Parameterizing Stock instance deriving

Motivation

See the original request on haskell-cafe ML: https://mail.haskell.org/pipermail/haskell-cafe/2019-September/131503.html

Proposal

Allow this syntax:

data E = ...
  deriving stock Eq via myTransformer

myTransformer :: E -> E
myTransformer = ...

This would mean that myTransformer is applied to the values before they are passed to the derived class functions ((==), etc.).

This extension would require a GHC proposal.