Proposal: Add conspicuously missing Functor instances for tuples

[Eric Seidel]

On Mon, Jan 18, 2016, at 15:31, Eric Seidel wrote:
> 
> On Mon, Jan 18, 2016, at 15:19, David Feuer wrote:
> > I would think the easiest
> > way
> > would be to add a "suppressed instance" structure to the type checker. It
> > would basically be a stripped-down analogue of the big instance database
> > the type checker uses for instance resolution. When resolving an
> > instance,
> > the type checker would first check the suppressed instance set for it; if
> > it found it there, it would fail or warn as appropriate. So I could write
> > something like
> > 
> > {-# SuppressInstance Functor ((,) a) #-}
> > 
> > But I could also write something more subtle like
> > 
> > {-# WarnInstance Semigroup v => Monoid (HashMap k v) #-}
> > 
> > which would only warn me if I use the Monoid instance for a HashMap whose
> > values form a semigroup.
> 
> We might be able to make this work as a type-checker plugin (which would
> be quite nice at least for experimentation). I don't recall off the top
> of my head if the constraint solver calls plugins before or after it
> tries to discharge the constraints on its own, but if plugins get access
> to constraints first we could write one that simply marks undesired
> instances as insoluble.

Ah, plugins are run after the built-in solver, bummer.