[Haskell] Polymorphic types in RHS of type instances

[José Pedro Magalhães]

Hi,

[Moving to glasgow-haskell-users at haskell.org]

I would also like to know the answer to this question. While I can imagine
it has something to do with type checking/inference, it is not immediately
clear to me where the problem lies.


Thanks,
Pedro

On Sat, Feb 5, 2011 at 12:25, Steven Keuchel <steven.keuchel at gmail.com>wrote:

> Hi list,
>
> I was wondering why GHC doesn't allow usage of polymorphic types in
> the right-hand side of type instance declarations for type families.
> The GHC user guide states: "The right-hand side of a type instance
> must be a monotype (i.e., it may not include foralls) [...]", but it
> doesn't state the reason.
>
> I stumbled upon this limitation when I was trying to generically
> calculate Johann's and Ghani's interpreter (transformers) for nested
> data types from their "Initial Algebra Semantics is Enough!" paper.
>
> Cheers,
> Steven
>
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