Allow ‘unsafe’ deriving strategy, deriving code with ‘unsafeCoerce’

This

newtype M m a = M (m a)
  deriving newtype
    Functor

produces something similar to

instance Functor m => Functor (M m) where
  fmap :: forall a a'. (a -> a') -> (M m a -> M m a')
  fmap = coerce (fmap @m @a @a')

but as wiki:Roles2 points out, this fails for methods such as join_:

class MonadJoin m where
  join_ :: m (m a) -> m a

newtype M m a = M (m a)
  deriving newtype
    MonadJoin 
 -- Couldn't match representation of type ‘m (M m a)’ with that of ‘m (m a)’

I think the user should be given the option to respect the roles or not by supplying unsafe wiki:Commentary/Compiler/DerivingStrategies

newtype M m a = M (m a)
  deriving unsafe newtype
    MonadJoin

-- -( Produces `unsafeCoerce' instead of `coerce' )-

-- instance MonadJoin m => MonadJoin (M m) where
--   join_ :: forall a. M m (M m a) -> M m a
--   join_ = unsafeCoerce (join_ @m @a)

This lets us (newtype-) derive a host of instances that we currently can not: Traversable, Distributive (currently forbidden due to distribute), Trace, ProfunctorMonad / ProfunctorComonad, ...

It does not seem like lenses (FunctorWithIndex, FoldableWithIndex, TraversableWithIndex) work with this though

Trac metadata

Trac field	Value
Version	8.0.1
Type	FeatureRequest
TypeOfFailure	OtherFailure
Priority	normal
Resolution	Unresolved
Component	Compiler
Test case
Differential revisions
BlockedBy
Related
Blocking
CC
Operating system
Architecture