Re: [Haskell-cafe] Inheritance and Wrappers

OK, what about this as a use case then. I want to create a type class 'Term' with only one function in it. The function returns a 'termTag' which labels the *"kind"* of a value in a DSL. class Term a where termTag :: a -> String A user of this type-class can happily provide an instance without any other type class requirement. However, I want those types which are instances of Data to be an instance of Term automatically. On top of that, I don't want to stop the user from creating a special instance for their data type. I want to be able to write the following instance to accomplish that: instance Data t => Term t where termTag = show . toConstr And if the user wants to write a more specific instance, they should be welcome to do so: instance Term UserDT where termTag (...) = ... I am not very much interested in the technical details about how things currently are, I am more interested in a discussion about why (if?) this would be considered a *design flaw*? Best, On 1 February 2011 12:18, Stephen Tetley wrote:

On 1 February 2011 11:47, Ozgur Akgun wrote:

...

So, is there a way to declare an AbGroup instance for the types with num instances only?

No - as Henning says its then no more useful than simply a function:

add :: (Num u) => a -> a -> a add = (+)

'Overarching instances' i.e. classes with one implementation are a design flaw, not a virtue [*] so this hypothetical extension isn't welcome...

{-# LANGUAGE OverarchingInstances #-}

[*] Of course, there might be some valid cases for things in Oleg Kiselyov's typecast style, but modelling numericals doesn't look like one of them.

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-- Ozgur Akgun