Hello all, I was recently thinking about a generic programming application (auto-generating UI code from data definitions) that would benefit from the ability to "annotate" data definitions. For example, let's say you have a type data Foo = Foo {barred :: Boolean} You are (auto-)generating a UI to display and manipulate `Foo`s. Perhaps the generic UI code defaults to making a check-box for Boolean values, but you would rather have it be a radio button. It would be advantageous to be able to write something like e.g. data Foo = Foo {barred :: Boolean # RenderMode RadioButton} In many languages, you have the ability to annotate data fields in a way which has no bearing whatsoever on the first-order behavior or representation of the type, and is only reflected via the language's generic programming API. For example, in Java, you have class Foo { @RenderMode(RadioButton) bool barred; } And in Rust, you have struct Foo { #[render_mode(radio_button)] barred : bool } In Haskell, I do not believe we have any such system in place for interfacing with Generics. The closest I am familiar with is from the optparse-generic package, where you write code of the form data Foo w = Foo {barred :: w ::: Boolean RenderMode RadioButton} See https://hackage.haskell.org/package/optparse-generic-1.4.8/docs/Options-Gene...:: :: This is basically a clever way to sort of re-create what we have available in Rust or Java, using type families, but with the annoying downside that it makes interacting with your type in normal code much more cumbersome, and is also relatively confusing to read and write. Compare Rust's Clap package, which is very similar to optparse-generic, but more convenient to work with because the annotations don't impact normal usage of the type. See https://docs.rs/clap/latest/clap/#example Would it be technically feasible to add such a thing to GHC's Generics support? I could imagine something like updating the GHC.Generics Meta type's MetaCons constructor to MetaCons Symbol FixityI Bool *[Annotation]* Best, Will