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

[Gábor Lehel]

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.
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