This package introduces a type Ann a to annotate data types with
information which doesn't influence the behaviour of your
program. These annotations can then be displayed, as assistance to the
user.
You are writing a programing language, and representing binder as [de Bruijn indices](https://en.wikipedia.org/wiki/ De_Bruijn_index). Nevertheless you want to keep the variable names written by the user, to be able to interact with them on these terms (e.g. in error messages). With 'Ann' it would look like this:
data Term
= Var Int
| App Term Term
| Lam (Ann String) Term
deriving (Eq)Thanks to the 'Ann' type, you can derive the intended equality: the user's choice of variable doesn't change the term (this is called α-equivalence).
The Validation
applicative can be made into a monad. Specifically Validation (Ann e) is a monad, as I explained in a Twitter
thread.