[Haskell-cafe] redefining a type class for one instance?

[Edward Amsden]

On Tue, Apr 12, 2011 at 3:13 PM, Henning Thielemann
<lemming at henning-thielemann.de> wrote:
>
> On Tue, 12 Apr 2011, Edward Amsden wrote:
>
>> I am trying as an experiment to write something as follows:
>>
>> data Wrapper a b where
>>  Pure :: (a -> b) -> Wrapper a b
>>  Nullable :: (Maybe a -> b) -> Wrapper a b
>>
>> class Liftable a b where
>>  liftTo :: (b -> c) -> a b c
>>
>> instance Liftable Wrapper a where
>>  liftTo = Pure
>
> Would it be ok, to erm wrap 'a', say
>
> newtype Generic a = Generic a
>
> instance Liftable Wrapper (Generic a) where
>  liftTo = ...
That rather defeats the purpose, as the point is to have a type level
dispatch on whether or not we can use a certain constructor or just a
general "all-purpose" constructor. This requires the user to decide to
wrap 'a' in Generic and not do the same for Maybe a.

>
>
>> instance Liftable Wrapper (Maybe a) where
>>  liftTo = Nullable
>



-- 
Edward Amsden
Student
Computer Science
Rochester Institute of Technology
www.edwardamsden.com