Module type_fn

Type-level functions.

Type-level functions come in two flavors: injective, and non-injective

Injective

An injective function is any function f for which a != b implies f(a) != f(b).
(For both injective and non-injective functions, f(a) != f(b) implies a != b)

The InjTypeFn trait encodes injective type-level functions, requiring the type to implement both TypeFn and RevTypeFn.

Example: injective function

typewit::inj_type_fn!{
    struct Upcast;
     
    impl u8 => u16;
    impl u16 => u32;
    impl u32 => u64;
    impl u64 => u128;
}
let _: CallInjFn<Upcast, u8> = 3u16;
let _: CallInjFn<Upcast, u16> = 5u32;

Because Upcast is injective, it is possible to query the argument from the returned value:

let _: UncallFn<Upcast, u16> = 3u8;
let _: UncallFn<Upcast, u128> = 5u64;

Non-injective

The TypeFn trait allows implementors to be non-injective.

Example: non-injective function

typewit::type_fn!{
    struct Bar;
     
    impl<T> Vec<T> => T;
    impl<T> Box<T> => T;
}

Bar is non-injective because it maps both Vec<T> and Box<T> to T.

Re-exports

• pub use crate::inj_type_fn;
• pub use crate::type_fn;
• pub use crate::type_fn;

Structs

• FnIdentity
  Type-level identity function
• FnRev
  Reverses an InjTypeFn, its arguments become return values, and its return values become arguments.
• GRef
  Type-level function from T to &'a T
• GRefMut
  Type-level function from T to &'a mut T
• Invoke
  Type-level function which implements TypeFn by delegating to F

Traits

• InjTypeFn
  An injective type-level function
• RevTypeFn
  The inverse of TypeFn, for getting the argument of a TypeFn from its return value.
• TypeFn
  A function that operates purely on the level of types.

Type Aliases

• CallFn
  Calls the F type-level function with T as its argument.
• CallInjFn
  CallFn with an additional F:InjTypeFn<A> requirement, which helps with type inference.
• UncallFn
  Queries the argument to a F: TypeFn from its return value.