[Haskell-cafe] Inheritance and Wrappers

John Lato jwlato at gmail.com
Tue Feb 1 12:41:34 CET 2011

> From: Steffen Schuldenzucker <sschuldenzucker at uni-bonn.de>
> On 01/31/2011 08:58 PM, MattMan wrote:
> > [...]
> >
> > data Wrapper a = Wrap a
> > instance (Num a) =>  AbGroup (Wrapper a) where
> >       add (Wrap i) (Wrap j) = Wrap(i+j)
> >
> > However, this is clumsy.  Is there something else I can do?  Thanks
> This is the normal approach. You can do funny things with the
> OverlappingInstances extension, but it is probably not what you want.
> The problem is that the compiler only considers the heads of the
> instance declarations when determining which instance to use for a
> specific type. So an instance like this:
>  > instance (Num a) => AbGroup a where ...
> means: Some type matching 'a' (that is, any type) is an instance of
> 'AbGroup' if and only if it is an instance of 'Num'.

I would word this differently.  I would say this instance means: Some type
matching 'a' (that is, any type) is an instance of 'AbGroup'.  It is an
error to use AbGroup methods if 'a' does not have a Num instance in scope.

The important point is that this declares an AbGroup instance for every
type, not just types with Num instances.  Which means that this instance is
much less useful than you would hope.

I would probably use either Henning's approach of separate functions or an
automatic deriving mechanism.  I've never understood why Haskellers are so
averse to writing new instances anyway.

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