Re:Rust: “Explain GATs Like I'm 5 Years Old”
by gpderetta on Monday November 07, 2022 @10:24AM
(#33506963)
You can generalize this:
A: type level functions from types to (associated) types (i.e. normal generics)
B: type level functions from types to type level functions (either As or Bs) (GAT).
with generic parameters (i.e. higher kinded types, or template template parameters in C++) you could have type level functions that map type functions to (types or type functions), but once you have GATs you can fake them by passing to first order functions a type that has an associated type.
Associated types are "outputs", they are controlled by the impl (`<Vec<String> as IntoIterator>::into_iter()` will always return a `std::vec::IntoIter<String>`, because that is how the trait is implemented for `Vec<T>`).
You can often use generic parameters for "outputs" as well (you could have a world where you had `impl<T> IntoIterator<IntoIter<T>> for Vec<T>`). However, this prevents the compiler from inferring the "output" type since there is no longer a guarantee that it is unique for a given set of "inputs".