typewit/type_fn/injective.rs
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//! Injective type-level functions
use crate::TypeFn;
use core::marker::PhantomData;
/// An [injective] type-level function
///
/// This trait is implemented automatically when both
/// [`TypeFn`] and [`RevTypeFn`] are implemented, and the function is [injective].
/// `InjTypeFn` cannot be manually implemented.
///
/// # Properties
///
/// These are properties about `InjTypeFn` that users can rely on.
///
/// For any given `F: InjTypeFn<A> + InjTypeFn<B>` these hold:
///
/// 1. If `A == B`, then `CallInjFn<F, A> == CallInjFn<F, B>`.
/// 2. If `CallInjFn<F, A> == CallInjFn<F, B>`, then `A == B`.
/// 3. If `A != B`, then `CallInjFn<F, A> != CallInjFn<F, B>`.
/// 4. If `CallInjFn<F, A> != CallInjFn<F, B>`, then `A != B`.
///
///
/// # Examples
///
/// ### Macro-based Implementation
///
/// ```rust
/// use typewit::{CallInjFn, UncallFn, inj_type_fn};
///
/// let _: CallInjFn<BoxFn, u32> = Box::new(3u32);
/// let _: UncallFn<BoxFn, Box<u32>> = 3u32;
///
/// inj_type_fn!{
/// struct BoxFn;
///
/// impl<T: ?Sized> T => Box<T>
/// }
/// ```
///
/// ### Non-macro Implementation
///
/// ```rust
/// use typewit::{CallInjFn, RevTypeFn, TypeFn, UncallFn};
///
/// let _: CallInjFn<BoxFn, u32> = Box::new(3u32);
/// let _: UncallFn<BoxFn, Box<u32>> = 3u32;
///
///
/// struct BoxFn;
///
/// impl<T: ?Sized> TypeFn<T> for BoxFn {
/// type Output = Box<T>;
///
/// // Asserts that this impl of `TypeFn` for `BoxFn` is injective.
/// const TYPE_FN_ASSERTS: () = { let _: CallInjFn<Self, T>; };
/// }
///
/// impl<T: ?Sized> RevTypeFn<Box<T>> for BoxFn {
/// type Arg = T;
/// }
///
/// ```
///
/// [injective]: mod@crate::type_fn#injective
pub trait InjTypeFn<A: ?Sized>: TypeFn<A, Output = Self::Ret> + RevTypeFn<Self::Ret, Arg = A> {
/// Return value of the function
type Ret: ?Sized;
}
impl<F, A: ?Sized, R: ?Sized> InjTypeFn<A> for F
where
F: TypeFn<A, Output = R>,
F: RevTypeFn<R, Arg = A>,
{
type Ret = R;
}
/// The inverse of [`TypeFn`],
/// for getting the argument of a [`TypeFn`](crate::type_fn::TypeFn)
/// from its return value.
///
/// # Properties
///
/// These are properties about `RevTypeFn` that users can rely on.
///
/// For any given `F: RevTypeFn<R> + RevTypeFn<O>` these hold:
///
/// 1. If `R == O`, then `UncallFn<F, R> == UncallFn<F, O>`
///
/// 2. If `R != O`, then `UncallFn<F, R> != UncallFn<F, O>`
///
/// Disclaimer: this trait **does not** by itself ensure that a function is
/// [injective],
/// since `RevTypeFn<Ret>` can't know if `Self::Arg` is the only argument
/// that could produce `Ret`.
///
/// # Examples
///
/// ### Macro-based impl
///
/// ```rust
/// use std::ops::Range;
///
/// use typewit::{RevTypeFn, UncallFn};
///
/// let array = [3usize, 5];
///
/// // Getting the argument of `ArrayFn` from its return value
/// let value: UncallFn<ArrayFn<2>, [usize; 2]> = array[0];
///
/// assert_eq!(value, 3usize);
///
/// typewit::inj_type_fn!{
/// struct ArrayFn<const N: usize>;
/// impl<T> T => [T; N]
/// }
/// ```
///
/// ### Manual impl
///
/// ```rust
/// use std::ops::Range;
///
/// use typewit::{CallInjFn, RevTypeFn, TypeFn, UncallFn};
///
/// let array = [3usize, 5];
///
/// // Getting the argument of `ArrayFn` from its return value
/// let value: UncallFn<ArrayFn<2>, [usize; 2]> = array[0];
///
/// assert_eq!(value, 3usize);
///
/// struct ArrayFn<const N: usize>;
///
/// impl<T, const N: usize> TypeFn<T> for ArrayFn<N> {
/// type Output = [T; N];
///
/// // Ensures that this impl of `TypeFn` for `ArrayFn` is injective.
/// const TYPE_FN_ASSERTS: () = { let _: CallInjFn<Self, T>; };
/// }
/// impl<T, const N: usize> RevTypeFn<[T; N]> for ArrayFn<N> {
/// type Arg = T;
/// }
/// ```
///
/// ### Non-injective function
///
/// As mentioned above, this trait doesn't make a function [injective].
///
/// In the example below, `NonInjective` isn't injective, because it maps different
/// arguments to the same return value:
///
/// ```rust
/// use typewit::{CallFn, RevTypeFn, TypeFn, UncallFn};
///
/// let _: CallFn<NonInjective, Vec<u8>> = 3u8;
/// let _: CallFn<NonInjective, String> = 5u8;
///
/// let _: UncallFn<NonInjective, u8> = ();
///
///
/// struct NonInjective;
///
/// impl<T> TypeFn<T> for NonInjective {
/// type Output = u8;
/// }
///
/// impl RevTypeFn<u8> for NonInjective {
/// type Arg = ();
/// }
/// ```
///
/// [injective]: mod@crate::type_fn#injective
pub trait RevTypeFn<Ret: ?Sized>: TypeFn<Self::Arg, Output = Ret> {
/// The argument to this function with `Ret` as the return value.
type Arg: ?Sized;
}
/// Queries the argument to a `F: `[`TypeFn`] from its return value.
///
/// # Example
///
/// ```rust
/// use typewit::UncallFn;
///
/// let vect = vec![3u32, 5, 8];
/// let value: UncallFn<VecFn, Vec<u32>> = vect[1];
/// assert_eq!(value, 5u32);
///
/// typewit::inj_type_fn!{
/// struct VecFn;
/// impl<T> T => Vec<T>
/// }
/// ```
pub type UncallFn<F, Ret> = <F as RevTypeFn<Ret>>::Arg;
/// [`CallFn`](crate::CallFn) with an additional `F:`[`InjTypeFn<A>`] requirement,
/// which helps with type inference.
///
/// # Example
///
/// ```rust
/// use typewit::{InjTypeFn, CallInjFn};
///
/// // inferred return type
/// let inferred_ret = upcast(3u8);
/// assert_eq!(inferred_ret, 3);
///
/// // inferred argument type
/// let inferred_arg: u32 = upcast(5);
/// assert_eq!(inferred_arg, 5);
///
/// // Because the return type is `CallInjFn<_, I>`,
/// // this can infer `I` from the return type,
/// fn upcast<I>(int: I) -> CallInjFn<Upcast, I>
/// where
/// Upcast: InjTypeFn<I>,
/// CallInjFn<Upcast, I>: From<I>,
/// {
/// int.into()
/// }
///
///
/// typewit::inj_type_fn!{
/// struct Upcast;
///
/// impl u8 => u16;
/// impl u16 => u32;
/// impl u32 => u64;
/// impl u64 => u128;
/// }
/// ```
///
/// As of October 2023, replacing `CallInjFn` with `CallFn` can cause type inference errors:
///
/// ```text
/// error[E0277]: the trait bound `Upcast: TypeFn<{integer}>` is not satisfied
/// --> src/type_fn/injective.rs:132:32
/// |
/// 11 | let inferred_arg: u32 = upcast(5);
/// | ------ ^ the trait `TypeFn<{integer}>` is not implemented for `Upcast`
/// | |
/// | required by a bound introduced by this call
/// |
/// = help: the following other types implement trait `TypeFn<T>`:
/// <Upcast as TypeFn<u16>>
/// <Upcast as TypeFn<u32>>
/// <Upcast as TypeFn<u64>>
/// <Upcast as TypeFn<u8>>
/// ```
pub type CallInjFn<F, A> = <F as InjTypeFn<A>>::Ret;
macro_rules! simple_inj_type_fn {
(
impl[$($impl:tt)*] ($arg:ty => $ret:ty) for $func:ty
$(where[$($where:tt)*])?
) => {
impl<$($impl)*> crate::type_fn::TypeFn<$arg> for $func
$(where $($where)*)?
{
type Output = $ret;
}
impl<$($impl)*> crate::type_fn::RevTypeFn<$ret> for $func
$(where $($where)*)?
{
type Arg = $arg;
}
};
} pub(crate) use simple_inj_type_fn;
////////////////////////////////////////////////////////////////////////////////
/// Reverses an [`InjTypeFn`], its arguments become return values,
/// and its return values become arguments.
///
/// # Examples
///
/// ### Permutations
///
/// The different ways this function can be combined with [`CallFn`] and
/// [`UncallFn`]
///
/// ```rust
/// use typewit::type_fn::{CallFn, FnRev, UncallFn};
///
/// let _: CallFn<FnRev<Swap>, Right> = Left;
/// let _: UncallFn< Swap, Right> = Left;
///
/// let _: CallFn< Swap, Up> = Down;
/// let _: UncallFn<FnRev<Swap>, Up> = Down;
///
/// typewit::inj_type_fn!{
/// struct Swap;
///
/// impl Left => Right;
/// impl Up => Down;
/// }
///
/// struct Left;
/// struct Right;
/// struct Up;
/// struct Down;
/// ```
///
/// [`CallFn`]: crate::CallFn
pub struct FnRev<F: ?Sized>(PhantomData<fn() -> F>);
impl<F: ?Sized> FnRev<F> {
/// Constructs a `FnRev`.
pub const NEW: Self = Self(PhantomData);
}
impl<F> FnRev<F> {
/// Constructs a `FnRev` from `&F`
pub const fn from_ref(_f: &F) -> Self {
Self::NEW
}
}
impl<F, A: ?Sized> TypeFn<A> for FnRev<F>
where
F: RevTypeFn<A>
{
type Output = UncallFn<F, A>;
}
impl<F, R: ?Sized> RevTypeFn<R> for FnRev<F>
where
F: InjTypeFn<R>
{
type Arg = CallInjFn<F, R>;
}
#[test]
fn test_fnrev_equivalence(){
fn _foo<A, F: InjTypeFn<A>>() {
let _ = crate::TypeEq::<CallInjFn<FnRev<F>, F::Ret>, UncallFn<F, F::Ret>>::NEW;
let _ = crate::TypeEq::<UncallFn<FnRev<F>, A>, CallInjFn<F, A>>::NEW;
}
}