[Haskell-cafe] What are the problems with instances for polymorphic types?

[Niklas Haas]

On Sun, 15 Jun 2014 21:47:52 +0200, Gábor Lehel <glaebhoerl at gmail.com> wrote:
> In other words instances for forall-types, such as:
> 
>     instance Foo (forall a. [a]) where ...
> 
> It feels obvious to me that there *would* be problems with this, but I'm
> curious about what, exactly, they are.
> 
> Could someone familiar with the matter either elaborate on them, or refer
> me to an existing explanation, a previous discussion, or something of the
> sort?
> 
> I *don't* have any kind of use case in mind, I'm merely seeking a better
> understanding of the type-system issues involved.
> 
> (I attempted Google, but didn't have much success.)
> 
> Thanks in advance.

It seems to me that it may be possible to get more information about
this by searching for issues related to ImpredicativeTypes, which seem
to be similar. (In fact, one could simulate instances like these by
implementing type classes using ImplicitParams + ImpredicativeTypes +
explicit instance records)