[Haskell-cafe] Synthetic values?

[Alexey Khudyakov]

On 09.02.2011 23:16, Cristiano Paris wrote:
> On Wed, Feb 9, 2011 at 19:33, Alexey Khudyakov
> <alexey.skladnoy at gmail.com>  wrote:
>> ...
>> If Private is not exported one cannot add instances to PRead.
>
> Nice trick. I would have thought of hiding the classes PRead and
> PWrite but I'm not sure if it could break the code.
>
It shouldn't but it would be impossible to write signatures for 
polymorphic functions which use thoose type classes.

foo :: PWrite p => Sealed p Int  <-- impossible to write
foo = ...