Functor, Applicative, Monad, Foldable, Traversable instances for (, , ) a b

[Henning Thielemann]

On Fri, 31 Mar 2017, Sven Panne wrote:

> So a reluctant +1 for b), but only *after* we have a flag/pragma. A 
> strong -1 for adding the instances before such a flag/pragma.

Generally we should first have a way to limit damage before adding more 
type unsafety. Currently re-adding safety belts lags years behind 
reduction of type safety.

Btw. the ticket is https://ghc.haskell.org/trac/ghc/ticket/11796


> Next battle: What will be the default for the flag/pragma? >:-)

I'd prefer that this warning is enabled by default. Next best solution is 
to integrate the warning in Wall. There are people who even find that too 
annoying.


Using the new selective conversion of warnings to errors we can also get 
back type errors:
    https://ghc.haskell.org/trac/ghc/ticket/11219