[Haskell-cafe] generalized newtype deriving allows the definition of otherwise undefinable functions

[Wolfgang Jeltsch]

Am Dienstag, 9. März 2010 15:54:16 schrieb Jan-Willem Maessen:
> On Mar 9, 2010, at 5:53 AM, Max Cantor wrote:
> > Isn't this just an extension of the notion that multi-parameter
> > typeclasses without functional dependencies or type families are
> > dangerous and allow for type-naughtiness?
> 
> I wondered the same thing, but came up with an analogous problematic case
> that *only* uses generalized newtype deriving:
>
> […]

Originally, I had a more restricted example in mind which is similar to yours. 
However, I wanted to generalize the problem and therefore introduced the 
general Iso class which made MultiParamTypeClasses and FlexibleInstances 
necessary. The actual problem is related neither to MultiParamTypeClasses nor 
to FlexibleInstances.

Best wishes,
Wolfgang