Hi,
[Moving to glasgow-haskell-users@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
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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