[Haskell-cafe] Generic "annotations" on type definitions

[William Yager]

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-Generic.html#t::
::

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
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