Hi, [Moving to [email protected]]
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 <[email protected]>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 > [email protected] > http://www.haskell.org/mailman/listinfo/haskell >
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