[RFC] Introduce As typeclass: composable errors

[chshersh]

I propose to introduce the following typeclass:

class As s a where
    as    :: a -> s
    match :: s -> Maybe a

It represents the ability to convert errors of type a to errors of type s and match back. Obvious law: match . as = Just. It can be used not only for errors, but this is its main goal.

So the type signature of a function like this:

queryNamed
    :: (WithError PgNamedError m, WithDb env m, FromRow res)
    => Sql.Query
    -> [NamedParam]
    -> m [res]

will become

queryNamed
    :: (WithError err m, As err PgNamedError, WithDb env m, FromRow res)
    => Sql.Query
    -> [NamedParam]
    -> m [res]

with a trivial instance for PgNamedError:

instance As PgNamedError PgNamedError where
    as    = id
    match = Just

[vrom911]

I see how helpful it could be with our architecture. Also, having possible errors that function can throw on type-level is also sounds good 👍

[chshersh]

@vrom911 I believe this approach can be implemented now and our projects still can use monomorphic PgNamedError without need to migrate to this approach. What do you think, should we do it now and use it later when we're ready?