See http://ghc.haskell.org/trac/ghc/wiki/DefaultSuperclassInstances | -----Original Message----- | From: Haskell-Cafe [mailto:haskell-cafe-bounces@haskell.org] On Behalf | Of Henning Thielemann | Sent: 26 August 2013 20:07 | To: Frantisek Farka | Cc: Haskell Cafe | Subject: Re: [Haskell-cafe] Extending Type Classes | | | The problem of refinement of type classes annoys me from time to time | when I work on the NumericPrelude. It is an experimental type class | hierarchy for mathematical types. Sometimes a new data type T shall be | implemented and it turns out that you can implement only a part of all | methods of a certain class. Then a natural step is to split the class | into | two classes A and B: 'A' contains the methods we can implement for T and | 'B' contains the remaining methods and 'B' is a sub-class of 'A'. | First, this means that all client code has to be rewritten. Second, | code | for instances becomes very lengthy, because over the time code tends to | contain one instances for every method. However the many small instances | actually carry information: Every instance has its specialised | constraints. E.g. you would certainly try to use only Applicative | constraints in an Applicative instance and not Monad constraints. | However, | if there is a way to define Applicative and Monad instances in one go, | the | Applicative instance may get Monad constraints. | | _______________________________________________ | Haskell-Cafe mailing list | Haskell-Cafe@haskell.org | http://www.haskell.org/mailman/listinfo/haskell-cafe