On Mon, Jun 6, 2011 at 7:52 AM, Yitzchak Gale
Scott Lawrence wrote:
More specifically, I have
class Model m a | m -> a where ... class Entropy d where ... instance (Model m a) => Entropy m where ...
The first line requires MultiParamTypeClasses and FunctionalDependencies... the third requires UndecidableInstances... Is this likely to cause a problem?
Yes.
You almost never want to use UndecidableInstances when writing practical programs in Haskell. When GHC tells you that you need them, it almost always means that your types are poorly designed, usually due to influence from previous experience with OOP.
Your best bet is to step back and think again about the problem you are trying to solve. What is the best way to formulate the problem functionally? That will lead you in the right direction. Please feel free to share more details about what you are trying to do. We would be happy to help you work out some good directions.
Are you sure you weren't thinking of OverlappingInstances here? I haven't seen as much scorn heaped upon Undecidable.
Regards, Yitz
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