> Consider:
>
> --
> data T a = T
> --
>
> Is there anything in the Prelude or a standard library that
> looks like
> this? Should there be? What should it be called? 'Singleton'?
> 'Reified'?
> 'T'?
>
> I looked but couldn't find anything. Such a simple type
> constructor is in
> fact very useful when you need to pass types around to
> disambiguate class
> instances. For instance:
>
> --
> class HasTypeName t where
> getTypeName :: T t -> String
>
> instance HasTypeName Int where
> getTypeName T = "Int"
>
> instance HasTypeName () where
> getTypeName T = "()"
>
> intName = getTypeName (T :: T Int)
The type T is undoubtedly useful, but I think the example you give isn't
a good one. It works perfectly well without the T data type:
class HasTypeName t where
getTypeName :: t -> String
instance HasTypeName Int where
getTypeName _ = "Int"
instance HasTypeName () where
getTypeName _ = "()"
intName = getTypeName (undefined :: Int)
(this is just like the Typeable class in the Dynamic library, BTW)
Cheers,
Simon
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