Enum typewit::const_marker::BoolWit

source ·

pub enum BoolWit<const B: bool> {
    True(TypeEq<Bool<B>, Bool<true>>),
    False(TypeEq<Bool<B>, Bool<false>>),
}

Expand description

Type Witness that Bool is either Bool<true> or Bool<false>.

Example

Making a function that takes a generic Foo and calls methods on Foo<false> or Foo<true> depending on the value of the const B: bool parameter.

use typewit::{const_marker::{Bool, BoolWit}, MakeTypeWitness};
 
 
assert_eq!(call_next(Incrementor::<GO_UP>(4)), Incrementor(5));
assert_eq!(call_next(Incrementor::<GO_UP>(5)), Incrementor(6));
 
assert_eq!(call_next(Incrementor::<GO_DOWN>(4)), Incrementor(3));
assert_eq!(call_next(Incrementor::<GO_DOWN>(3)), Incrementor(2));
 
 
const fn call_next<const B: bool>(incrementor: Incrementor<B>) -> Incrementor<B> {
    typewit::type_fn! {
        // type-level function from `Bool<B>` to `Incrementor<B>`
        struct IncrementorFn;
        impl<const B: bool> Bool<B> => Incrementor<B>
    }
 
    // The example below this one shows how to write this match more concisely
    match BoolWit::MAKE {
        // `bw: TypeEq<Bool<B>, Bool<true>>`
        BoolWit::True(bw) => {
            // `te: TypeEq<Incrementor<B>, Incrementor<true>>`
            let te = bw.project::<IncrementorFn>();
 
            // `te.to_right` casts `Incrementor<B>` to `Incrementor<true>`,
            // (this allows calling the inherent method).
            // 
            // `te.to_left` casts `Incrementor<true>` to `Incrementor<B>`
            te.to_left(te.to_right(incrementor).next())
        }
        // `bw: TypeEq<Bool<B>, Bool<false>>`
        BoolWit::False(bw) => {
            // `te: TypeEq<Incrementor<B>, Incrementor<false>>`
            let te = bw.project::<IncrementorFn>();
 
            // like the other branch, but with `Incrementor<false>`
            te.to_left(te.to_right(incrementor).next())
        }
    }
}
 
 
#[derive(Debug, Copy, Clone, PartialEq, Eq)]
struct Incrementor<const GO_UP: bool>(usize);
 
const GO_UP: bool = true;
const GO_DOWN: bool = false;
 
impl Incrementor<GO_DOWN> {
    #[track_caller]
    pub const fn next(self) -> Self {
        Self(self.0 - 1)
    }
}
 
impl Incrementor<GO_UP> {
    pub const fn next(self) -> Self {
        Self(self.0 + 1)
    }
}

Using `polymatch` for conciseness

The polymatch macro can be used to more concisely implement the call_next function.

const fn call_next<const B: bool>(incrementor: Incrementor<B>) -> Incrementor<B> {
    typewit::type_fn! {
        struct IncrementorFn;
        impl<const B: bool> Bool<B> => Incrementor<B>
    }
 
    // expands to a match with two arms, 
    // one for `BoolWit::True` and one for `BoolWit::False`,
    // copying the expression to the right of the `=>` to both arms.
    typewit::polymatch! {BoolWit::MAKE;
        BoolWit::True(bw) | BoolWit::False(bw) => {
            let te = bw.project::<IncrementorFn>();
            te.to_left(te.to_right(incrementor).next())
        }
    }
}

What happens without `BoolWit`

If the call_next function was defined like this:

const fn call_next<const B: bool>(incrementor: Incrementor<B>) -> Incrementor<B> {
    incrementor.next()
}

it would produce this error

error[E0599]: no method named `next` found for struct `Incrementor<B>` in the current scope
  --> src/const_marker/const_witnesses.rs:20:17
   |
7  |     incrementor.next()
   |                 ^^^^ method not found in `Incrementor<B>`
...
38 | struct Incrementor<const WRAPPING: bool>(usize);
   | ---------------------------------------- method `next` not found for this struct
   |
   = note: the method was found for
           - `Incrementor<false>`
           - `Incrementor<true>`