Warning about certain instances (Was: Add conspicuously missing Functor instances for tuples)

[Bart Massey]

IMHO managing import of instances using the normal module import mechanisms
is the only sane way to go. I seem to recall SPJ wanting to do this many
years ago: I don't remember the details of the discussion or why it wasn't
actually done. It does seem hard, but would be so worth it.

On Wed, Feb 17, 2016 at 2:30 AM Henning Thielemann <
lemming at henning-thielemann.de> wrote:

>
> On Mon, 18 Jan 2016, Eric Seidel wrote:
>
> > 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.
>
> Nice to see that you have concrete suggestions how to implement such a
> warning. Once I proposed an extension for forbidding certain instances:
>    https://ghc.haskell.org/trac/ghc/ticket/7775
>
> I was forwarded to the "instance chain" proposal that would solve my
> problem:
>    https://ghc.haskell.org/trac/ghc/ticket/9334
>
> This ticket also contains a solution that requires only existing
> extensions. However this solution does not allow to only warn about a
> problematic instance or to prevent the use of such an instance only
> locally.
>
> A proper solution would allow to define a set of forbidden instances and
> import that in other packages, but does not affect packages later in the
> import chain. A way for explicit import of instances was discussed every
> now and then. This might also provide a way to get rid of specific
> instances.
>
>
> However, every solution seems to be a long way to go. Eventually, it looks
> wrong to me to add security holes first and then start to fix them, or
> defer that for an indefinite time.
>
>
> Btw. I see that the problem was already discussed on Reddit:
>
> https://www.reddit.com/r/haskell/comments/3qcg2d/proposal_forbidden_instances/
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