Can you give an example of what you'd like to write, but can't? Simon
From: glasgow-haskell-users-boun...@haskell.org [mailto:glasgow-haskell-users-boun...@haskell.org] On Behalf Of José Pedro Magalhães Sent: 04 April 2011 10:53 To: GHC users Cc: Steven Keuchel Subject: Re: [Haskell] Polymorphic types in RHS of type instances Hi, [Moving to glasgow-haskell-users@haskell.org<mailto: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 <steven.keuc...@gmail.com<mailto:steven.keuc...@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 _______________________________________________ Haskell mailing list hask...@haskell.org<mailto:hask...@haskell.org> http://www.haskell.org/mailman/listinfo/haskell
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