Haskell Foldable Wats

[Phil Ruffwind]

> 2.  to implement a language extension in GHC that allows source
> language to specify a certain instance will never be defined and a
> type error shall be thrown statically at any use site, or

How about a feature that allows GHC to warn whenever a certain
instance is used? That way, if you don't like a certain instance, you
can put {-# FORBIDINSTANCE Foldable ((,) a) #-} in your modules and
get warnings whenever that instance is used.