Structs
An
ApplyFn is a function from A to B wrapped in something Rust’s type
system can more easily digest. Arguments for
Apply::apply() are required to be of this type rather than
an arbitrary type matching Fn(A) -> B.Traits
Apply takes an F<Fn(A) -> B> (or, rather, an F<ApplyFn<'a,A, B>>
specifically) and applies it to an F<A> to produce an F<B>.