_eq!(returnal::<&str>(), "hello");


const fn returnal<'a, R>() -> R
where
    RetWitness<'a, R>: MakeTypeWitness,
{
    match MakeTypeWitness::MAKE {
        RetWitness::U8(te) => {
            // `te` (a `TypeEq<R, u8>`) allows coercing between `R` and `u8`,
            // because `TypeEq` is a value-level proof that both types are the same.
            // `te.to_left(...)` goes from `u8` to `R`.
            te.to_left(3u8)
        }
        RetWitness::Str(te) => {
            // `te` is a `TypeEq<R, &'a str>`
            // `te.to_left(...)` goes from `&'a str` to `R`.
            te.to_left("hello")
        }
    }
}

// This macro declares a type witness enum
typewit::simple_type_witness! {
    // Declares `enum RetWitness<'a, __Wit>` 
    // (the `__Wit` type parameter is implicitly added after all generics)
    enum RetWitness['a] {
        // This variant requires `__Wit == u8`
        U8 = u8,
   
        // This variant requires `__Wit == &'a str`
        Str = &'a str,
    }
}

Indexing polymorphism

This function demonstrates const fn polymorphism and projecting TypeEq by implementing TypeFn.

(this example requires Rust 1.61.0, because of the I: SliceIndex<T>, bound)

use std::ops::Range;

use typewit::{HasTypeWitness, TypeEq};

fn main() {
    let array = [3, 5, 8, 13, 21, 34, 55, 89];

    assert_eq!(index(&array, 0), &3);
    assert_eq!(index(&array, 3), &13);
    assert_eq!(index(&array, 0..4), [3, 5, 8, 13]);
    assert_eq!(index(&array, 3..5), [13, 21]);
}

const fn index<T, I>(slice: &[T], idx: I) -> &SliceIndexRet<I, T>
where
    I: SliceIndex<T>,
{
    // `I::WITNESS` is `<I as HasTypeWitness<IndexWitness<I>>>::WITNESS`,
    match I::WITNESS {
        IndexWitness::Usize(arg_te) => {
            // `arg_te` (a `TypeEq<I, usize>`) allows coercing between `I` and `usize`,
            // because `TypeEq` is a value-level proof that both types are the same.
            let idx: usize = arg_te.to_right(idx);

            // using the `TypeFn` impl for `FnSliceIndexRet<T>` to 
            // map `TypeEq<I, usize>` 
            // to  `TypeEq<SliceIndexRet<I, T>, SliceIndexRet<usize, T>>`
            arg_te.project::<FnSliceIndexRet<T>>()
                // converts`TypeEq<SliceIndexRet<I, T>, T>` 
                //      to `TypeEq<&SliceIndexRet<I, T>, &T>`
                .in_ref()
                .to_left(&slice[idx])
        }
        IndexWitness::Range(arg_te) => {
            let range: Range<usize> = arg_te.to_right(idx);
            let ret: &[T] = slice_range(slice, range);
            arg_te.project::<FnSliceIndexRet<T>>().in_ref().to_left(ret)
        }
    }
}

// This macro declares a type witness enum
typewit::simple_type_witness! {
    // Declares `enum IndexWitness<__Wit>` 
    // (the `__Wit` type parameter is implicitly added after all generics)
    enum IndexWitness {
        // This variant requires `__Wit == usize`
        Usize = usize,
   
        // This variant requires `__Wit == Range<usize>`
        Range = Range<usize>,
    }
}

/// Trait for all types that can be used as slice indices
/// 
/// The `HasTypeWitness` supertrait allows getting a `IndexWitness<Self>`
/// with its `WITNESS` associated constant.
trait SliceIndex<T>: HasTypeWitness<IndexWitness<Self>> + Sized {
    type Returns: ?Sized;
}
impl<T> SliceIndex<T> for usize {
    type Returns = T;
}
impl<T> SliceIndex<T> for Range<usize> {
    type Returns = [T];
}

type SliceIndexRet<I, T> = <I as SliceIndex<T>>::Returns;

// Declares `struct FnSliceIndexRet<T>`
// a type-level function (TypeFn implementor) from `I` to `SliceIndexRet<I, T>`
typewit::type_fn! {
    struct FnSliceIndexRet<T>;

    impl<I: SliceIndex<T>> I => SliceIndexRet<I, T>
}

When the wrong type is passed for the index, the compile-time error is the same as with normal generic functions:

error[E0277]: the trait bound `RangeFull: SliceIndex<{integer}>` is not satisfied
  --> src/main.rs:43:30
   |
13 |     assert_eq!(index(&array, ..), [13, 21]);
   |                -----         ^^ the trait `SliceIndex<{integer}>` is not implemented for `RangeFull`
   |                |
   |                required by a bound introduced by this call
   |
   = help: the following other types implement trait `SliceIndex<T>`:
             std::ops::Range<usize>
             usize

Cargo features

These are the features of this crates:

• "rust_1_61": allows the typewit crate to use Rust 1.61.0 features.

• "rust_stable": enables all the "rust_1_*" features.

• "alloc": enable items that use anything from the standard alloc crate.

• "const_marker"(enabled by default): Enables the const_param module, and all items that depend on it.

• "mut_refs": turns functions that take mutable references into const fns. note: as of April 2023, this crate feature requires a stable compiler from the future.

• "nightly_mut_refs"(requires the nightly compiler): Enables the "mut_refs" crate feature and the const_mut_refs nightly feature.

None of the crate features are enabled by default.

No-std support

typewit is #![no_std], it can be used anywhere Rust can be used. You need to enable the "alloc" feature to enable items that use anything from the standard alloc crate.

Minimum Supported Rust Version

typewit supports Rust 1.57.0.

Features that require newer versions of Rust, or the nightly compiler, need to be explicitly enabled with crate features.