[Haskell-cafe] Supplying a default implementation for a typeclass based on another class

Svein Ove Aas svein.ove at aas.no
Sun Mar 1 15:12:34 EST 2009

On Sun, Mar 1, 2009 at 6:42 PM, Martin Huschenbett <huschi at gmx.org> wrote:
> Hi,
> you could do something like
>> instance (Show a,Read a) => Binary a where
>>  put = put . show
>>  get = fmap read get
> But then you will need the following language extensions: FlexibleInstances,
> OverlappingInstances, UndecidableInstances
Well, isn't there a good chance it'll end up picking that instance
even when a more specific one is available, then?

I think the problem here is that I don't know of any way to inform GHC
that any particular instance is "less specific".

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