A Fn, also known as Reader or Environment, is a type over a unary javascript function. ie. (a: number) => string can be a Fn. As an algebraic data type, the associated type class instances for Fn are limited to single variable inputs so they will look like (a: number) => string, with only one argument. The purposes of a Fn are many and varied, some common purposes are: computation, reading values from a shared environment, and sub-computations in a modified environment. In many ways Fn is a more powerful abstraction than State, and indeed the State monad in fun is exactly State<S, A> = Fn<[S], [A, S]>.

Currently, there is no implementation of Chain recursion or trampolining for Fn implemented, but it is likely to be a future feature. Once implemented Fn will gain some much needed stack safety.